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FOREWORD

Anyone who works at the German Parliament, whether they are an elected representative or a member of the administration staff, comes into contact every day with one of the most striking stone witnesses to German history: the Reichstag Building.

What takes place within the building itself is already worth a look – this is after all where policy is conceived and implemented for a population of 83 million. It is rare, however, for a European parliamentary building to reflect the destiny and fate of the nation it was designed to serve in a more vivid, dramatic and unsparing way than the Reichstag Building in Berlin does.

The Hohenzollern had no love for the building, shortly after the Nazis seized power it was set ablaze, and when the Red Army ultimately seized it on 1 May 1945 after many days of battle, it was left in ruins. Soviet soldiers left victors’ graffiti on the walls of the building and while these inscriptions were initially covered up during the years of division in Germany, when the building was restored following the fall of the Wall, they were made visible once more.

All of this is history set in stone.

The city of Berlin is awash with these stone witnesses to history: the Olympic Stadium, the main building of the Messe, the Brandenburg Gate, the New Guardhouse, the newly constructed Berlin City Palace, the towers at Frankfurter Tor – to name but a few. Successful and less successful attempts at restoration can be found everywhere, in the east of the city as much as in the west. They all reflect a piece of urban history, a piece of German history. They are object lessons and stone-hewn witnesses to success and failure, to acts of heroism and barbarity. It is the responsibility of the state to preserve such buildings in order to make the history of Germany tangible for future generations too.

And that is why it is a particular honour for me to welcome you to a congress such as “Monument Future, Decay and Conservation of Stone”, a meeting of academics, conservators, architects and engineers, here in Göttingen.

Researching the implications of climate change alone on the condition and life expectancy of all buildings, whether they are privately owned or public property, is a task for generations. Even a layperson can recognise that environmental influences increasingly affect not just the patina of the stone. Many things that would have survived for a long time to come if the air were clearer are already crumbling.

There is no doubt that new methods of digitalisation enable a much more exact means of monitoring changes in stone structures. It is fascinating to observe how interdisciplinary paths and solutions develop during a congress such as this, viewing the stone of a building almost as an object undergoing transformation, the changes to which we can influence.

6Your work is historic preservation in the best sense of the term. Too much has been destroyed in this country, even in the aftermath of war in numerous cases and without necessity, many say without any sense of reason and understanding. That is why we need to preserve what remains for us. Your congress makes an important contribution towards achieving this.

For those who are dedicated to preserving these stone witnesses to history are dedicated to the future of those who have learned from history and who wish to continue to learn from it.

Thomas Oppermann Vice-President of the German Bundestag

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FOREWORD

Photographs of crumbling stone figures – along with impressive photos of the demolition of historical buildings – became symbols of environmental pollution and the increasing loss of built cultural heritage in the late 1960s and early 1970s.

The ever increasing criticism and concern about the loss of built cultural heritage in Europe led to a rise in civic and state engagement. The “Stone” congresses, the first of which took place in 1972, were part of the international heritage movement and continue to be so today.

Two years later, in 1974, the founding of the Deutsches Nationalkomitee für Denkmalschutz (DNK, German National Committee for Monument Protection) in the Federal Republic of Germany was an important step towards the acceptance of monument protection as a social and national objective.

First the DNK prepared the German activities for the European Cultural Heritage Year 1975. In the following years the DNK gave a lot of impetus into society, but acted itself as a seismograph for conservation and restoration issues, too.

Since then, the DNK has assumed responsibility for bundling the expertise of science, craft, preservation and conservation expertise. In 1984, for example, a trip with journalists was devoted to the topic of stone deterioration.

Today, half a century after these years of departure, many of the demands of that time have been met. The cooperation between basic research and owners, preservation authorities, architects and engineers, craftsmen and restorers is particularly pronounced in the field of natural and artificial stone.

Nevertheless there are new challenges to be overcome: the climate change with droughts, high temperatures, heavy rain and storms is affecting stone, plaster and mortar. Emissions from industry and car traffic will decrease, but will not disappear. Thus, we cannot rest on our laurels.

The “Stone” congress is a long-standing and important platform for bundling and disseminating new findings on stone conservation topics.

I admire the level of expertise and commitment with which scientists and practical users continuously share their results.

The “Stone” congress is a prime example of the transfer of knowledge between basic research and practical application on built heritage and back.

I also wish “Stone 2020” to continue this remarkable tradition. May the shared insights benefit our heritage in Europe and the world!

Dr. Gunnar Schellenberger State Secretary for Culture and President of the German National Committee for Monument Protection

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FOREWORD

Our built cultural heritage is a great source of inspiration. It provides us with a sense of belonging and identity in a complex world. And much of this heritage – from buildings to monuments and sculptures – is made of stone. Therefore, it is key for preserving our cultural heritage for future generations to better understand, manage and mitigate the deterioration processes that affect stone-based structures and thus rob us of our past.

Preserving stone requires a deep understanding of the various environmental factors that affect the material and innovative approaches to counter the decay they cause. This is why the German Federal Environmental Foundation (Deutsche Bundesstiftung Umwelt DBU) has been a funding partner for research in this field for many years. With our funding programme “Environment and Cultural Heritage” we are supporting pioneering projects at the intersection of environmental science and cultural heritage protection. Since the foundation began its work in 1990, we have provided funding for nearly 830 cultural heritage projects. Special emphasis is placed on problems caused by anthropogenic environmental factors such as sulfur dioxide, nitrogen, or climatic changes.

The contributions in this anthology present an inspiring overview of cutting-edge endeavours in the field of stone conservation. Every four years, the International Council on Monuments and Sights (ICOMOS) brings together experts from a variety of disciplines, ranging from conservators and restoration specialists to engineers and geoscientist to engage in discussion. The topics of exchange cover all relevant aspects from novel modelling and simulation techniques, to better understand environmental impacts on different stones, to original approaches in damage assessment and mitigation. Best-practice examples offer valuable insights into state-of-the-art conservation and restoration procedures and techniques.

Among the participants are several experts working in DBU-funded projects. It fills us with pride to be able to contribute to this important field of research and witness how our support furthers the discussion and understanding of stone conservation and decay.

The DBU thanks the Georg August University Göttingen (Geoscientific Center) and the University of Kassel (Department of Civil and Environmental Engineering) as well as the International Scientific Committee for Stone (ISCS) of ICOMOS for compiling these proceedings.

And we would like to express our gratitude towards all participating experts and researchers for their contribution to the preservation of our collective cultural heritage.

Sincerely yours,

Alexander Bonde Secretary General, German Federal Environmental Foundation (DBU)

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PREFACE

“Monuments Future: Decay and Conservation of Stone”

14th International Congress on the Deterioration and Conservation of Stone

Siegfried Siegesmund¹ and Bernhard Middendorf²

1 Geoscience Centre of the University Goettingen, Goldschmidtstr. 3, 37077 Goettingen, Germany

2 University of Kassel, Department of Structural Materials and Construction Chemistry, Moenchebergstr. 7, 34125 Kassel, Germany

Cultural heritage creates identity and promotes a sense of community even in times of globalisation. At the very latest, the devastating fire at Notre-Dame de Paris shows that a loss of cultural hertitage of this importants is painfully affecting society.

For more than hundred years, statutory monument protection in Germany has been an important part of our cultural self-image and public awareness. Fortunately, since then the protection of historical monuments has enjoyed increasing acceptance among the population. With the recognition of the cultural significance, there has been a change in the appreciation of architectural monuments as cultural monuments. Efforts are now increasingly focused not on the destruction or removal, but on the preservation of our cultural heritage. Events on the occasion of the annual “Open Monument Day” are evidence of this gratifying trend.

Natural stone was the world’s most durable and valuable building material of past millennia. This is why most of our preserved historical monuments are made of natural stone. The diversity of natural stone deposits worldwide has a decisive and unmistakable influence on the regional cultural landscapes. Environmental influences such as natural weathering and the impact of anthropogenic environmental pollutants unfortunately led to growing and serious damage to many monuments. Since the 1970s, the public has become increasingly aware of this environmentally-induced loss of cultural assets. Ever since buildings were and are constructed of natural stone, problems of stone decay have been known. The signs are alarming.

The international scientific “Committee for Stone (ISCS)” of ICOMOS organizes every four years the International Congress on the Deterioration and Conservation of Stone. After congresses in La Rochelle, Athens, Venice, Louisville, Lausanne, Torun, Lisbon, Berlin, Venice, Stockholm, Torun, New York and Paisley, the 14th Interntional Congress was planned to take place from 7th to 12th 2020 at the Universities of Göttingen and Kassel in Germany. The conference is considered the most important forum for the entire field of stone conservation and stone decay and is recognized trend-setter for innovative developments and methods. The worldwide available knowledge about stone decay and stone conservation is summarized in the proceedings presented here and reflects the current state of the art.

In recent years, many individual results have been achieved worldwide in the field of natural stone conservation which, despite increasing digitisation, 12have not been published and are therefore not publicly accessible. In the meantime, however, a new level which requires a summary of the knowledge achieved in order to be able to specialist knowledge on a broad basis in the practice of monument conservation and building maintenance can be transferred. This was also the aim and the topic of the multi-day symposium and was subtitled Monuments Future: Decay and Conservation of Stone.

The conference is aimed at monument conservators, restorers, architects, engineers and natural scientists and will be divided into several thematically different blocks. This conference is the main gathering for the dissemination of knowledge in the field of stone deterioration issues. It represents and captures the state-of-the-art in the field of stone conservation and cultural heritage conservation with regards to the following topics:

— Characterisation of damage phenomena of stone and related building materials (plaster, rendering, mortar etc.)

— Methods for the investigation of stone decay; in-situ and non-destructive testing

— Long-term monitoring of stone monuments and buildings

— Simulation and modelling of decay

— Technology and development of improved treatments and use of stone in new buildings

— Assessment of long-term effects of treatments

— Impact of climate change on stone decay of Cultural Heritage

— Reports about stone conservation: case studies and projects

— Digitalization and documentation in stone conservation

— Open Topics

It is a great pleasure for the Executive Committee to gratefully acknowledge the members of the Permanent Scientific Committee and the Local Scientific Committee for their reviews of each contribution followed by revision by the authors. The contents and accuracy of the papers are the responsibility of the authors!

​Permanent Scientific Committee

— Miloš Drdácký (President, Czech Republic)

— Ann Bourges (France)

— Susanna Bracci (Italy)

— Philippe Bromblet (France)

— Hilde De Clercq (Belgium)

— Katerina Delegou (Greece)

— Julie Desarnaud (Belgium)

— Robert J. Flatt (Switzerland)

— Christoph Franzen (Germany)

— John Hughes (UK)

— Takeshi Ishizaki (Japan)

— Heiner Siedel (Germany)

— Ákos Török (Hungary)

— Johannes Weber (Austria)

​Local Scientific Committee

— Prof. Dr. Siegfried Siegesmund (University of Göttingen, Germany) – Executive Committee

— Prof. Dr. Bernhard Middendorf (University of Kassel, Germany) – Executive Committee

— Dr. Michael Auras (Institute for Stone Conservation (IFS), Mainz, Germany)

— Prof. Dr. Paul Bellendorf (University of Bamberg, Germany)

— Dr. Angela Ehling (Federal Institute for Geosciences and Natural Resource, BGR, Berlin)

— M. A.; M. Sc. Constanze Fuhrmann (The German Federal Environmental Foundation (DBU), Osnabrück, Germany)

— Dr. Luigi Germinario (University of Saitama, Japan)

— Prof. Dr. Gabriele Grassegger (Bau-Consult – Consulting and Development, Winnenden, Germany)

— PD Dr. Michael Hoppert (University of Göttingen, Germany)

— Prof. Dr. Roman Koch (University of Erlangen, Germany)

— PhD Rubén López-Doncel (Universidad Autónoma de San Luis Potosi, Mexico)

— M. A. Johanna Menningen (University of Göttingen, Germany)

— M. Sc. Christopher Pötzl (University of Göttingen, Germany)

13— Dr. Enrico Sassoni (University of Bologna, Italy)

— M. Sc. Tim Schade (University of Kassel, Germany)

— Prof. Dr. Erwin Stadlbauer (Lower saxony state office for the preservation of historical monuments, Hannover, Germany))

— Prof. Dr. Rolf Snethlage (Natural stone, building physics and building chemistry in monument conservation, Bamberg, Germany)

— Dr. Robert Sobott (Laboratory for the preservation of historic buildings and monuments Naumburg, Germany)

— PhD Luís Sousa (University of Trás-os-Montes e Alto Douro Vila Real, Portugal)

— Prof. Dr. Michael Steiger (University of Hamburg, Germany)

— Dr. Wanja Wedekind (University of Göttingen, Germany)

— Dr. Eberhard Wendler (Specialist laboratory, Munich, Germany)

— Dr. Alexander Wetzel (University of Kassel)

The organizing committee also wish to thank all authors for their efforts in preparing the abstracts and contributions, as well as for their collaboration which was the key to the success of these proccedings.

Furthermore we like to thank the German Federal Environmental Foundation (Deutsche Bundesstiftung Umwelt – DBU, AZ- 34720/01), the German German Science Foundation (Deutsche Forschungsgemeinschaft – DFG, AZ-Si 438/53-1) and our commercial sponsors KÄRCHER, REMMERS, TUBAG, FOCUS GmbH, IBZ – Salzchemie GmbH & Co. KG for their support. We also thank the publisher Mitteldeutscher Verlag (MDV) for the compilation of the proceedings.

The editors would like to express their sincere thanks to these persons who, through their tireless efforts, have significantly supported the creation of the proceedings: Tim Schade, Johanna Menningen, Christopher Pötzl, Christian J. Gross, Marie-France Hesse, Alexandra Jung14

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TABLE OF CONTENT

CHARACTERISATION OF DAMAGE PHENOMENA OF STONE AND RELATED BUILDING MATERIALS (PLASTER, RENDERING, MORTAR ETC.)

R. Snethlage

Risk Number: Documentation and Objective Assessment of Environmental Damage to Marble and Sandstone Sculpture

R. Bugini, L. Folli

New marbles for the Italian architecture (1920–40)

K. Ekinci, A. Karagöl, G. G. Küçükosmanoğlu, I. Polat Pekmezci

Characterization and deterioration of materials of Rumelifeneri Fortress in Istanbul

A. Kück, Ch. Pötzl, R. López-Doncel, R. Dohrmann, S. Siegesmund

Tuffs in pre-Columbian and colonial architecture of Oaxaca, Mexico

J. Diaz, B. Menéndez

Sustainable lime restoration mortars: Physical properties and durability assessment

C. Noël, B. Menéndez

Comparison of physico-chemical properties between ancient and restoration renders in the Vexin Français area (NW of Paris)

S. Ahmadinezhad, A. Sansonetti, A. Pane, D. Biondelli

Study of decay patterns and damage assessment of the Achaemenian rock-reliefs of Naqsh-e Rustam

16E. Vasanelli, A. Calia, G. Quarta, D. Melica, M. Masieri, A. Monte, F. Micelli

Effect of fire on the durability of a porous calcarenite: the case study of a tobacco factory in Tricase (Lecce, Southern Italy)

M. Drdácký, D. Frankeová, Z. Slížková

Variations of characteristics of sandstone subjected to weathering and conservation interventions

N. Freudenberg, T. Frühwirt, K.-J. Kohl, M. Kutz, H. Siedel, J. Wichert

Thermal behavior of building sandstone: laboratory heating experiments vs. real fire exposure

L. Schröer, T. De Kock, N. Boon, V. Cnudde

Exploring microbial communities inhabiting gypsum crusts of weathered natural building stones

M. Haselberger, M. Milchin, K. Fuchs, G. Enkhbat, Ts. Tsolmon, J. Weber, G. Krist

Stone Cultural Heritage in Mongolia – Model-like Study and Condition Assessment of the Site of Ikh Khöshööt

M. Morales Demarco, E. Cocorel

Characterization and deterioration assessment of Kösseine Granite in Palacio Salvo, Montevideo, Uruguay

A. Eckart, R. Sobott, H. Kletti, T. Burkert, W. Jäger

2000 years of use of plaster as building and restoration material for the „Takht-e Soleyman“ in Western Azerbaijan, Iran

A. Kück, Ch. Pötzl, R. López-Doncel, R. Dohrmann, S. Siegesmund

Effects of zeolites and swellable clay minerals on water-related properties and thermal dilatation in volcanic tuff rocks

M. Saba, J. Lizarazo-Marriaga, N. L. Hernandez-Romero, C. Tedeschi, E. Quiñones-Bolaños

Physico-chemical characterization of the Cartagena Wall and quarry material stone used for its restoration

Á. Török, L. Germinario, R. López-Doncel, Ch. Pötzl, S. Siegesmund

Comparative analysis of volcanic tuffs from Europe, Asia and North-America

W. Wedekind, E. Harutyunyan, S. Siegesmund

The Cathedral-ruins of Zvartnots and Avan (Armenia) – a comparative study on decay and restoration

17L. Sousa, J. Menningen, S. Siegesmund

Notes of a bowing behavior on limestone

L. Lazzarini, E. Tesser

The four sculpted columns of the St. Mark basilica’ ciborium, Venice: marbles, polychromy, past treatments

METHODS FOR THE INVESTIGATION OF STONE DECAY; IN-SITU AND NON-DESTRUCTIVE TESTING

R. Livingston, C. Grissom, Y. Gorokhovich

Portable XRF study of the geographic distribution and growth rate of mn-rich rock varnish

B. Sacchi, E. Cantisani, T. Salvatici, C. A. Garzonio

Microdrilling resistance measurements system and mortar penetrometer: two methods for evaluating in situ mortar resistance

Ma H., Dai Sh., Zhou Y., Qian B., Tang Zh., Zhang G., Xiang J., Zhen G.

Ultrasonic testing of the dolomite marble statue of Soong Ching-ling with respect to the depth of cracks and deterioration state

T. Waragai, T. Takemura

Acoustic emission behavior of rocks subjected to temperature changes

St. Pirskawetz, J. Menningen, S. Siegesmund

Acoustic emissions – insights into the decay mechanisms of thermally treated marbles

D. Benavente, M. de Jongh, J. J. Galiana-Merino, C. Pla, J. Martinez-Martinez, M. Lee, M. E. Young

Automatic estimation of the P- and S-wave onset-times in weathered sandstones by salt crystallisation

H. Yang, D. W. Kim, C. H. Lee, J. H. Yoo

A study on nondestructive diagnosis and conservation scheme of Korean dinosaur egg fossil sites in Hwaseong Gojeongri

H. Y. Park, S. Y. Park, C. H. Lee, J. H. Park

Material Characteristics and Non-Destructive Diagnosis of Stone Standing Buddha and Stone Chamber of Mireukri Temple Site in Chungju, Korea

18Ch. Pötzl, A. D. Öcal, T. Cramer, S. Siegesmund

Decay of Pre-Columbian phallic stone monoliths

St. Pfefferkorn, Ch. Franzen

Evaluation of effective compression strength of natural stone by drilling resistance measurements

L. Germinario, C. T. Oguchi

Historical man-made caves in Japan: Vulnerability of rocks and cultural assets in the underground environment

F. Dursun

Material Characterization And Decay Of The Limestones Used In Historical Structures Of Mardin, Turkey

M. Drdácký, M. Eisler, R. Krompholz

In-situ investigation of stone consolidation effects with immersed ultrasonic double-probe

F. Tiefensee, C. Degel, P. Weber, W. Bost, M. Moses, M. Schmieger

A new system for fast ultrasound-tomography at marble sculptures

D. M. Freire-Lista, L. Sousa

Fuente de Cibeles of Madrid and decay of Montesclaros marble

O. Sánchez Rovira, D. Giovannacci, J.-D. Mertz, J. Wassermann, B. Ledésert, R. Hébert, Y. Mélinge

Water content estimation using non-destructive tools applied to archaeological materials

W. Wedekind, E. Harutyunyan, N. Novakovic, S. Siegesmund

Experimental conservation and first investigations on the weathering of Geghard Monastery (Armenia)

W. Wedekind, Ch. Schmidt

The three funerary Stelae Project

C. Tedeschi, M. Taccia

Monitoring and evaluation of damaging of traditional cleaning techniques of stones with 3D optical microscopy profilometry

M. Deprez, G. De Schutter, V. Cnudde, T. De Kock

Freeze-thaw weathering: Digging deeper in the temperature and length evolution of natural stones

19G. Cardani

The evaluation of the masonry quality in historical load-bearing stonework

P. Kozub

Karsten Tube Penetration Test – new findings and new evaluation method

S. T. Park, S. H. Kim, C. H. Lee

Evaluation on Reusability and Damage of Stone Properties for Repair of Wolnamsaji Temple Site Stone pagoda for Gangjin Korea in 13th Century

LONG-TERM MONITORING OF STONE MONUMENTS AND BUILDINGS

S. Hamilton, C. Graham, Ch. Gerdwilker, J. Hepher, S. Mirashrafi, E. Hyslop

Challenges of investigating decay in an enclosed bedrock monument: Fossil Grove, Glasgow

O. E. Gilbert, L. Mol, O. Campbell, T. Blenkinsop

The Influence of Angle of Ballistic Impact on Stone Weathering

J. H. Park, C. H. Lee, Y. H. Jo

A Study on Conservation Management System of Fortress Cultural Heritage by Behavior Monitoring and GIS

S. H. Kim, C. H. Lee

Change of Environmental Stability by Improved Airtightness in the Korean Ancient Royal Tombs from the 6th Century Baekje Kingdom

A. G. Costa

Long-Term Monitoring Of Processes Affecting Stone Materials Applied To Historical And Contemporary Buildings In Brazil

J. Menningen, S. Siegesmund, R. Krompholz, Y. Rieffel

Comparative, Non-destructive analysis of the marble sculptures General von Bülow and von Scharnhorst in Berlin

O. A. Cuzman, S. Rescic, P. Tiano

A new portable technique to evaluate the compatibility of protective treatments applied on monumental stones

S. Laue, C. Schaab, D. Drese, D. Krauthäuser, G. Helfmeier, J. Vogt

Long-Term investigations and monitoring of the salt loaded Crypt of St. Maria im Kapitol, Cologne

20K. Wilhelm, D. Gulotta, A. Leslie, M. Krus, V. Thome, R. Kilian, H. Viles

Reviving ‘Asterixe’: a unique long-term archive for built heritage research

S. Siegesmund, M. Morales Demarco, R. Wittenbecher, M. Pfeifer, K.-J. Stein

Koelga Marble as building veneer at the Town Hall of Wermelskirchen (Germany): Weathering dynamics, bowing potential and risk assessment: Part I

S. Siegesmund, M. Morales Demarco, R. Wittenbecher, M. Pfeifer, K.-J. Stein

Koelga Marble as building veneer at the Town Hall of Wermelskirchen (Germany): Weathering dynamics, bowing potential and risk assessment: Part II

A. Bordi, M. Matteini, F. Piqué

2-step DAP consolidation of marble busts on the facade of Lugano’s Cathedral

I. Kisilewicz

The Impact of Natural Climate Hazards impact on the Restoration of Dragomirna Church

J. M. Quadrizius, E. Wendler, J. Meinhardt

Improved Evaluation of Karsten Water Uptake Measurements: Visualisation of the Soaking Front by Absorption of Dye Solutions

Y. Praticò, F. Girardet, R. J. Flatt

New insights on the causes of contour scaling in Swiss sandstones

SIMULATION AND MODELLING OF DECAY

Xiaolin C.

Performance Simulation of Solid Brick Construction under the Impacts of Increased Precipitation as a Consequence of Climate Change

F. Dursun, T. Topal

Effect of weathering tests on microfracture properties of the massive and vesicular basalts

Mengjun J., Ling H.

The implication from the Na2SO4 Crystallization behaviors on Dafosi sandstones during salt-loaded hygrothermal aging cycles

21J. Bartholdy, P. Klenz Larsen, J. Bredal-Jørgensen

The damaging effect of “eco-friendly” deicing salts on building materials

A. Stahlbuhk, M. Steiger

Investigations on the Supersaturation and Damage Potential of Nitrate Salts Relevant for Objects of Cultural Heritage

E. Huby, C. Thomachot-Schneider, P. Vàzquez, G. Fronteau, J. Menningen, S. Siegesmund

Dilatation behavior of building stones repaired with a restoration mortar

J. Frick, N. Sakiyama, B. Hejazi, F. Lehmann, S. Gambarelli, J. Ožbolt, H. Garrecht

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CHARACTERISATION OF DAMAGE PHENOMENA OF STONE AND RELATED BUILDING MATERIALS (PLASTER, RENDERING, MORTAR ETC.)

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RISK NUMBER: DOCUMENTATION AND OBJECTIVE ASSESSMENT OF ENVIRONMENTAL DAMAGE TO MARBLE AND SANDSTONE SCULPTURE

Rolf Snethlage

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

Naturstein in der Denkmalpflege Bamberg, Germany

Summary

Expert reports form the basis for the decision-making concerning conservation measures on marble and sandstone sculptures. However, in the dialogue between the experts, quantitative results in the expert reports are transformed into lingual assessments using subjective expressions such as “weakly/strongly affected” or “little/highly endangered”. In this way, primarily objective measurement values obtain a subjectively tinged interpretation.

Against this background it is the goal of the Risk Number concept to replace a subjective assessment with a preferably objective and quantitative evaluation. The Risk Number is defined as the product of a Measurement Number M(i) and a Valuation Number B(i):

R(i) = M(i) * B(i)

The Measurement Number M(i) constitutes a measured parameter (i) in the form of a dimensionless value. It is calculated by scaling the actual measurement values to the original condition of the object.

The Valuation Number B(i) indicates the effect of the relevant parameter (i) regarding weathering grade and risk of damage.

For the calculation of the Overall Risk Number R(total) of a sculpture, 104 different parameters are evaluated, which are assigned to 10 Excel data sheets representing the following categories:

1 Object Description

2 Art Historical Evaluation

3 Restoration History

4 Environment / Exogenous Risk Factors

5 Natural Stone / Endogenous Risk Factors; calculated separately for marble and sandstone

6 Damage Caused by Human Beings / Vandalism

7 Mechanical Damage / Loss of Material

8 Surface Alteration

9 Ultrasonic Diagnosis

10 Evaluation of Overall Risk Number

For each of these parameters and for each data sheet a separate Risk Number R(i) is calculated. These are summarized to the Overall Risk Number R(total), which expresses the risk the object under consideration is exposed to. The design of the calculation method causes the Risk Number to be a number between zero and one.

Due to a purely mathematical implementation of the measured values, the Risk Number represents an objective and quantitative assessment of the state of damage condition and risk damage.

So far, the application of a Risk Number is intended for marble and sandstone sculptures. An overall 36Risk Number R(total) has been calculated for altogether 8 sculptures and 4 tombstones.

The sculptures include: Apollo, Vestal Verging (both Carrara marble, Neue Kammern, Sanssouci Palace garden, Potsdam), the Flora Aeolus (both Laas marble, Garden of Nymphenburg Palace, Munich), Hera (original), Hera (copy), Rhea (original), and Rhea (copy) (all Cotta sandstone, Baroque Garden, Großsedlitz near Dresden).

The Tombstones from Jewish Cemeteries are: Baiersdorf/Erlangen: Bernhard Ehrenbacher, Siegmund Sulzberger, Loew Ganz (all Schilfsandsteine), No. 1091, Double tomb (red Buntsandstein)

The Total Risk Numbers R(total) calculated for these objects vary from 0.36 (tombstone No. 1091 to 0.64 (sandstone sculptures Hera and Rhea).

Introduction Risk Number: Information Content

The decision for conservation measures on marble or sandstone sculptures is usually made after a dialogue between the owner, heritage conservators, restorers and external experts weighing arguments and determining procedures. In oral and written presentations, the involved parties use expressions about the state of conservation and expected damage progress such as the sculpture is “very/at the highest” at risk, the marble weathered to a “low/medium/highest” degree, the surface of the sculpture is “little/hardly/strongly” affected threatening a loss of “significant/irreplaceable” artistic details. Although these evaluations derive from exact observations and measurements, they are nevertheless highly influenced by the individual and subjective estimation of the rapporteurs. As a result, decisions taken are difficult to retrace later on.

Considering this precondition, the Risk Number concept aims at developing a calculation method on a metrological and objective basis to record the condition and degree of endangerment for an outdoor sculpture. All observations and measured values recorded on an object and its environment are contained in one single figure, the Risk Number.

The concept of assessing the damage risk of historical objects is not completely new. Similar approaches regarding risk evaluation were reported by WAENTIG 2014; DELGADO RODRIGUES & GROSSI 2004; 2007; REVEZ 2010. These authors, however, did not scale the calculated risk to the limits Zero and One because the calculated risk is not easy to rate.

Defining and calculating the Risk Number R(i) and the Valuation Number B(i)

Per definition the Risk Number R(i) is the product of a Measurement Figure M(i) and an Valuation Number B(i):

R(I) = M(i) * B(i)

This definition derives from the general risk definition as it is used in the insurance industries.

The Measurement Figure M(i) constitutes a measured parameter (i) in the form of a dimensionless value. Two different methods of recording the individual Measurement Figure M(i) are used:

— Metrological definable parameters: They are defined by the alteration from the initial zero state to the actual state.

— Numerically recorded parameters: These parameters do not describe the deviation from an initial state. They are available as measured values, which, however, have to be classified according to agreed upon criteria. One example is the quantity of precipitation, where the measured values obtained from environmental stations are scaled to numbers between zero and one according to the rain intensity.

According to the calculation method M(i), it will always be a number between zero and one.

The Valuation Number B(i) indicates how much the respective parameter affects the weathering process. It is limited to values between 0 and 10, which are attributed by the user. For example, a parameter with a great impact on the weathering process is rated 10, another with less impact maybe only 5. In the calculation process, these Value Numbers B(i) are fixed and must not be changed 37because otherwise no comparison would be possible between different investigations.

Several exogenous (climate, exposition) and endogenous (stone properties) parameters influence the integrity of stone figures in the outdoor climate. For calculating their Risk Number R(i) it is necessary to measure these parameters quantitatively. Altogether we use 104 different parameters, which are allocated to 10 Excel data sheets according to their meaning. The 11th data sheet serves for the calculation of the Overall Risk Number R(total), which comprises the individual Risk Numbers R(i) of the other data sheets.

1 Object Description

2 Art historical Evaluation B(AE)

3 Restoration History R(RH)

4 Environment / Exogenous Risk Factors R(E)

5 Natural Stone / Endogenous Risk Factors R(N-M) for Marble

6 Natural Stone / Endogenous Risk Factors R(N-S) for Sandstone

7 Damage Caused by Human Beings / Vandalism R(V)

8 Mechanical Damage / Loss of Material R(M)

9 Surface Alteration R(SF)

10 Ultrasonic Diagnosis R(US)

11 Overall Risk Number R(total)

Function of Data Sheets

Content and purpose of the data sheets and the calculation of the Risk Number will be explained using the sculpture of Hera (original) in the Baroque garden, Großsedlitz near Dresden. Bullet impacts during World War II caused severe damages to the figure.

Because of limitations of space, only one data sheet can be shown. The endogenous natural stone properties decisively determine the course of weathering due to mineral content, structure, and thermal as well as hygric expansion. The data sheet shown refers to the sandstone sculpture Hera/Juno in the Baroque Garden of Großsedlitz near Dresden. It can be seen how the different parameters are evaluated and in which way the Risk Number R(i) of the whole sheet is calculated.

On the last data sheet “Overall Valuation” the Partial Risk Numbers are combined to assess the Overall Risk Number R(total). For this purpose, the Partial Risk Numbers R(i) of all data sheets except Nr. 1 (Object Description) and Nr. 2 (Art Historical Evaluation) are summed up and a mean value is formed:

R(total) = [R(RH) + R(E) + R(N) + R(V) + R(M) + R(SF) + R(US)] / 7 = < 1

Figure 1: Data Sheet for endogenous stone properties. Example Hera in the Baroque Garden of Großsedlitz.

38Due to the mean value formation the Overall Risk Number R(total) is also a number between 0 and 1. Zero stands for no risk and one for a very high risk. This scaling gives an immediate indication of the degree of a risk for the sculpture under consideration. Again, it has to be emphasized that the Overall Risk Number R(i) is formed with objective and quantitative measurement values and is therefore free of any subjective evaluation. It is ideal for decision-making, especially for determining a ranking within a group of sculptures. Furthermore, in combination with the Overall Risk Number R(total) the Risk Numbers R(i) of the data sheets can indicate the areas of a sculpture at particular risk.

Risk number: Evaluation and Comparisons

For testing the concept of the Risk Number R(i), 12 objects have been selected, four statues of marble, four of sandstone and four sandstone tombstones from the Jewish Cemetery in Baiersdorf/Erlangen (see Figure 2).

Figure 2: The objects chosen for testing the Risk Number.

The Overall Risk Number R(total) in data sheet 11 of the Excel work sheet is automatically calculated. Although this data sheet lists both the arithmetic mean of the Risk Number and the quadratic mean, only the arithmetic is considered in the following because the overall statement remains the same even if the quadratic mean provides slightly different values.

The individual Risk Number calculations of the 12 examined objects yielded surprising results in an initial, unbiased assessment, which, however, have proved to be very useful.

The Overall Risk Numbers of the individual sculptures and tombstones are depicted in Figure 3.

The object with the lowest Risk Number is tombstone No. 1901, the one with the highest is Vestalin. In addition, at the upper and lower ends of the scale the realistic maximum and minimum limits for marble and sandstone are presented. These limits indicate possible maximum and minimum overall Risk Numbers for these rock types. The range extents from 0.24 to 0.88 for marble and from 0.18 to 0.79 for sandstone. The reason for these limits lies in the fact that certain data sheets or parameters cannot reach a value like zero or one.

For example, data sheet “Environment – Exogenous Risk Factors” could theoretically assume a value of 1 in the case of very extreme climate, but never a value of zero, because a “non-climate” does not exist. This also applies to data sheets “Natural Stone – Endogenous Risk Factors”. Even in a very favourable climate, thermal and hydric expansion and water absorption exert an influence on the weathering.

Figure 3: Overall Risk Numbers of the selected objects.

39The calculation system for the Overall Risk Number permits model calculations for open air exposure or location in a store. Relevant for this are the data sheets “Environment – Exogenous Risk Factors”, “Vandalism” and certain rock properties such as thermal and hygric expansion. These parameters vary according to the location of the sculpture. Comparisons have shown that the risk reduces by 0.22 to 0.25 for marble objects and 0.18 to 0.22 for sandstone objects when moving them from outdoors to a depot.

Whatever the value of an object’s Risk Number, the question arises as to how large the numerical change regarding the Overall Risk Number must be in order to be able to draw a valid conclusion as to a greater or lesser overall risk. As we could see, the values of some sculptures are very close to each other, for example Aeolus (R(total) = 0.60) and Flora (R(total) = 0.59). The question is whether 0.01 points is significant or not. Simple considerations demonstrate that even small differences are important. Because, for calculating the Overall Risk Number R(total) the sum of the individual Risk Numbers R(i) is divided by seven so that even a small difference of 0.01 can be significant in one of the data sheets.

More straightforward, three types of endangerments can be defined: “little endangered” – “endangered” – highly endangered”. These categories range approximately from 0.2 to 0.4, 0.4 to 0.6 and 0.6 and 0.8. Figure 4 shows that the marble sculptures Vestalin, Flora and Äolus are highly endangered.

Conclusion

This report has shown that the concept of the Risk Number accurately represents the degree of endangerment to outdoor sculptures. The difference often found between a visually subjective evaluation and the objectively calculated Risk Number initially led to the conclusion that the Risk Number would not accurately reflect the overall situation. The fact, however, that the Risk Numbers describe the conditions of environment and stone properties as they really are, means in consequence that the Risk Number values represent the risk of endangerment for the investigated sculptures objectively and realistically.

Figure 4: Simplified categories for marble: “little endangered” – “endangered” – highly endangered”.

Model calculations also permit a prediction about the decrease of the risk for a sculpture when moving it from an outdoor exposure to a depot. In these cases, the risk decreases by about 20 %, which is a considerable reduction. However, the risk in a depot cannot be completely zero either, because the sculptures still underlie a climatic effect, albeit to a lesser extent. Also, some risk factors such as low ultrasound velocity remain, even if there will probably be no further deterioration in a depot.

Overall results show that the Risk Number is an appropriate tool to all owners who wish to examine their sculpture stock and determine the risk of endangerment. Although the Risk Number does not give specific instructions about what measures are required, a high number indicates the main risks. It is therefore a useful tool in the discussion about necessary conservation measures.

Acknowledgements

The author is indebted with great thanks to Prof. Dr. Rainer Drewello (University of Bamberg), Dipl. Rest. Carolin Pfeuffer (Europäisches Zentrum für Steinmetz und Steinbildhauer) and Dipl. Phys. Wolfram Köhler.40

References

Delgado Rodrigues J. & Grossi A. (2007). Indicators and Ratings for the Compatibility Assessment of Conservation Actions. Journal of Cultural Heritage, 8, 32–43. doi: doi:101016/j.culher.2006.04.007

Revez Maria J. (2016): Calculated Risk. The (In) compatibility of Built Heritage Cleaning Methods. Dissertation Faculty of Chemistry and Technology New University of Lisbon

Riegl A. (1903): Der moderne Denkmalkultus. Sein Wesen und seine Entstehung. In: A. Riegl: Gesammelte Aufsätze. Augsburg, Wien.

Waentig F., Dropmann M., Konold K., Spiegel E., Wenzel Ch. (2014): Präventive Konservierung. Ein Leitfaden. ICOM Deutschland – Beiträge zur Museologie Band 5. 96 Pages.

41

NEW MARBLES FOR THE ITALIAN ARCHITECTURE (1920–40)

Roberto Bugini, Luisa Folli

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

CNR-ISPC- Istituto per le Scienze del Patrimonio Culturale, via Roberto Cozzi 53, 20125 Milano, Italia

Abstract

The use of stone in the italian architecture improved dramatically in the period 1920–40, mainly for cladding on concrete structures. New methods of exploitation and supply made available different kinds of stone, never or only locally used before. Features of the stones are detailed together with examples from Milan.

Keywords: building stone, marble, 20th century architecture, Italy.

Introduction

The use of marble and natural stone in Italian architecture strongly increased in the third decade of 20th century, after a period (1900-20) when the “artificial stone”, made of Portland cement moulded in hundred of different shapes, was omnipresent (Biondelli 2004a). Many factors were involved: new architectural trends; rein-forced concrete framing where the stone lost any structural function, large availability of stone materials throughout the Italian territory; new methods of quarry working; cut of thin slabs suitable for cladding; improvement of trucks, trains, roads and railways. Moreover the stone industry was boosted by the policy (called Autarchia) planned by the Fascist government to hinder the Sanctions of the League of Nations (november 1935); so, despite a very low import from foreign countries (i. e. blue syenite from Larvik, Norway), dozens of new quarries were opened and the ancient ones were expanded. Acting in accordance with this policy, the reintroduction of the structural use of stone, in order to reduce the import of fuel and iron, (Portland cement and reinforced concrete), was also advised, but never applied (Peverelli 1939).

The return of the marbles on the building façades was really significant in Milan, the capital of the most important industrial area of Italy. A group of young architects was ahead of a very impressive urban change, juxtaposing huge scales or massive forms of Fascist public architecture and clean outlines or simple forms of residential buildings for the capitalist ruling class, according to the thesis of “Novecento” and “Razionalismo”.

Catalogue of stones and marbles

Different italian marbles never exploited in the past or only locally exploited were used in architecture, among many others. The configuration and shape of building stones were chosen according to the texture of each stone: slabs of various thickness (cladding or flooring, blending the gradation 42of colours or the arrangement of veins), moulded elements (decoration), squared blocks (pillar, column), monolithic pieces (column). The finishing of the surfaces was almost always the polishing.

Some significant stones of different nature, coming from different Italian regions (from Piedmont to Latium, from Friuli to Tuscany, from Lombardy to Liguria), are described in this study.

IGNEOUS

— Rosso Pantheon (red granite)

— Sienite della Balma (violet syenite)

— Diorite nera di Anzola (black amphibole gabbro)

— Porfido monumentale (purple ignimbrite)

SEDIMENTARY

— Zandobbio (pink dolomite)

— Ceppo di Grè/Ceppo Poltragno (conglomerate)

— Pietra di Chiampo (pink limestone)

— Pietra di Aurisina (grey limestone)

— Pietra di Finale (pink to yellow limestone)

— Rosso Amiata (red limestone)

— Portasanta di Caldana (red limestone)

— Travertino di Rapolano (light carbonate deposit)

METAMORPHIC

— Verde Alpi (green, white veined ophicalcite)

— Rosso di Lèvanto (red or green ophicalcite)

— Verde Roja (green clayey schist).

MARBLE

— Marmo di Valle Strona (grey, veined)

— Marmo di Lasa (white, veined)

— Fior di pesco carnico (grey, pink spots)

— Calacatta, Cipollino (white or green, veined)

Each stone is discussed in order to enhance: macroscopic and microscopic features together with decay phenomena caused by weathering agents (Biondelli 2004b); geological settings (Ispra 1976, Ispra 2012); quarry sites (Peverelli and Squarzina 1939, Pieri 1966); use and application in architecture, mainly in Milan, but also in other important cities (Grandi 1980, Gramigna 2001, Pierini 2017).

Rosso Pantheon

Igneous rock (granite). Colour: dark red, scattered of light grey spots. Minerals: quartz, potash feldspar and plagioclase. Decay morphologies: scaling. Geology: “Serie del Canavese” (Permian-Triassic) including red and grey granites barren of micas. Quarry: few kilometres west of Valperga (Cuorgnè, Canavese, Torino province). Use: mainly polished slabs for façade cladding of residential and public buildings (Palazzo INA, portals and cladding in the porch, P. Portaluppi, 1936, Milan; Palazzo del Popolo d’Italia, balcony, G. Muzio 1938/42, Milan).

Sienite della Balma

Igneous rock (syenite). Colour: violet with black spots. Minerals: potash feldspar (orthoclase), plagioclase, amphibole (hornblende), biotite, piroxene (augite), quartz. Decay morphologies: scaling. Geology: Valle Cervo pluton (late-alpine, Oligocene) of the “Sesia-Lanzo Zone” (meta-morphic rocks, mainly micaschists). Quarry: near the village of Balma (valle del Cervo, Biella province). Use: polished slabs for cladding of residential and public buildings (Palazzo delle Colonne, façade made of very thick slabs with point chiseled surface in a T-shaped building with a porch of ten couples of shafts of Granito di Santo Stefano (Sardinia), G. Muzio – G. Greppi 1940, Milan).

Diorite nera di Anzola

The commercial name refers to a wrong petrographic classification: this is a igneous coarse grained rock (amphibole gabbro). Colour: black with some light grey spots. Minerals: plagioclase, hornblende. Decay morphologies: scaling. Geology: “Dioritico – kinzigitica” forma-tion of the “Ivrea-Verbano Zone”. Quarry: near the village of Anzola (val d’Ossola, Verbano province). Use: squared blocks and slabs (Banca Popolare, framework of openings, G. Greppi 1931, Milan); another use was for funerary purposes (Mausoleo Cadorna, M. Piacentini 1932, Pallanza; Cemeteries, Milan).

A stone, of the same colour but unlike origin (migmatite, Gneiss Valcondria), was quarried in val Chiavenna (Sondrio) and used in buildings.

Porfido monumentale

Igneous rock (rhyolitic ignimbrite). Colour: purple to dark red showing clear phenocrysts and some elliptic coarse-grained patches. Minerals: quartz and 43plagioclase. Decay morphologies: scaling. Geology: “Vulcaniti di Auccia” forma-tion (lower Permian). Quarry: near the pass of “Croce Domini” (Bienno, val Camonica – val Trompia, Brescia province). Use: thick slabs for cladding or slabs for flooring (Palazzo delle Colonne, polished thick slabs deeply carved by Giacomo Manzù to represent the “coats of arms” of the Lombard provinces).

Pietra di Zandobbio

Carbonate rock (dolomite). Colour: light pink tending to whitish with fine cracks featuring a typical network. Mineral: dolomite. Decay morphologies: surface erosion with widening of the crack network; sulphate skin formation. Geology: “Dolomia di Zandobbio” formation (Hettangian - Rhaetian) of the sedimentary series of Southern Alps. Quarry: near the villages of Zandobbio (Bergamo proince). Use: firstly during the Renaissance period (i. e. Cappella Colleoni, 1472–76; Biblioteca Angelo Maj, early 17th century); later (20th century) greatly imple-mented (Palazzo Littorio, cladding made of slabs with sawed face, A. Bergonzo 1940, Bergamo) and it was also spread in Milan for residential and public building.

Ceppo di Grè/Ceppo Poltragno

Clastic rock (diamictite with ocraceous matrix and calcite cement, clasts with angular corners). Colour: grey with some yellowish hues. Mineral: clasts of Dolomia principale (Haupt dolomit), size from 0.05 to 1.0 m. Decay morphologies: roughening on clasts and matrix, soot deposition on cavities, sulphate skins formation. Geology: “Complesso di Poltragno”, subdivided in two units (Unità di Poltragno and Unità di Gré), including slope deposits and alluvial deposits (lower and middle Pleistocene). Quarry: near Castro (Bergamo province), on the mountain rising above the north-western shore of lake Iseo. Use: slabs with sawed face for cladding of residential and public buildings (Casa dei Giornalisti, G. Muzio 1936, Milan; Palazzo Vittoria, E. Frisia 1935, Milan). Besides, this conglomerate is employed today (i. e. the façade cladding on the new building of Università Bocconi, Grafton architects 2008, Milan).

Pietra di Chiampo

Organic carbonate rock (shell limestone, biomicrite – packstone) wiht different varieties according to the colour and the texture (Rosa, Perla, Mandorlato, Paglierino). Colour: white to yellow to pink ground scattered with small light spots (shells of Foraminifera). Mineral: calcite. Decay morphologies: surface erosion, sulphate skin formation. Geology: “Calcari nummulitici” formation (Eocene) of the sedimentary series of the Western Venetia. Quarry: few kilometres near Chiampo, a village of the same valley (Vicenza province); limestone in alternating with basalt. Use: slabs for cladding (Palazzo Popolo d’Italia) or for flooring and stairs (Intendenza di Finanza, Genio Civile 1935, Milan).

Pietra di Aurisina

Organic carbonate rock (limestone, biosparite-wackestone) with different varieties according to the texture (Fiorito, Granitello). Colour: grey with evidence of shells of Bivalves (Rudistae). Mineral: calcite. Decay morphologies: surface erosion, sulphate skin formation. Geology: “Calcare di Aurisina” formation (upper Creta-ceous) of the Karst of Trieste. Quarry: near Aurisina (Nabrežina) in the north-western part of Karst (Trieste province). Use: early uses are documented in Roman architecture of north-eastern Italy and also in Milan (funerary stelae and blocks of urban walls); nevertheless the stone was used in the 20th century as blocks with point chiseled face (main body of Stazione Centrale, U. Stacchini 1912/31, Milan; former Banca Commerciale now Ragioneria Comunale, L. Beltrami 1913/23, Milan).

Other sedimentary rocks coming from Karst, with similar origin (organic limestone) and composition, were employed in 20th century architecture for flooring and cladding: Gabria Tomadio, Lipos, Repen.

Pietra di Finale

Organic carbonate rock (shell limestone; bio-sparite / packstone). Colour: pinkish to yellowish with some variegations with evidence of shells of Bivalves (Chlamys). Mineral: calcite. Decay morphologies: surface erosion; sulphate skin formation. Geology: “Calcare di Finale Ligure” formation (lower 44Miocene) of post-orogenetic deposits of the “Bacino del Finalese”. Quarry: near Orco-Feglino, few kilometres north of Finale Ligure (Savona). Use: thick point chiseled slabs mainly employed for cladding (Università L. Bocconi, G. Pagano 1941, Milan; Palazzo del Toro, E. Lancia 1939, Milan).

Rosso Amiata

Nodular carbonate rock with ammonite shells (limestone, biomicrite). Colour: dark red with white veins and pink shades. Minerals: calcite, haematite. Decay morphologies: surface erosion, sulphate skin formation, chromatic alteration. Geology: “Calcare ammonitifero” formation (lower Lias) of the non-metamorphic series of Tuscany. Quarry: south of Roccalbegna (Grosseto province). Use: polished slabs mainly for flooring in residential and public buildings (Casa Wassermann, a four-storeys urban residence with an accurate choice of coloured stones for flooring, P. Portaluppi 1934, Milan).

Portasanta di Caldana

Carbonate rock (pseudo-breccia, limestone). Colour: red to purple with white, purple or pink irregular spots and very thin grey or purple veins. Mineral: calcite. Decay morphologies: surface erosion, sulphate skin formation, chromatic alte-ration. Geology: “Calcare mas-siccio” formation (Hettangian, lower Lias) of the non-metamorphic series of Tuscany. Quarry: Caldana (south of Gavorrano, Grosseto province). Use: polished slabs mainly for flooring in residential buildings together with other coloured marbles, Casa Wassermann). A significant use is displayed on the great staircases of Milan’s Stazione Centrale (U. Stacchini 1912/31): the big shafts are made assembling moulded pieces of curvilinear shape and pieces of fluted shape.

Travertino di Rapolano

Carbonate stone with spongy appearance. Colour: creamy-white or yellowish; Mineral: calcite. Decay morphologies: roughening, sulphate skin formation, soot deposits in the cavities. Geology: carbonate deposition from hot springs (Upper Pleistocene). Quarry: near Serre di Rapolano, east of Sienna (Tuscany). Use: the stone was taken in great consideration during the Thirties as blocks and thick slabs with sawed face for cladding (Main Atrium of the Stazione Centrale; Ca’ Brütta, lower part of façades, G. Muzio 1922; Palazzo della Borsa, colonnaded façade, P. Mezzanotte 1932, Milan). This stone is very similar to Travertino romano (Lapis Tiburtinus, coming from Tivoli-Guidonia, Rome), largely used in ancient Rome and by Baroque architects, later (since the third decade of 20th century) spread in the whole Italian territory.

Verde Alpi

Metamorpihc rock (ophicalcite). Colour: dark green colour with several light green elements and a large grid of white calcitic veins. Minerals: serpentine, calcite and magnetite. Decay morphologies: roughening, chromatic alteration. Geology: “Unità Ofiolitiche dello Chenaillet” (Jurassic). Quarry: near Cesana Torinese in the valle di Susa, near the French border (Torino province). Use: mainly polished slabs for flooring (Casa Wassermann).

Other ophicalcites (Verde Champ de Praz, Verde Issorie), with similar texture and composition and quarried in the eastern part of Valle d’Aosta (Chatillon), were also largely employed for flooring in the 20th century architecture.

Rosso di Lèvanto

Metamorphic rock (ophicalcite). Colour: dark red ground with very irregular white calcitic veins (a variety shows a dark green ground instead of red). Minerals: serpentine, calcite. Decay morphologies: roughening, chromatic alteration. Geology: “Ofioliti liguri”, a group including serpentinite, serpentinized peridotite, gabbro euphotide, diabase and ophicalcite (Upper Jurassic – Lower Cretaceous). Quarry: spread along the coast of Eastern Liguria (Lévanto, Bonassola etc. La Spezia province). Use: mainly polished slabs for cladding, moulded elements were also employed as jambs and lintel in portals and doors (Palazzo di Giustizia, flooring).

Verde Roja

Metamorphic rock (clayey schist, easily divisible into slabs). Colour: green with darker silicate veins. Minerals: quartz, mica, chlorite. 45Decay morphologies: scaling. Geology: “Scisti gneiss-sici” formation (Permian). Quarry: upper valley of the river Roja (Colle di Tenda) a former Italian territory assigned to France (dép. Alpes Maritimes) after the WWII. Use: mainly unpolished slabs for cladding or flooring (Casa Fiocchi, cladding, M. Fiocchi 1925 and Stazione Centrale, flooring together with other coloured stones, Milan).

Marmo di Valle Strona

Regional metamorphic rock (marble, coarse grain-size). Colour: grey with darker veins. Minerals: calcite, muscovite. Decay morpho-logies: disaggregation, sulphate skin formation. Geology: lenses in the “Dioritico-kinzigitica” formation of the “Ivrea-Verbano Zone”, spread from Valle Strona (Piedmont) to Canton Ticino (Switzerland). Quarry: near the village of Sambughetto (valle Strona, Verbano province). Use: mainly polished slabs for cladding (Palazzo di Giustizia, façades totally coated with this marble). It is worth to note the use of this marble in Naples, very far from the quarry site: Palazzo delle Poste (cladding of the curved façade, G. Vaccaro 1936); Banco di Napoli (cladding of the main hall, M. Piacentini 1940).

Marmo di Lasa

Regional metamorphic rock (marble, medium to fine grain-size). Colour: white with bands of various colours (grey to black due to graphite; green to chlorite; pink to zoisite); sometimes groups of little elongate black spots with shaded rims are present and an appropriate cut may produce a particular graphic effect (called Fantastico). Minerals: calcite; graphite, chlorite and zoisite. Decay morphologies: surface erosion, sulphate skin formation. Geology: Laas Unit (micaschist, banded paragneiss and marble) of Ortles-Campo Nappe, Austro-Alpine System (Pre-Permian metamorphic Basement). Quarry: above the village of Lasa-Laas (val Venosta-Vinschgau, Bolzano-Bozen province), the most important one (Weisswasser) was located at an altitude of 1,600 metres and the marble came down the hill using a incline railway. Use: mainly polished slabs for cladding; in some cases, the slabs are disposed in “open book style” (Casa Rustici, G. Terragni 1935, Milan; Torre Rasini, E. Lancia and G. Ponti 1934, Milan).

Fior di pesco carnico

Low grade metamorphosed crystalline limestone (marble, fine grain-size). Colour: light grey ground sometimes with pinkish or purplish spots and coarse grained veins. Mineral: calcite, opaque. Decay morphologies: surface erosion, sulphate skin formation. Geology: limestone of organic origin (Devonian) of the “Paleozoico carnico”. Quarry: near Pierabec, north of Forni Avoltri (Udine province). Use: mainly polished slabs for cladding (Palazzo della Provincia, atrium, G. Muzio 1942, Milan). Other building stones, coming from the same area in the north-western corner of this province, were also used in the 20th century architecture (Rosso Porfirico, upper Jurassic of Verzegnis or Persichino, upper Devonian of Timau).

Calacata and Cipollino

Regional metamorphic rock (calcite marble, very fine grain-size). Different varieties of marble are distinguished, mainly for colour and arrangement of the veins. Colour: white ground with gold-yellowish irregular veins (Calacata), green or white ground with undulating bands of light to dark green (Cipollino). Mineral: calcite. Decay morphologies: disaggregation, cracking, sulphate skin formation. Geology: “Autoctono toscano metamorfico” made of different epimetamorphic formations from Carboniferous to Paleogene; in particular these marbles are referred to “Cipollini” (lower Cretaceous – Oligocene). Quarry: different sites of Apuanian Alps according to the varieties. Calacata: Carrara district; Cipollino: Versilia – Lucca province, Cardoso and Arni districts.

Use: mainly polished slabs for façade cladding (Cipollino: former Palazzo della Montecatini, G. Ponti and A. Fornaroli 1936, Milan) of for interior cladding and for flooring (Calacata: Casa Wassermann).

Conclusion

The increase of the use of natural stones and marbles in the Thirties of 20th century, after two decades 46of “artificial stone”, was led by architects of the milanese school (Lancia, Muzio, Ponti, Portaluppi etc.). The use of natural stone was in agreement with the policy focused to improve the utilization of products of the Italian territory; this policy brought the opening of new quarries, but also brought the exploitation of ancient ones using up-to-date methods. The architects mainly paid attention to employ each stone for a specific purpose (cladding, floor, upright structural member, decoration etc.) according to its features (origin, mineralogy, texture, workability, resistance). The combined use of different stones in a single building was the obvious consequence of this option. In addition, the orientation of the cut of veined or brecciated blocks was accurately chosen in order to obtain slabs where the disposition of veins and colours was improved. Finally, in some cases, the stone elements were designed and manufactured to be exactly set only in one particular position of the whole building.

References

Biondelli D., Bugini R., Folli L., Saltari V. 2004a. I materiali del liberty a Milano. In: Biscontin G, Driussi G (eds) Architettura e materiali del Novecento. Arcadia, pp 27–36.

Biondelli D., Bugini R., Folli L., Saltari V. 2004b. I materiali di Piero Portaluppi. In: Biscontin G, Driussi G (eds.) Architettura e materiali del Novecento. Arcadia, pp 37–48.

Gramigna G., Mazza S. 2001. Milano – Un secolo di architettura milanese. Hoepli, p. 597.

Grandi M., Pracchi A. 1980. Milano – Guida all’architettura moderna. Zanichelli, p. 421.

ISPRA Istituto Superiore Protezione e Ricerca Ambientale 2012. Cartografia Geologica d’Italia scala 1:50000. http://www.ispra.it.

ISPRA Istituto Superiore Protezione e Ricerca Ambientale 1976. Cartografia Geologica d’Italia scala 1:100000. http://www.ispra.it.

Peverelli G. (ed.) 1939. Atti Convegno Nazionale presso Mostra Autarchica del Minerale Italiano. Il marmo, p. 127.

Peverelli G., Squarzina F. (eds.) 1939. Marmi Italiani. Federazione Fascista Industriali, p. 156.

Pieri M. 1966. Marmologia. Hoepli, p. 693.

Pierini O. S., Isastia A. 2017. Case milanesi 1923– 1973. Hoepli, p. 512.

47

CHARACTERIZATION AND DETERIORATION OF MATERIALS OF RUMELIFENERI FORTRESS IN ISTANBUL

Kadir Ekinci, Ahsen Karagöl, Gulberk G. Küçükosmanoğlu, Isil Polat Pekmezci

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

Istanbul Technical University, Turkey

Abstract

The historic fortresses and bastions that constitute the defense system of Bosporus, have ceased to be used by military units after losing their strategic importance in time and later they have been abandoned. Rumelifeneri Fortress which is a part of this defense system dates back to the 18th century. Today the most prominent problems of the fortress are; the ongoing process of deterioration caused by environmental impacts, due to its location by the sea and the uncontrolled influx of visitors. In terms of the study, to make a complete description of deterioration types, firstly a deterioration mapping is prepared. Samples were taken from different parts of the structure and chemical, physical and petrographic analyses were conducted in order to analyze the characterization of the original materials and types and degrees of deterioration. As a result of the analyses, the types and degrees of deterioration observed in building stone basaltic andesites, the main building material of the structure, were identified and classified. The characterization of the mortars and plasters are also completed; such as binder, aggregate ratios and the aggregate size distribution and the types of the aggregates. According to these results, suggestions and interventions have been developed for the parts of the structure that should be primarily conserved. This study aims to draw attention to these structures which are an important part of the historical and cultural heritage of Eurasia, by analizing characterization and deterioration of materials of Rumelifeneri Fortress.

Keywords: Cultural Heritage, Deterioration of Igneous Rock, Military Architecture, Mortar Characterization, Stone Conservation, Waterfront Fortresses

General Information and Description of the Site

The Bosphorus, forming the continental boundary between Asia and Europe, is an approximately thirty km long natural waterway connecting the Black Sea to the Marmara Sea. Fortifications were built on both sides of the Bosphorus starting from the Byzantine period. Rumelifeneri Fortress is an authentic example of the Bosphorus’ defense system dating back to 18th century during the Ottoman period. The fortress was built on a promontory volcanic rock near Rumelifeneri Lighthouse located at the northernmost point on the European side of the Bosphorus. (Figure 1).

The fortress has a rectangular plan with two beveled corners on the seafront with approximate dimensions of 55 m to 70 m (Karadağ 2003) (Figure 2). Stones were used as ashlar stones on faces and as rubble in the core of the casemate walls.

48Bonding timbers were used all around the walls on the springing line level of embrasure arches and 1.85 meters above this level. Arches of embrasures and the dome in the eastern tower were built with bricks measuring 34 × 34 × 3 centimeters.

Figure 1: Rumelifeneri Fortress (Url-1 2017).

Figure 2: Plan of Rumelifeneri Fortress (Eyüpgiller and Yașa 2019), the location of samples.

Scope of the Study and Methodology

This study reveals the characteristics and deteriorations of the stones and mortars of Rumelifeneri Fortress within detailed field studies and laboratory analyses. Visual observations on the site indicate that the northeastern façade represents nearly all of the deteriorations. Thus, a mapping of the façade was prepared to obtain complete identification of geology of the area and all deteriorations based on the architectural survey and rectified photographs (Figure 3).

The geological formation of the area is called “Garipçe Formation” which is from Upper Cretaceous and Lower Eocene era. The fortress is located on the lava of Rumelifeneri which consists of andesite and basaltic andesite. The location has two layers of lava, the later phase lava is grey and black colored while the earlier phase lava is reddish and dark brownish. (Yavuz and Yılmaz 2010). Thus, these two different basaltic andesite lavas were identified as Lava 1 (reddish colored) and Lava 2 (grey colored). ICOMOS – ISCS Illustrated Glossary on Stone Deterioration Patterns was used for defining the deterioration types (ICOMOS 2008) and different levels of stone deteriorations (differential erosion, alveolization and coving) were shown with different shades of the same color for each stone type. Besides, discolouration (black), calcite encrustation, salt crust, lichens and graffiti were also observed on the façade and indicated on the mapping. In addition, all of the deterioration types were gathered together on a table to create a deterioration glossary for the building (Figure 4).

Figure 3: Mapping of deteriorations on northeastern façade, April 2019.

Figure 4: Deterioration types encountered on Rumelifeneri Fortress.

Laboratory studies were carried out for characterization of the materials used on the building. Within this context 13 samples (10 mortar and 3 stone) 49were taken from both sound and deteriorated parts of the building. Locations of the samples were indicated on the scaled drawings (Figure 2, 3). The details about the samples can be seen in Table 1. The sampling was followed by laboratory analysis. Chemical and physical analyses were conducted on the samples to determine the characteristics of the original materials and the causes and the depth of deterioration. Acid loss test, sieve analysis, ignition loss test, protein and oil spot tests were performed on mortars. The water soluble salt content of the mortars was also analyzed. (Pekmezci and Ersen 2010; Güleç and Ersen 1998; Middendorf et al. 2005; Teutonico 1988; KUDEB 2011). The stone samples were examined by using SEM-EDS.

Table 1: Description of the samples.

Sample No	Description
M1	Masonry mortar with yellowish colored binder, stone and seashell aggregate under 5mm
M2	Masonry mortar with white colored binder, sand and seashell aggregate under 3mm
M3	Masonry mortar with yellowish colored binder, stone aggregate under 8mm
M4	Masonry mortar with white colored binder, stone and seashell aggregate under 5mm
M5	Strong masonry mortar with white colored binder, stone & seashell aggregate under 8mm
M6	Strong joint mortar with white colored binder, stone & seashell aggregate under 2mm
M7	Masonry mortar with white colored binder, stone and seashell aggregate under 6mm
M8	Strong mortar with yellowish colored binder, stone & seashell agg. under 4mm
M9	Plaster with pinkish colored binder and brick aggregate under 4mm
M10	Strong masonry mortar with yellowish colored binder, stone aggregate under 1mm
S1	Gray colored basaltic andesite lava, discolouration (black)
S2	Greenish colored basaltic andesitic breccia, lichens
S3	Dark brownish-reddish colored basaltic andesite lava, coving

Test Results

“Garipçe Formation” consists of green, greenish-gray, purple and black colored andesite, basaltic andesite, agglomerate, volcanic breccia, lava and tuff combined rocks occurred in Upper Creteaous geological era (Angı et al. 2018). Macroscopic and microscopic views of samples from “Garipçe Formation” can be seen in Table 2.

Acid loss and ignition loss tests applied on mortars and various binder/aggregate ratios were observed (Table 3). As the ignition loss test results indicate that the samples have lower ratios of CaCO3 than acid loss test results, it can be said that mortars of the castle had carbonated aggregates.

Aggregates of mortars were observed by optical microscopy after acid loss tests. Regarding the observations on aggregates, mortars have mainly siliceous aggregates with a little amount of brick. Observations show that the siliceous aggregates are composed of mainly volcanic rocks, quartz and 50feldspar. On the other hand, aggregates of Sample M9 (cistern wall plaster) are mainly composed of bricks with a little amount of siliceous aggregates. Salt and SEM-EDS analysis indicate that samples taken from the places facing the sea have a high amount of chlorine and conductivity (Table 4).

Table 2: Macroscopic and microscopic views of stone samples from Rumelifeneri Fortress.

Discussion

Rumelifeneri Fortress is one of the fortresses on Bosphorus that was completely neglected after the Second World War. In addition to the effects of vandalism, the fortress is also open to the corrosive effects of natural conditions such as precipitation, wind and the waves. Previous studies indicate that the local stones of Rumelifeneri area are weaker than other igneous building stones of Turkey in terms of mechanical properties and considered to be sensitive against weather conditions (Akgür and Mahmutoğlu 2015). Furthermore, most of the samples have a high amount of chlorine as a result of being on the seaside and the conductivity test results clearly show the effects of being faced to the sea.

As a result of these facts, deteriorations such as differential erosion, alveolization, coving are seen on building stones. In ICOMOS Glossary it is mentioned that these deteriorations are generally found on sedimentary and volcanic stones, due to inhomogeneities in physical or chemical properties of the stone such as heterogeneous stones containing harder and/or less porous zones. Salt crust formations can also be observed in lower parts of the northeastern façade that are washed by the waves. Besides, calcite encrustation linked to water leached from joints is seen on this façade (ICOMOS 2008). On the other hand, the samples taken from the places where visitors lit fire such as the northeast façade and the cistern have sulfate and carbonate besides chlorine. Discolouration can be observed in these areas as a result of being exposed to fire. Besides, the presence of protein and oil on the samples M6 and M8 can be related to the action of visitors and some biological formations. These results show that measures should be taken to preserve the building against vandalism.

As intervention proposal, stuccoes can be employed which are composed of stone itself (crushed) and lime (Torraca 2005) for the stones highlighted on the mapping with the deterioration types differential erosion and alveolization, while the ones highlighted with coving can be integrated with natural 51stones of same type from the source in the area. Mechanical cleaning methods can be suggested for the deteriorations such as salt crust and calcite encrustation. For deteriorations such as discoloration and graffiti, several cleaning methods should be examined for each case before implementation phase. For biological formations such as lichens, an interdisciplinary research is needed.

Table 3: Macroscopic and microscopic views of stone samples from Rumelifeneri Fortress. (B: Binder, A: Aggregate, CA: Calcareous Aggregate, PBW: Physically Bound Water, SBW: Structurally Bound Water, Ct: Calcite, Q: Quartz, V: Volcanic, C: Carbonates/Limestone, F: Feldspars, B: Brick particles and dust).

Table 4: Results of water soluble salts, protein, oil and conductivity analysis. (–: Undet., +: Small amt., ++: Pres., +++: Large amt., ++++: Abun.).

A color difference on courtyard level could be observed on masonry mortars of the façades facing the sea. Masonry mortars below this level are yellowish (M1 and M3) while the ones above are whitish (M4). This difference could be seen also in test results (B : A ratios, sieve analysis, aggregate distribution). According to the observations, not only M1 and M3 but also M10 have similar properties, and this fact can be evaluated as a hint for explaining the phases of the construction process. On the other hand, M2 can be re-evaluated as a joint or repair mortar as it has different properties from M1 and M3, despite being below the courtyard level. Besides, M5, M6 and M7 were taken from brick masonry parts have the same B : A ratio and similar aggregate properties. This indicates that the same mortar was used in those parts.

Ignition loss and acid loss tests results indicated that lime was used as binder in all of the mortar samples. As a result of macroscopic and microscopic observations on aggregates, mortar samples have mostly volcanic rocks, quartz, feldspars and meshed brick as aggregate in different sizes.

Despite its conservation problems, Rumelifeneri Fortress has also several advantages. As the northern part of the Bosporus has the same geology, similar building materials were used in the construction of northern Bosporus fortresses from 52both European and Asian sides such as Garipçe, Poyraz, Kilyos and Riva (Akgür 2015). Thus, similar deteriorations and conservation problems are encountered in these buildings. As a result of this situation, the mapping and glossary prepared in this study can be taken as a reference for other fortresses of the zone for material and deterioration analysis and also intervention proposals. Besides, as Rumelifeneri Fortress was built on its building stone sources, there is no problem with supplying original material for its restoration and conservation works. On the other hand, this source area can be used as a laboratory for monitoring the results of several intervention methods and defining the correct methods for conservation works. All in all, Rumelifeneri Fortress can be chosen as a pilot area to preserve Bosporus fortresses and the intervention methods should be monitored on the natural stone source area which is facing the same conditions (weather, sea salts, visitors, etc.) as the fortress. After ascertaining the suitable intervention methods on the source area and necessary awareness raising activities, conservation and restoration works shall start on Rumelifeneri Fortress and the other fortresses of Bosporus.

Acknowledgements

We thank Burcu Bas, er Gürer, Ph. D. student from Istanbul Technical University, Faculty of Architecture for her contributions at the beginning of the study, Dr. O. Serkan Angı from ITU, Faculty of Mines for his precious support for our studies and Prof. Dr. Lütfi Öveçoğlu from ITU, Faculty of Chemical and Metallurgical Engineering and his laboratory team for their support about SEM-EDS analysis in their laboratories.

References

Akgür B., 2015, İstanbul Boğazı’nın Batısındaki Kretase Volkanitlerinin Malzeme Özellikleri ve Yapı Taşı Olarak Kullanılabilirliğinin Araştırılması, İstanbul Teknik Üniversitesi (Unpublished master’s thesis), İstanbul.

Akgür B., Mahmutoğlu Y., 2015, Garipçe Piroksenli Andezitinin Malzeme Özellikleri ve Doğal Yapı Taşı Olarak Kullanılabilirliği, MÜHJEO’2015: Ulusal Mühendislik Jeolojisi Sempozyumu, 3–5 September 2015, KTÜ, Trabzon.

Angı O. S., Yavuz O., Çiftçi, E., 2018, Geo-Lithological and Architectonical Properties of Indigenous Building and Ornamental Stones Used in Landmark Structures in Istanbul from Past to Present, İstanbul Yerbilimleri Dergisi, V.28, I.1, 163–196, Y. 2015–2017.

Eyüpgiller K. K. & Yaşa Y., 2019, İstanbul Bahr-i Siyah Karadeniz Boğazı Kale ve Tabyaları, Kitabevi Yayınları, İstanbul.

Güleç A., Ersen A., 1998, Characterization of Ancient Mortars: Evaluation of Simple and Sophisticated Methods, Journal of Architectural Conservation – 1, 56–67.

ICOMOS-ISCS, 2008, Illustrated glossary on stone deterioration patterns, English-French version.

Karadağ R. E., 2003, Rumeli Feneri Kalesi Restorasyon Projesi, İstanbul Teknik Üniversitesi (Unpublished master’s thesis), İstanbul.

KUDEB, 2011, Restorasyon ve Konservasyon Laboratuvarları, Şan Matbaası, İstanbul.

Middendorf B., Hughes J. J., Callebaut K., Baronio G., Papayianni I., 2005, Investigation Methods for the Characterisation of Historic Mortars – Part 2: Chemical Characterisation, RILEM TC 167-COM, Materials and Structures 38, 771–780

Polat-Pekmezci, I., Ersen, A., Characterization of Roman Mortars and Plasters in Tarsus (Cilicia-Turkey), 2nd Historic Mortars Conference HMC2010 and RILEM TC 203-RHM Final Workshop, 22–24 September 2010, Prague.

Teutonico J. M., 1988, A Laboratory Manual for Architectural Conservators, ICCROM, Rome.

Torraca, G., 2005, Porous Building Materials: Materials Science for Architectural Conservation – 3rd Edition, ICCROM, Rome.

Yavuz O. & Yılmaz Y., 2010, İstanbul Kuzeyi Volkanitlerinin Jeolojik, Petrografik ve Mineralojik Özellikleri, İtüdergisi/D Mühendislik Vol:9, Issue:3, 38–46 June 2010, İstanbul.

URL-1, 2017, Rumeli Feneri ve Topçu Kalesi Drone Çekimi, https://youtu.be/lOpEL5ftKIs Access date: 24.01.2020.

53

TUFFS IN PRE-COLUMBIAN AND COLONIAL ARCHITECTURE OF OAXACA, MEXICO

Alexandra Kück1, Christopher Pötzl1, Rubén López-Doncel2, Reiner Dohrmann3, Siegfried Siegesmund1

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Geoscience Centre of the Georg August University Göttingen, Germany

2 Geological Institute of the Autonomous University of San Luis Potosí, Mexico

3 Federal Institute for Geosciences and Natural Resources (BGR), Hanover, Germany

Abstract

Volcanic tuff was of great importance in the ancient culture of Zapotecs and Mixtecs as a construction material. The historical buildings in both Mitla and the historical center of Oaxaca were erected with a great variety of volcanic tuff rocks, many of which are not quarried anymore. These tuffs were compared and evaluated regarding their petrophysical properties and weathering behavior. Analyses of the petrography, pore space, water transport and water storage as well as mechanical properties were performed.

The results of the investigations show that the rocks have high sensitivity to water, linked to high porosities and high amounts of capillary pores. Additionally, very variable behavior towards hydric expansion and salt bursting provokes different responses to weathering and decay. To protect the historical buildings in Oaxaca, it is therefore necessary to control the exposure to water or to find suitable conservation measures for the stones.

Introduction

The state of Oaxaca has a very long history of architectural construction and an important archaeological heritage. The Zapotecs and Mixtecs were the leading cultures in the region until the Aztecs invaded in 1428, and finally conquered by the Spanish conquistadors in 1521, occupying Oaxaca (Blanton et al., 1999).

The convent of Santo Domingo de Guzman was built in the 16th century by the Dominican Order (Urquiaga 2000). The Cathedral of Oaxaca, also called ‘Catedral de Nuestra Señora de la Asunción’, was built from 1553 to 1733. There were several periods of reconstruction in the history of the cathedral, for example in 1696, 1723, 1891 and 1999 (Casanova and Pino 2004).

The pre-Hispanic archaeological site of Mitla (about 45 km southeast of Oaxaca) is about 1800 years old and was first mentioned in the literature of the 16th century (García 2016; Bernal 1963).

The historical center of Oaxaca de Juárez is UNESCO world heritage site known for its cultural tradition and its history of art and architecture. Both the city center of Oaxaca and the archaeological site of Mitla were built with a great variety of volcanic tuff rocks (Fig. 1).

This study focusses on the deterioration behavior of the tuffs in both Colonial and pre-Hispanic architecture. A variety of tuff rocks have been tested regarding their petrography, pore space properties, water transport- and storage properties, mechanical properties and weathering behavior.54

Figure 1: Historical buildings in Oaxaca. a): Church of Santo Domingo de Guzmán, b): Oaxaca Cathedral, c): Archaeological site of Mitla (‘El Palacio’).

Sampling

Cantera Verde Oaxaca (CVO), Cantera Amarilla Oaxaca (CAO) and Cantera Rosa Oaxaca (CRO) are samples from the city of Oaxaca de Juárez. Cantera Verde Etla (CVE), Cantera Amarilla Etla (CAE) and Cantera Rosa Etla (CRE) are used nowadays as replacement stones (originating 20 km north of Oaxaca de Juárez). The samples from Mitla are called MG (Mitla Gris) and MR (Mitla Rosa).

Methods

The archaeological site of Mitla, the church of Santo Domingo de Guzmán and the Oaxaca Cathedral were mapped for lithology, intensity of damage and weathering features (Fig. 2). For most walls a representative area was selected, and each rock was mapped individually to make a semi-quantitative analysis.

In accordance to the German industrial norms, several laboratory tests were conducted for the analysis of the petrophysical and moisture parameters as porosity, density, capillary water absorption, water vapor diffusion, hygroscopic water sorption, 55ultrasonic velocity, rebound hardness and tensile strength. For the analysis of weathering properties, tests on the thermal expansion, hydric expansion and salt bursting were performed. The petrophysical properties were analyzed parallel (X-direction) and perpendicular to the bedding plane (Z-direction). A detailed description of the laboratory analysis can be found in (Siegesmund and Dürrast 2011).

Figure 2: Mapping in Oaxaca and Mitla. a): Lithological map of Santo Domingo de Guzmán, b): Weathering features at the eastern side of the Oaxaca Cathedral, c): Intensity of damage at the southern side of ‘el palacio’ at the archaeological site of Mitla.

Results

Table 1 summarizes the test results for the petrophysical and moisture properties as well as the weathering properties of all samples.

Mapping

The historical buildings in Oaxaca were mostly built with CVO. Delicate parts like ornaments and window sills were built with the softer CAO. CVE was used as a replacement for CVO during different phases of restoration (Fig. 2a).

The most important weathering features in Oaxaca are loss of components, loss of matrix and bursting. The base parts of the buildings often show scaling and flaking accompanied by salt crust formation. Upper wall parts and protruding areas are strongly affected by moisture and form a characteristic organic black crust (Fig. 2b). CVO is dominated by loss of components when pumice clasts dissolved. CAO and CVE are strongly affected by fracturing, when in contact with water (Kück et al. 2020b). CRO and CRE are the only samples which show bedding parallel scaling.

In Mitla most of the building stones are original and mostly in good condition. The southern and eastern side of the buildings have the highest damage intensities linked to high temperature changes during the day of up to 20 °C (Fig. 2c). The northern side experiences a lot of moisture, but the temperature difference during the day is low (3 °C). The western side is the least weathered side.

Petrophysical and moisture properties

The effective porosity of all analyzed tuffs varies from 12 % (MG) to 41 % (CAO). The Mitla tuffs show the lowest values (12–15 %). The sample with the highest matrix density is MR with 2.6 g/cm3, while MG has the lowest density with 2.2 g/cm3 (Tab. 1). The saturation degree S of the samples ranges from 0.59 (MG) to 0.92 (MR, CVO). After Hirschwald (1912) most samples are weathering and frost resistant except MR and CVO. The pore size distributions were measured for all samples (Fig. 3). It is noticeable, that CVO and MR have almost no capillary pores, but a high share of micropores.

Figure 3: Comparison of the ratio of micropores and capillary pores.

The samples from Mitla show the lowest capillary water absorption (0.7 kg/m2√h–1.7kg/m2√h). The samples from Oaxaca show moderate w-values ranging from 1.0–2.8 kg/m2√h (CV ) to 6.4–8.5 kg/m2√h (CAO). CVO has the highest water vapor diffusion resistance factor μ with 21.9, followed by MR with 19.0. The sample with the lowest water vapor diffusion resistance is CAO with 7.4. Hygroscopic water sorption was measured from 20–95 % rh. The highest sorption value was measured for CVE with 7.75 wt.-% at 95 % rh. The lowest sorption at 95 % rh was measured for CRO with 1.74 wt.-%. Between 20 and 75 % rh all samples show a linear weight increase with varying gradient. Above 75 % rh the curves show a sudden increase in slope. The highest ultrasonic velocities were measured for MR (3.7–3.8 km/s) and CVO (3.2–3.6 km/s). The lowest ultrasonic velocity was measured for CRE (1.6–2.3 km/s).

The tensile strength of all samples varies between 1.3–1.7 MPa (CRE) and 6.4–6.9 MPa (CVO). MR has the highest surface hardness with 528–604 HL followed by CVO (603–632 HL). The lowest surface hardness was measured for CAO with 330–337 HL.56

Table 1: Petrophysical properties and weathering behaviour of the studied samples.

Weathering properties

Thermal dilatation was tested in two heating cycles each in both a dry and wet environment. The samples CVO, CVE and CAE show pronounced differences between the first and second heating cycle (Fig. 4). All samples expand during heating and then shrink again during cooling. The samples from Oaxaca and Etla have negative residual strain, especially after the second heating cycle. At about 70–80 °C the samples shrink significantly. The samples from Mitla reach their initial state after cooling and the first and second heating cycle are similar. CAE has the highest thermal expansion with 0.78–0.87 mm/m. The tuff varieties from Oaxaca and Etla have the lowest thermal expansion (0.17–0.22 mm/m).

After two dry heating cycles the samples were saturated with water and heated in two wet heating cycles. After the first hour in a wet environment the samples already showed intense hydric expansion with a maximum of more than 2 mm/m. After that first intense expansion the samples 57underwent a thermohydric dilatation. For thermohydric dilatation CRO and CRE show much higher anisotropies than for just thermal dilatation. The highest thermohydric expansion was measured for CVO (2.90 mm/m) and CVE (1.98–2.25 mm/m). The samples from Mitla have the lowest thermohydric expansion.

Figure 4: Thermal and thermohydric expansion.

The second dry cycle shows a significant decrease of residual strain, with negative values representing thermal contraction. After the first wet heating cycle the samples show a decrease in residual strain. Especially CVO, CVE and CAE show this feature. CAO and CAE do not recover from contraction after the second drying cycle. The Mitla samples are rather unaffected by both thermal and thermohydric dilatation.

Figure 5: Maximum hydric expansion values.

Figure 5 shows the hydric expansion of all tested samples. CVO expands the most of all samples, when in contact with water. The maximal hydric expansion is reached by CVO in the Z-direction with 2.36 mm/m. The smallest hydric expansion was measured for MR with 0.04 mm/m in the X-direction. The sample with the highest anisotropy is MR with 73 %. Most samples show higher hydric expansion in the Z-direction, except for CAO, CAE and MG. All samples reach maximal hydric expansion already during the first hours.

The effect of salt weathering on all samples is presented in Figure 6. Most of the samples show a weight increase at the beginning of the salt bursting test. The most resistant sample is MR, while CRE, CRO and CVO show low resistance towards salt weathering. Table 1 shows the number of salt cycles until at least 30 % weight loss is reached.

Discussion and conclusion

The combination of the mapping results and the experimental tests allow an evaluation of the rocks. The samples from Mitla are in general very resistant to most forms of weathering. They have low porosities and low w-values with low hydric 58and thermohydric dilatation. Both MG and MR are relatively resistant to salt weathering compared to the other analyzed tuff samples.

Figure 6: Photographic documentation of the salt bursting test.

The pore space has a very important impact on the process of salt weathering. A general dependence between the share of micropores and the number of salt cycles could be observed in the samples. MR and CVO are the strongest samples with high surface hardness, ultrasonic velocities, tensile strength and low porosity with almost no capillary pores present. CVO is not very resistant to salt weathering and shows the highest hydric expansion and strong thermohydric dilatation with non-reversible changes due to the dehydration of clay minerals. This makes CVO less suitable as a building stone. Nevertheless, CVE is even more affected by moisture than CVO. With the highest sorption, high hydric expansion and low tensile strength as well as low ultrasonic velocity CVE is strongly affected by loss of components and fracturing/bursting. CAO and CAE are less affected by component loss and show strong back weathering and fracturing due to a high amount of clay minerals, which makes the yellow tuff varieties rather soft samples. The red tuffs (CRO and CRE) are the only ones which show anisotropic behavior in the weathering features that correlate with relatively high anisotropic properties.

The results suggest, that moisture and thermal changes are the most severe damaging factors for tuffs from Oaxaca. The southern and eastern sides of most buildings are the ones with the highest temperature changes, and therefore are weathered the strongest. The northern side usually experiences a lot of moisture, while the western side is in general the least weathered building side. The tuff varieties from Mitla are very suitable building stones and have remained in the buildings for a long time. The green tuff from Oaxaca (CVO) is a historical building stone of good quality but needs to be protected from moisture. The replacement stone from Etla (CVE), which is used today, is a suitable building rock but is certainly of a lower quality than CVO. Therefore, conservational treatment should be considered for CVE.

Acknowledgements

This work was supported by the German Research Foundation (Si-438/52-1) and the German Federal Environmental Foundation (AZ20017/481). We would like to thank J. L. Noria Sánchez, A. E. Andrade Cuautle and M. Á. González for the support of the on site work. For the laboratory support and helpful comments we thank K. Wemmer, J.Menningen, W. Wedekind and C. Gross.

References

Bernal, I. (1963), Otra tumba cruciforme de Mitla, Estudios de cultura náhuatl, Vol. 4, pp. 223–232.

Blanton, R., Feinman, G., Kowalewski, S. and Nicholas, L. (1999), Ancient Oaxaca, Vol. 2, Cambridge University Press.

Casanova, J. P., and Pino, L., (2004) Templos históricos de Oaxaca, 1. ed.: Oaxaca de Juárez, H. Ayuntamiento de Oaxaca de Juárez, H. Ayuntamiento Oaxaca de Juárez (Series), Volume 2.

García, N. M. R. (2016), Mitla: Su desarrollo cultural e importancia regional, Fondo de Cultura Económica.

Hirschwald, J. (1912), Die Prüfung der natürlichen Bausteine auf ihre Wetterbeständigkeit, W. Ernst & Sohn, Berlin.

Kück, A., Pötzl, C., López-Doncel, R. A., Dohrmann, R. and Siegesmund, S. (2020b), Effects of zeolites and swellable clay minerals on water related properties and thermal dilatation in volcanic tuff rocks, in Siegesmund, S. and Middendorf, B. (Eds.), Monument future: Decay and conservation of stone, Göttingen, Kassel, Mitteldeutscher Verlag, Halle.

Siegesmund, S. and Dürrast, H. (2011), Physical and mechanical properties of rocks, in Siegesmund, S. and Snethlage, R. (Eds.), Stone in architecture, Springer, pp. 97–225.

Urquiaga, J. (2000), La restauración del ex Convento de Santo Domingo, Oaxaca, Consejo Nacional para la Cultura y la Artes, México.

SUSTAINABLE LIME RESTORATION MORTARS: PHYSICAL PROPERTIES AND DURABILITY ASSESSMENT

José Diaz, Beatriz Menéndez

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

Geosciences and Environment Cergy, Cergy-Pontoise University, France, jose.diaz-basteris@u-cergy.fr, beatriz.menendez@u-cergy.fr

Abstract

In order to improve the ecological footprint of the restauration mortars while keeping their efficiency, we have tested several combinations of natural hydraulic lime and air lime with natural aggregates and additives. Repair mortars should be compatible with the support (stones, bricks) and protect original materials from environmental agents; aesthetical and historic aspects must not be neglected. We started by “simple” mixtures of lime and natural siliceous sand or crushed calcareous limestone. Aggregates granulometry was 0/2 mm in all the cases. More “complex” mortars were also prepared by incorporating the additives pinecone resin, semimilled cones of pine, milled glass waste, chamotte (brick production residue).

Different physical properties have been measured in all mortars: porosity, density, capillarity absorption, mechanical strength (flexural and compression), P and S waves velocities from which dynamic Young’s modulus and Poisson’s ratio were inferred. Durability of mortars has been estimated by salt crystallization and frost/thaw cycles.

Keywords: Restoration mortars, sustainable Cultural Heritage conservation, Building materials, Durability

Introduction

Restoration mortar designates a group of products made to repair a damaged masonry. They are used to rebuild the lost parts of buildings, monuments or sculptures that have disappeared and to make replicas of architectonic or sculptural elements of high cultural value, placed inside buildings or museums to ensure their conservation. Mortars for restorations must be carefully selected before application. They have to be able to interact with the ancient stone and match in a chemical, physical and mechanical way with the pre-existing materials (Rota Rossi Doria, 1986; Torney, 2016). The choice of the mortar must take into account the support properties and the environment in which the mortar will be applied. Nowadays, in the development of new commercial restoration mortars, their environmental impact and their durability in future conditions are not always considered.

Our work aims at proposing solutions to produce more ecological and durable mortars. This research will allow the development of a series of new restoration mortars which design will be done in a “circular economy” philosophy.

To develop new sustainable formulas, we tested different additives available in Paris region, with a low CO2 footprint and capable to improve the 60mechanical properties of resulting mortars. Inorganic additives such as crushed bricks, crushed glass and slags have been identified in historical mortars. The use of recycled materials is considered as a zero CO2 emission contribution to the mortar footprint; only the energy used to transform these materials: grinding or cleaning is considered in the total environmental balance. For this reason we included recycled additives in our formulations. Crashed bricks (Aalil et al 2019) and waste glass powder (GWP) have been used in construction works because they improves the durability and the compressive strength of lime mortars (Carsana et al. 2014). GWP provides a pozzolanic behaviour to lime mortar (Edwards et al. 2007).

Many studies have been done about the influence of kind of binder, composition and particle size of aggregates on the physical and chemical properties of mortars. For example, the use of limestone sand as aggregate is recommended to ensure a better compatibility with natural stones (Szemerey-Kiss et al. 2011).

In ancient cultures like Mayans and Romans, organic additives were used to improve the properties of the mortars (Villaseñor et al. 2009; Ordóñez et al. 2019). For example, the tannin and sugar content of the pinecones ensured water resistance and plasticity of mortars (Rampazzi et al 2016, Arcolao 1998). In restoration, it is very important to dispose of mortars with different ranges of physical and mechanical properties using natural additives and without using any cement. (Russlan et al. 2018).

Materials and Methods

The amount of raw materials is limited and must be preserved and managed with caution. One important aspect of the presented work is to save natural raw materials when formulating restauration mortars. In order to increase the sustainability of the restoration mortar industry, the use of waste materials will be enhanced.

Different mixtures were prepared in order to determine the influence of components in the physical properties of harder mortars. Two binders, four types of sand (aggregates) and four additives were employed in the fabrication of studied mortars. The binders are: aerial lime (Cl90) and hydraulic lime (NHL5) both provided by Socli company. The aggregates are: silica sand (Leroy Merlin), calcareous sand provided by Rocamat company, silico-calcareous sand (Italcementi group) and fine silica sand (Sibelco). Four different additives were used: chamotte provided by Briqueterie d’Allone, ground glass provided by the Fédération du Verre, ground pine cone and water/oil solution of pine cone.

Mortars were prepared in the laboratory at average temperature of 24 °C and relative humidity of 40 %. The preparation process consists in several steps: 1) sands were dried at 60 °C during 24 h, 2) weighing of the components, 3) dry mixing using an electric mixer (Rubimix 9), 4) addition of water and mechanical mixing for 3 minutes, 5) mortars are moulded in prismatic casts, 40 mm × 40 mm × 160 mm and placed in hermetic plastic boxes for 7 days to preserve 90 % humidity, 6) samples are unmoulded and put in a humidity chamber at 65 % for 21 days following the standard EN 1015-2. Finally, the samples were stored at laboratory conditions.

The different formulations obtained are presented in Table 1. To name the mortars we use the initials of the components: H for hydraulic lime, A for aerial lime, C for calcareous sand, S for silica sand, F for silico-calcareous sand, D for fine silica sand, 61G for waste glass powder, B for chamotte (crushed brick), P for pinecone and R for the resin of the pinecone.

Figure 1: Mortars samples at 180 days. The different formulations obtained are presented in Table 1.

The first mortar prepared was the HFD after evaluation it was decided to increase the amount of the binder, to increase the resistance of 20 %. The mortars HS and AS are performed to compare binders, the mortar HB is prepared to test the additive, the mortar HSD is prepared to test the effect of granulometry to increase de resistance.

The HSC and HSCR mortars start from the same base with 30 % of binder but one is prepared with water and the other with the pine resin solution, mortar HC was prepared to test the calcareous sand, and HSG for test the WPG.In future work more formulas will be prepared to make comparisons.

Several properties have been measured in the harden mortars (90 or 180 days old). Porosity and density have been measured by the triple weight method following the standard EN 1936. Compressive and flexural strength were obtained in 4 × 4 × 16 cm³ samples, according to EN 1015-11 standard. Capillarity water absorption tests were performed following the standard EN 1015-18. Durability of the samples was estimated by salt crystallisation and frost resistance tests following standards EN 12371 and EN 12370. Dynamic Young’s modulus (E) and Poisson’s coefficient (n) from P and S wave velocities using the next equations (Baron 2007):

In order to validate the obtained formulations, their physical properties have been compared with three commercial products (Altarpierre, Artopierre and Lithomex TM) previously studed in our laboratory and largely employed in France and others parts of the world.

Results and Discussion

Granulometry of the mortars produced in this work was compared to that of the commercial products to compare if it has an impact in the durability of the mortar. The particle diameter of all mortars ranges from 0.08 to 0.8 mm. Particle size distribution of mortar HB is a little different from the others due to the use of chamotte as aggregate. Mortar AS shows smaller particle size due to the use of air lime.

Porosity and density, water capillarity absorption and relative amount of mixing water necessary to obtain the expected workability are presented in Table 2. The mean density measured at 90 days is 1,720(109) kg/m³, HSP mortar presents the smallest value with 1534 kg/m³ and the highest value corresponds to mortar HSG with 1,873 kg/m³. These two mortars have additives, pinecone and waste glass powder respectively, which shows that the use of the correct additives is able to modify 62the density of a restoration mortar. The mean porosity at 90 days is 33.7(4) %; HSG mortar, with 29.4 %, presents the lowest value and the highest value corresponds to HSP mortar, with 40.9 %. As expected, mortar with the lowest density has the highest porosity.

Table 1: Restoration mortar formulations in weigh percentage. WGP: Waste glass powder, PN: Pine cone, PCRS (Pine Cone Resin Solution), CHAM:Chamotte (Crushed brick waste).

Figure 2: Granulometric Curves.

The mean capillarity coefficient at 180 days is 1.73(0.53) kg/m²min^½, with a minimum of 0.99 kg/ m²min^½ for mortar HSP and a maximum of 2.57 Kg/ m²min^½ for mortar HB. These results are in accord with the results obtained by Margalha et al. (2011) in mortars with hot lime mix.

Table 2: Physical characteristics of mortars. ρ: density (kg/m³), n: open porosity (%), C: Capillarity coefficient (Kg/m²min^½), W: mixing water (g water/g dry mortar).

Mortar	ρ	n	C	W
HFD	1,783 (18)	30.2 (0.5)	2.39 (0.02)	0,16
HSD	1,713 (20)	31.2 (0.3)	2.02 (0.18)	0,18
HS	1,762 (13)	30.1 (0.2)	1.99 (0.02)	0,14
HB	1,541 (23)	37.9 (0.3)	2.57 (0.53)	0,27
HSCR	1,764 (8)	33.7 (0.8)	1.42 (0.20)	0,15
HSC	1,787 (26)	33.2 (0.2)	1.24 (0.45)	0,15
HSG	1,873 (26)	29.4 (0.1)	1.18 (0.1)	0,15
HSP	1,534 (14)	40.9 (0.1)	0.99 (0.1)	0,15
HC	1,669 (34)	38.4 (0.2)	1.97 (.13)	0,14
AS	1,771 (3)	31.5 (0.1)	1.58 (.10)	0,18

The mean mixing water is 17(4)% in weight, the smallest value corresponds to mortar HC with 14 %, and the highest to mortar HB with 27 %.

Mechanical properties at 90 days are shown in Table 3. The mean compressive strength at 90 days is 2.36(1.3)MPa; with a minimum of 0.68 MPa (mortar HSP) and maximum of 4.30 MPa (mortar HSG). These results are in the same range than those of some commercial mortars: compression strength at 90 days, Lithomex of 8.3–9.0 MPa; Conserv, 0.97 MPa, Altarpierre, 15.6 MPa (Torney et al.2014; Lopez-Arce et al 2016). In another experimental study, comparing 160 mortars values go from 0.5 MPa to 15.20 MPa with a mean value of 3.76 MPa (Apostolopoulou et al., 2019).

The HSG mortar has the strongest mechanical properties. This mortar was fabricated with waste glass powder and the obtained results are in accord with those of Carsana et al. (2014) and Edwards et al. (2007). The mean flexural strength at 90 days for all the mortars is 0.86(.22) MPa, with a minimum value for mortar HSP and AS. Considering that AS mortar was made with aerial lime this value is in accord with the work of Margalha et al. (2011). Mortar HSG shows the highest value with 1.29 MPa.

Table 3: Mechanical properties of mortars. CS: Compressive Strength (MPa) at 90 days; FS: Flexural Strength (MPa) at 90 days; E: Young modulus (GPa), υ: Poisson coefficient.

Mortar	CS	FS	E	u
HFD	2.04 (0.06)	0.82 (0.25)	6.3 (0.73)	0.13 (0.04)
HSD	1.76 (0.33)	0.94 (0.18)	6.43 (0.20)	0.15 (0.05)
HS	1.20 (0.05)	0.77 (0.02)	7.28 (0.37)	0.21 (0.03)
HB	1.91 (0.20)	0.91 (.25)	5.14 (0.71)	0.18 (0.03)
HSCR	3.51 (0.07)	1.07 (0.01)	8.91 (0.45)	0.21 (0.01)
HSC	3.63 (0.37)	1.04 (0.04)	10.12 (0.71)	0.19 (0.07)
HSG	4.3 (1.2)	1.29 (0.15)	11.45 (0.15)	0.26 (0.02)
HSP	0.68 (0.08)	0.48 (0.04)	2.58 (0.32)	0.32 (0.01)
HC	3.62 (0.50)	0.82 (0.06)	8.03 (0.90)	0.17 (0.07)
AS	1.01 (0.27)	0.69 (0.08)	6.29 (0.09)	0.15 (0.01)

63The use of organic additives, like resins, can improve the mechanical properties of mortars (Ordoñez et al. 2019). In our case, HSCR mortar shows only little improvement in the flexural strength compared to HSC mortar.

The mean dynamic Young’s modulus at 360 days is 7.25(2) GPa, the minimum is shown in mortar HSP with 2.58 GPa and the maximum in HSG with 11.45 GPa. These values are similar to those obtained by Nežerka et al. (2015).

The mean dynamic Poisson ratio at 360 days is 0.20(0.05), the minimum corresponds to mortar HSFD with 0.13, and the maximum to mortar HSP with 0.32. These values are similar to those of Palomar et al. (2015).

Frost Resistance results are presented in Figure 3. All mortar samples presented a slow and constant increase in weight during the thaw/frost test until the 13 cycles when the sample HFD started to lose weight. The mean mass variation per unit of mass is 0.11(0.04), with a maximum of 0.15 for the mortar HC. The mean final weight is 11(4)% higher than the initial one. This fact may indicate that mortars are still undergoing carbonation under these conditions, as carbonation produces a weight increase in mortars, clearly higher in mortars with higher amount of lime (Arizzi et al. 2012).

The results of salt crystallization tests are presented in figure 4. Weight decreases for all the samples during 16 cycles. Mortars AS and HSP collapse prematurely, with partial destruction at cycle 8. Mortar HSC shows the same partial destruction at cycle 13. In contrast, all the other mortars resist 16 cycles. In mortars HC, HB and HS, deterioration occurred at a much slower rate. These values are within normal boundaries according to Klisińska-Kopacza et al. (2013). Low weathering rate is related to the presence of hydraulic lime and hard minerals, such as silicates. Mineral additives can improve the durability of the mortars according to Theodoridou et al. (2014). The Mortar HSPR showed better durability in comparison with the mortar HSP. We can say that the addition of pinecone resin solution seems to be adequate to increase the durability of mortars.

Figure 3: Frost Resistance test. ΔM/M(%) vs number of cycles.

Figure 4: Salt Crystallization test. ΔM/M(%) vs number of cycles.

Conclusions

Correlations were established between mortars properties and the use of different additives. It can be concluded that employed recyclable additives will improve some mortars properties.

Pinecone fragments reduce significantly the capillarity coefficient but also its mechanical properties. The waste glass powder improves the compressive strength of materials and decreases porosity. Mortars with crushed brick waste as aggregate absorb almost twice more water than mortars with sand, this additive can be used as a red dye and has a high resistance in durability tests.

The Pinecone Resin shows an improvement in the durability of the mortars as well as a slight improvement in mechanical properties.

Future works will seek to combine different additives in order to obtain better mortars.

New methods to calculate the life cycle analysis are available with Open LCA methods, We plan to use design models and economic models in order to select the composition of new mortars we want to develop.

Acknowledgements

We thank the programs “Make Our Planet Great Again” and “Initiative d’excellence Paris Seine” for 64providing economic support for the development of this research; and the companies Socly, Rocamat, Briqueterie d’Allone and Fédération du Verre for providing the materials for this work.

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COMPARISON OF PHYSICO-CHEMICAL PROPERTIES BETWEEN ANCIENT AND RESTORATION RENDERS IN THE VEXIN FRANÇAIS AREA (NW OF PARIS)

Claire Noël, Beatriz Menéndez

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

Laboratoire Géosciences et Environnement Cergy, Université de Cergy Pontoise, 95000 Neuville-sur-Oise, France

Abstract

In the Paris area there are important gypsum resources which results in an extended use of gypsum in traditional architecture as façade renders. Traditional renders are still visible today in urban areas and in the surrounding countryside. This study concerns renders of the vernacular architecture of the Vexin Français, few tens of kilometres North-West of Paris and is based on the work carried out by Tiffanie Le Dantec in her PhD thesis. We focus our research on the analysis of ancient and new renders in order to propose suitable conservation or restoration solutions. This preliminary study compares several physical, chemical and mineralogical properties of ancient render samples and modern renders formulations used for façade restoration, in order to find possible relationships between the characteristics of studied formulations and their physical properties. The final goal is to estimate physical properties of ancient renders from their mineralogical composition.

Keywords: Traditional gypsum renders, renders conservation, renders restoration, mechanical properties, chemical analysis, hydric properties

Introduction

In the Vexin Français area, 40 to 70 km NW from Paris, two principal construction techniques are found: important buildings (churches, chateaux or big houses) are built in ashlar masonry, whereas smaller ones (farms, houses and other vernacular buildings) are made on random rubble masonry covered by renders. Nowadays many new owners of traditional buildings want to remove renders in order to show the stone masonry. This new tendency has two major disadvantages: it alters region landscape and it exposes masonry to weathering and decay induced by environmental conditions.

The long-term goal of the study is to provide proofs that traditional renders are much more adequate and durable when repairing stone façades than new cement base renders. Most of the façades in the Vexin area are made of rubble stones with a lime mortar and covered by a plaster of Paris, lime or plaster of Paris-lime render. The use of this kind of render has aesthetic purposes, helps to evacuate the humidity and has a protection role.

Our study focused on comparisons between ancient renders sampled in façades of the Vexin area and new formulations of renders supplied by Plâtrière Vieujot. This society has been producing plaster of Paris renders since 1880. One of its renders, “plâtre Briard” is used in rural areas of Paris region as external render 66because of its non-uniform aspect due to the presence of impurities like charcoal and small pieces of gypsum stone.

Traditional renders in the Vexin Français

Traditional houses in “Vexin Français” can be classified into three groups: big farms for growing of cereals with closed court-yards; workers’ houses and wine growers’ houses. Before the arrival of railway in the area, the price of raw materials forced people to build with local quarry materials: mainly limestones, sandstones in some areas or “meulière” (millstone). Ashlars and “meulière”, prestigious materials, were apparent but rubble stones were covered by renders.

Nowadays, many people think that plaster of Paris is a weak material, soluble in water (~2 g/l at 20 °C) and very sensitive to environmental conditions. Gypsum has been used in façades since antiquity and it has been usually employed in the last century in very different climate areas (Spain, France, Germany, Czech Republic, etc.). Some of these renders are well preserved. Blottas (1839) showed that gypsum at Montmartre (Paris, France) is a sulphurous limestone (“chaux sulfatée calcarifère”) composed of gypsum with more than 12 % of calcite. This mineralogical composition, (Blottas, 1839; Debauve 1884; Flavien 1887), does not correspond to the amount of calcite actually found in Parisian gypsum, varying from 2 % to 5 %. In traditional Plaster of Paris we find overcooked, undercooked and uncooked gypsum due to the inhomogeneity of the temperature distribution inside the kiln. It can be thought that the high durability of gypsum plaster with respect to outdoor environmental conditions comes from the presence of other minerals as clays (Sanz Arrauz and Domínguez, 2009).

Different kinds of renders have been used in the Paris area. Some of them are mixtures of plaster of Paris and aerial lime, around 12 % (Thénard, 1834; Le Dantec, 2016). In some cases the amount of lime in gypsum renders can be explained by a batching of plaster with lime water. Some gypsum-free mortars have been employed in the region, as mortars or as renders (Toussaint, 1841). Lime mortars are more difficult to apply, they harden slower than gypsum and it is less easy to achieve a flat surface. Rugosity of lime mortars is higher than that of gypsum renders due to the present aggregates and for this reason they are less used as finish renders in this region.

Materials and Methods

In this work we studied renders sampled in old façades and new renders formulations in order to compare their physical properties, composition and microstructure. New plasters are used in the rehabilitation of vernacular buildings but also in the restoration of buildings and monuments in historical cities.

Twelve fragments of façades from the Vexin Français area and about ten render formulations supplied by Plâtrière Vieujot were studied. Some façade samples come from the Tiffanie Le Dantec collection (TLD) (Le Dantec, 2019), while others were collected from the sites during this study (CN). They were taken before rehabilitation campaigns or from walls where the renders were about to be taken off. Some of the samples presented two different layers that were characterized separately. It is very difficult to date these renders by documents as most of the studied buildings are private houses or farms without any record on ancient restoration.

TLD collection consists in samples from:

— Théméricourt (THM, XVII century): A façade with two different states of weathering and different colour layers.

— Condécourt (CON, Château de Villette, 1663–1669): Façade with two layers of render, a grey layer and a white one.

— Hédouville (HED, XIX century): Two samples, one from the front façade with a grey upper layer and a white inner layer, and one from the rear façade with only one white layer.

— Fontenay Saint Père (FStP, XVIII century): White façade.

— Arthies (ART, probably XX and XIX centuries respectively): Two samples from one white farm façade, called “Moellons” et “Charbons”.

67CN collection

— Fontenay Saint Père (FStP): White façade, quite degraded

— Saint Clair sur Epte (StC): Two façades with lime render with large amount of aggregates.

— Genainville (GEN): Small part of a white render, almost completely disappeared.

— samples from non-well-maintained enclosure walls at Saint Gervais and Ambleville (W1, W2, W3) We observed façade samples under polarized light and digital incident light microscopy.

Formulations of new render samples of “Plâtrerie Viejot” have also been fabricated according to the experience of this company specialized in outdoor and indoor plaster renders. They reproduce in industrial kilns the traditional cooking of gypsum in “four culée” kiln type (XVIII and XIX centuries).

Water has been added manually to the powder until desired workability was achieved. This procedure corresponds to what is done during building works depending on the experience of the plasterer. For these renders only general results are presented as manufacturer consider the data confidential. Formulations consist in gypsum with different amount of lime but also with marble powder and charcoal.

Mineralogical compositions were determined by thermo-gravimetric analysis (TGA) and X-Ray powder diffraction (XRD). Porosity and apparent density were measured by the triple weight method (NF EN 1936); water vapour sorption/desorption tests were determined following the procedure presented in standard NF EN ISO 12571 with a Vsorp Plus apparatus. Water vapour permeability was measured by the standard NF EN 15803, method of wetcup. Compressive and flexural strength were measured following NF EN 13892-2 standard.

Most of new plaster samples have been measured about one week after their preparation, and some of them have been re-measured 30 days later. All samples were dried at 60 °C during 48 hours. Not all properties could be measured for all of the samples due to the limited size of historical samples and to the preparation procedure of new ones.

Results
Historic renders

The results cannot be considered representative of all the renders from the Vexin area because mainly gypsum renders were considered. The percentage of gypsum vs. lime renders in the area is unknown. Having survived weathering, sampled renders represent the most durable renders of XIX century. Experimental results show that samples from historic façades have a relatively uniform mineralogical composition (Table 1), close to formulations proposed nowadays. However, their physical properties are quite different, probably due to changes produced with time.

According to TGA analyses, in most of the historic samples, except StC, the amount of gypsum ranges from 80 to 93 % and the amount of calcite from 1 to 8 %. The latter may either be an impurity of the natural gypsum, indicate high temperature calcination, or correspond to the addition of lime or calcite aggregate to the plaster. The amount of non-detected minerals by TGA varies from 2 to 16 %. In order to identify this unspecified portion, XRD analyses were done. Quartz but also anhydrite, 68halite and aragonite were found. In StC samples, only calcite was found with over 80 % of unidentified minerals by TGA, which indicates a lime render with a large amount of aggregates.

Table 1: Thermo gravimetric analysis results for ancient samples. Gyp: Gypsum, Cal: Calcite, ND: Non-determined.

The grey colour of samples from CON and HED may come from the addition of charcoal. The change of colour from grey to white and a peak at 430 °C seem to indicate the presence of charcoal (Shi et al, 2012).

Thin sections and polished surfaces (Fig. 1) of samples from THM, CON, HED, FStP, ART and StC were observed. Figure 1. THM, CON and ART samples present two different layers. We can observe the differences in grain size between the two layers but also between different samples taken at the same façade. THM sample has a red-colour superficial layer which is partially detached. CON sample is the only one that contains agglomerations of small gypsum crystals as well as bigger crystals. In the THM sample the two layers appear detached by a fissure. In ART there is also a fissure parallel to the surface but in the inner layer. HED sample shows a thin layer in the surface with recrystallization zones. Quartz sand grains in the Saint Claire sample are a little bit bigger than gypsum crystals in the other samples. In most of the samples charcoal grains can be observed, only ART and FStP do not show any charcoal. In TGA results this last sample has only 2.5 % of “undetermined” components, which is the lowest value of all the samples. Porosity and density results for historic samples are shown in Figure 2. A general direct relationship is observed for samples FStP, ART, HED and THM, while the CON sample has a higher porosity with respect to its density which can be explained by its higher content of calcite compared to the other samples. Density of calcite and gypsum are 2,700 kg/m³ and 2,300 kg/m³, respectively. FStP has an extraordinarily high porosity and a low density that can be explained by the small grain size and the high gypsum content. Figure 2 In general, good correlation was found between density and porosity with the exception of the lime-rich sample mentioned above, a fact that supports the similarity in composition between the respective samples.

Figure 1: Digital optical incident light microscopy. All images are at the same magnification.

Water vapour absorption/desorption results for samples from historic renders are presented in Figure 3. A first group of samples absorbs much less water vapour than the others at 90 % relative humidity, THM (0.15 %), FStP (0.5 %) and ART (0.77 %). Two other samples present intermediate values, StC (1.66 %) and Heudouville (2.5 %). The 69CON sample has the highest value, 4 %. Samples with low water vapour absorption have higher contents of gypsum. CON samples, with the highest absorption, have particles with very fine crystals which can explain this result due to a high specific surface. HED has the highest percentage of undetermined minerals in TGA which may correspond to the charcoal content (Fig. 2). The StC sample is quite different from the others, with lime binder containing almost 85 % of quartz with low water vapour absorption properties.

Figure 2: Porosity and density values of ancient samples.

Figure 3: Water vapour absorption/desorption curves for some old renders.

The CON sample absorbs the maximum of vapour and has the highest hysteresis of desorption. 20 % of the absorbed water remains in the sample after the test. THM sample absorption is the lowest one but it keeps 18 % of the water after desorption. StC sample keeps 10 % and the three others around 4 %. The differences in the sorption/desorption hysteresis cannot be explained.

Laboratory samples

TGA analysis of laboratory made plasters was performed shortly after their preparation, in which lime had not time to carbonate. The results obtained are not comparable to samples from historic renders.

Porosity of new plaster samples goes from 58 % in samples of rough render for exterior walls with lime or charcoal grains, to 44 % in fat or pure gypsum renders.

In new render samples, the amount of absorbed water vapour by unit of mass goes from 0.18 to 0.50 i. e. values similar to THM, FStP and StC samples. The highest values correspond to samples with the highest content of lime or charcoal. Lowest values correspond to pure lime samples.

Water vapour permeability was measured for new mortars. Permeance in kg/(m² × s × Pa) goes from 1E-13 to 2E-13. The highest values correspond to samples with 20 % of lime and the lowest ones to pure plaster renders. Tests have been done at three different ages, 10, 25 and 90 days, and the permeability values slightly decrease with time. Compression and flexural strength were measured for several samples of new plaster. Compressive strength varies from less than 1 MPa in some samples of pure gypsum with charcoal to more than 6 MPa in samples of pure gypsum with or without lime. At the tested ages the admixture of lime does not produce any increase in strength, as more time is necessary to complete carbonation of lime.

The amount of mixing water controls mechanical and hydric properties. With increasing amounts of mixing water, compression strength decreases, porosity and water vapour absorption increase and density decreases. The most important variation has been observed in water vapour permeability. In several studied renders, a diminution of 15 % in mixing water induces a decrease of water vapour permeability between 30 and 40 %.

Conclusions

In this paper we present preliminary results in order to understand the properties of gypsum renders as a function of their composition, binder and aggregates, but also of the samples preparation, especially the amount of mixing water, It is difficult to draw general conclusions, as the sampling process is not easy and obtained samples cannot be considered as representative of the whole population of renders in the Vexin Français area.

Another difficulty of the study is the age of the samples. This study has been done during a Master degree research project of 6 months. Samples from buildings are at least 100 years old but laboratory samples have less than 3 months. This difference is especially important for samples with lime. In new 70samples we find some portlandite, not found in old samples, not even in Saint Clair sur Epte sample, fabricated entirely with lime binder.

Nevertheless, some conclusions can be inferred from this work:

a) Historical studied renders are very different from one to the other, even if most of them are composed of more than 80 % of gypsum. Aggregates granulometry and composition, amount of charcoal, number of layers, colour, etc. vary from one sample to the other.

b) The amount of calcite or quartz aggregates is more important in renders of farms or enclosure walls than in façades. Decorative elements are composed almost exclusively of gypsum. The composition also depends on the location of a building in respect to the areas of plaster of Paris production.

c) Mixing water and the amount of lime control the properties of renders. In this way, the plasterer’s skills determine the quality of the products.

d) Porosity cannot be directly related to the water absorption/desorption of plasters. Sorption/ desorption test seems more adequate to characterise hydric behaviour of plasters than porosity measurements.

e) Permeability of plaster is directly related to the capacity of samples to desorb the absorbed water upon drying.

f) It is difficult to compare new and old plasters due to the alteration of their properties with time.

Acknowledgements

For their help during this work we thank Guillaume SODEZZA (PNR Vexin Français), Marc POTIN and Keltoum BALBZIOUI (Plâtrière Vieujot), Mae PRADAL (UCP student) and Tiffanie LE DANTEC and Jean DUCASSE-LAPEYRUSSE (Cercle des Partenaires du Patrimoine – LRMH).

References

Blottas, M. (1839). Traité complet du toisé des ouvrages de maçonnerie. Carilian-Goeury et V. Dalmont, Paris.

Debauve, A. (1884). Procédés et matériaux de construction. Quatrième partie. Matériaux de construction. Vve Ch. Dunod, Paris. 680 p.

Flavien, F. (1887). Plâtre, in: Dictionnaire encyclopédique et biographique de l’industrie et des arts industriels. Lami,Tharel & Cie, Paris, pp. 393–404.

Le Dantec, T. (2016). Le plâtre en façade, une architecture francilienne historique. fabricA 10. hal-01611369.

Le Dantec, T. (2019) Les façades enduites au plâtre d’Île-de-France. Le déclin du plâtre extérieur, du XVIIe au XXe siècle. PdD. 555p. Université Paris-Saclay.

NF EN 1936 May 2007. Natural stone test method -Determination of real density and apparent density, and of total and open porosity.

NF EN ISO 12571 October 2013. Hygrothermal performance of building materials and products – Determination of hygroscopic sorption properties.

NF EN 15803 March 2010. Conservation of cultural property – Test methods – Determination of water vapour permeability.

NF EN 13892-2 September 2003. Methods of test for screed materials – Part 2: determination of flexural and compressive strength.

Sanz Arauz, D., Domínguez, L. V. (2009) Evolución de los morteros históricos de yeso al exterior en la España Central. Congreso Nacional de Historia de laConstrucción. Valencia V2:1329–1336, ISBN978-84-9728-316-8.

Shi, L., Liu, Q., Guo, X., Wu, W., Liu, Z. (2013) Pyrolysis behavior and bonding information of coal – A TGA study. Fuel Processing Technology 108, 125–132. doi.org/10.1016/j.fuproc.2012.06.023.

Thénard, L-J ; (1834) Traité de chimie élémentaire théorique et pratique, Paris, Crochard,p. 221.

Toussaint, C.-J. (1841). Nouveau manuel complet du maçon-plâtrier, du carreleur et du paveur, EncyclopédieRoret. Libr encyclop de Roret.

71

STUDY OF DECAY PATTERNS AND DAMAGE ASSESSMENT OF THE ACHAEMENIAN ROCK-RELIEFS OF NAQSH-E RUSTAM

Sahar Ahmadinezhad¹, Antonio Sansonetti², Andrea Pane³, Danilo Biondelli²

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Polytechnic University of Milan, Dep. of Architecture and Urban Studies (DASTU), Milan, Italy

2 Institute of Heritage Science (ISPC) Italian National Research Council (CNR), Milan, Italy

3 University of Naples Federico II, Architecture Dep.(DiARC), Naples, Italy

Abstract

Ancient rock reliefs in the necropolis of Naqsh-e Rustam (Iran) are important testimonies of the development of an outstanding monumental art over the centuries, in close relationship with their natural context. The rock reliefs underwent natural and anthropic decay processes in the course of time leading to the loss of fragments and in some cases to severe structural instability. This paper focuses on the oldest group of rock reliefs, dating back to the Achaemenian period; due to their location on the top of a sloping cliff, they are less accessible and more challenging by a conservation point of view. The reliefs have been studied in field and by means of a multi-analytical laboratory procedure, in order to identify the decay patterns, along with an assessment of the state of conservation. In order to frame the problem in a proper historical context, archival material including photographs, drawings and descriptions – created by western scholars between the 17th and the 20th century – were also analyzed focusing on any indications as regards deterioration problems. Fragments were studied by means of optical microscopy, SEM-EDX and XRD analyses. Among the main decay causes and mechanisms, the chemical dissolution of the stone substrate and the heavy microbiological subsurface growth play a major role. The layered aluminosilicate encrustations imply a continuous exposure of the limestone monument to the moisture ingress from the outer environment. Also, indications of the recent impact of atmospheric pollutants were observed, which is noteworthy, considering the distance of the monument from the urban areas.

Keywords: damage characterization, decay pattern, Iran, rock relief

Introduction

Naqsh-e Rustam archeological site is located in Marvdasht municipal territory, in the Fars province, Iran (Figure 1). The site served as a necropolis during the Achaemenian (550–330 BC) and Sassanian (224–651 AD) periods (Schmidt 1970). Naqsh-e Rustam contains the rock-cut tombs of four celebrated kings of ancient Persia, Darius the Great, Xerxes, Artaxerxes, and Darius II.

This paper deals with the conservation issues related to the outstanding bas-relief decorations of these tombs. During the recent years, reports about their progressive decay were repeatedly presented by local conservation experts as well as visitors. Different hypotheses were presented about the causes of threats and decay rates which may be 72put into two main classes. Those which relate the current situation of rock reliefs to the long-term exposure to the open-air weathering processes. On the other hand, it is argued that the rate of decay is drastically increased during the last decade due to anthropic factors such as air pollution, as well as climatic changes such as unexpected seasonal floods and land subsidence.

Due to the difficulties in getting access to the tombs which are located in the upper parts of vertical cliffs, no report, mapping or analytical data was in hand about their previous state of conservation. Therefore, not only a close examination of the stone substance had to be done, but also it was necessary to obtain a clear picture of the state of conservation of the reliefs in the past decades or even longer periods, if possible.

Another specific feature of these reliefs which were dealt with during the previous attempts for conservation in one of the tombs (Darius I) was the uncovering of polychromy. Therefore, it was important to consider the possibility of encountering such features also on other tombs and to discuss their decay problems as well. This study was carried out with the aim to support the decay mapping of rock reliefs, which can be used as a measure for systematic monitoring and mitigation of the risks in the framework of a preventive conservation program in the future. For this reason, a combined methodology consisting of different approaches were applied for data collection about the reliefs. In the meanwhile, it became possible to discover aspects of the original decoration, which was hidden for many years from the experts and visitors.

Research Method

Data were collected for this research through three approaches:

a) Mapping the current situation

b) Collection of historical data for assessing the decay phenomena in a time framework

c) Sampling from representative points and analytical studies for understanding the decay

The bas-reliefs are located at heights between 15 and 38 meters, on the perpendicular rocks. There-fore we inevitably required scaffoldings to gain access to the monuments. The local conservation teams had conventionally used rock climbing equipment under guidance of bouldering professionals for reaching the surface of interest. However, since 2018, scaffoldings have been mounted in front of the tomb of Xerxes, for preforming emergency conservation works.

Figure 1: Map of Iran with the location of the site and rock-cut tomb of Xerxes in Naqsh-e Rustam and its bas-relief decorations.

Close examination was done on the rock reliefs which showed various patterns of decay for the monument. These features were classified using ICOMOS-ISCS glossary on stone decay patterns and UNI 11187:2006 (ICOMOS-ISCS, 2008 & UNI, 2006) based on the mapping methodology developed by Fitzner et al., (1995). The orthophotograph output of laser scanning documentation was used to generate the base map on which the decay features were transported.

In the next step, the decay patterns and their distribution on the surface of rock reliefs were compared with historical resources to assess the changes occurred by the passage of time. The site was one of the main destinations of travelers and scholars since the 17th century. Based on this reality, a search was done for archival material including the site condition in the past. After identification and collection of the historical resources, we focused on any indication of deterioration problems. Not only the graphic sources, such as old photos and drawings, were compared with the decay map, but also, we consider older written materials such as travelers’ accounts with reference to the reliefs. Some of them were analyzed during the 20th century, however, these data were never studied from 73the conservation point of view. For example, the Engelbert Kampfer’s report about the site (1712) was neglected mostly because of the difficulty in translating its specific style of Latin, which was finally partially translated into German in the mid 20th century. However, those parts about Naqsh-e Rustam were put aside. After the completion of these phases, the decay processes were tentatively assessed, on the basis of the collected information, comparing them to the present state of the site, attempting to locate them on the map.

This approach was followed by sampling from representative points on the reliefs. Considering 3 different elevations and the variety of the decay features identified during the mapping process, in total 14 samples were taken from the rock monument.

Two samples were taken from the rock for its characterization. They were taken from the places where detachments had occurred, from the walls due to cracks. Two more samples were taken from the undamaged part of the rock from places more distant from the monument. Other samples were taken from the weathered surface layers (Table 1). Moreover, traces of polychrome decoration on the stone were revealed in the course of emergency conservation operations, which were covered for centuries under layers of soil depositions and crusts. The samples were studied by means of optical and SEM microscopic observations of thin sections and cross-sections, as well as SEM-EDX and XRD analyses. Observations on thin sections and cross-sections of samples were accomplished using a stereomicroscope Leitz Wild M420 at different magnifications and a Scanning electron microscope JEOL 5910 LV, source tungsten filament, coupled with X-ray spectrometer (EDS) in a dispersion of IXRF-2000 energy in order to observe the sequence of the superimposed crusts and samples with traces of polychromy. Analyses were conducted in low vacuum conditions. The compositional nature and distribution of elements on sample was investigated by EDS spectra and maps from 0 to 20 keV. X-ray diffraction was carried out on powdered samples by PANalytical X’Pert PRO X-ray diffractometer, with geometry goniometer θ–θ. The diffractograms were recorded between 3° and 75° 2 θ with a scanning speed of 0.21 θ/sec, using a Cu Kα radiation, a PW generator 3040/60 in the conditions of 40 kV and 40 mA, and a solid-state multi-detector X’Celerator PW3015/20, with Ni filter. Results were interpreted by the use of the X’Pert HighScore software.

In order to give a more comprehensive view to the problem of decay patterns in Naqshe-Rustam, we preferred to focus on one of the main weathering phenomena which predominate the decay patterns in this site and contributes to the progress of other decay problems. We finally discussed one of the specific findings about the remaining traces of surface polychrome decorations, which seem to be very rare and need specific care and conservation measures in the future.

Preliminary Results and Discussions
a) Mapping

Seven types of decay patterns were mainly identified in the close examination of the reliefs (Figure 2).

b) Historical documents

A considerably high number of historical records were identified about the preservation condition of the reliefs. This is due to the fact that the site was very popular among western scholars between 17th and 20th centuries.

Among the identified material, some decay phenomena were described in detail. A critical approach in reading the precedent visitors’ works was especially noted among most of the authors, which make it possible to follow the events through history. According to the first notes written about the site by Pietro Della Valle, the Italian traveler in 17th century (Masetti, 2018) “the very little fragility of the sculptures shows that they are so strong that can last as long as the mountains themselves” (Della Valle, 1843, p. 261). This condition as the beginning point of the survey may be compared with the evidence of decay, which are traced in the early 20th century photos and even the drawings from 18th and 19th centuries. However, it is more 74precise to compare the graphic resources with the present mapped condition. Figure 3 shows examples of specific phenomena, which were discovered.

Figure 2: Example of decay map of the rock reliefs decorations on the façade of Xerxes’s tomb.

c) Laboratory Analysis

The rock substance in Naqshe-Rustam is composed of calcite with little amount of magnesian calcite and even minor phases of quartz (Figure 4). According to the results of mapping activities, the external surface of rock-reliefs is mainly covered by encrustations.

Field observation and also the historical photos showed the link between the movement of water on the stone surface through cracks and fissures in the rock and the formation of crusts. We focus here on this phenomenon and through the results of the analytical approach will discuss the link among it and other forms of decay.

Encrustations are formed as several superimposed layers (with an overall thickness between 0.1 to 0.5 mm). While the intact stone has a color similar to white marble, the crusts color range between dark orange to bright ochres (Figure 5). The SEMEDX analyses showed that these encrustations are composed of Ca, Si, Al, with minor variations.

It was seen in the maps (Figure 2) that a kind of dark coloration is developing as small to medium size patches on the reliefs, especially on the previously encrusted surfaces. This feature is present as a very thin layer, rich in Sulphur, which is formed on the external surface of the overall encrustation. While the older encrustations are basically close to the rock composition with small amounts of aluminum-silicates contamination, the newer phenomenon is connected with a composition of gypsum and bassanite highlighted as minor phases in XRD analyses.

The appearance of crusts in these cases does not change so much (Figure 6). However, the progressive formation of sulphate layers can lead to black crust formation, as an impact of high atmospheric pollution (Fronteau et al., 2010). We still need to complete our studies on the air pollution data for this area.

Figure 3: Comparison between an old photo shot no later than 1939 (Schmidt, 1970) and recent condition of the reliefs, showing the progress of weathering and material loss.

Figure 4: XRD patterns of rock samples of Xerxes’s tomb. Main phases: calcite, minor phases: magnesian calcite.

75

Figure 5: Encrustation on the reliefs, (a) Sampling location in the pathway of moisture with crusts of different colors, (b) the polished cross section of crusts, (c) and (d) SEM elemental maps of Si and S for the same sample.

Another secondary decay phenomenon related to the formation of crusts regards the presence of microrganisms. They form a dark film, which is also accompanied by pitting effects (Sohrabi et al., 2017). Microscopic observations showed that this is originally the same encrustation layer which is then contaminated with biological growth. The presence of micro-organisms in the pores of the encrustation layer caused a greyish or darker color. In places near the cracks and fissures conducting the rainwater, it went deeper and caused the detachment of stone pieces from the rock. Therefore, cross sections showed the presence of biologic material not only on the surface layer but also on the opposite side of the sample (Figure 6).

Finally, a very interesting feature was identified in a few samples under the encrustation. SEM/EDX analysis shows a considerable amount of phosphorous in this layer (Figure 7). It is found that this feature is linked to the technique, which was used for polychrome decoration. The first finding of such feature was given nearly two centuries ago by the French archaeologist who discovered traces of blue paint on Darius tomb, under a thick layer of what was described as a calcareous cover, but currently we may classify as a crust layer (Dieulafoy, 1885: 227). There was no report about the polychromy on other tombs of Naqshe-Rustam, especially the Xerxes’s. However, during the conservation activities in the site a few traces of red paint were identified on the rock reliefs, which was given to the authors for analytical studies. We found the same layer with a high amount of P, interpreted as a ground under the paint in those samples.

Similar phosphorous containing material, probably as a product of burning bones, was found in Persepolis, as a ground layer for painting, in constructions attributed to Xerxes era (Ridolfi et al., 2018).

We need to continue the mapping and also to 76compare analytically our findings on Xerxes tomb with the earlier polychromy on the tomb of Darius. Moreover, this discovery enabled us to distinguish the traces of P-rich ground layer of the original polychromy from other superficial depositions on the rock reliefs. This provides an important measure for future conservation works in order to avoid errors such as overcleaning.

Figure 6: Black Biofilm on the reliefs, (a) sampling location, (b) the cross-section of the stone surface with biofilm, (c) and (d) SEM graphs of the surface of same sample showing the biological growth in the porous structure of the crusted surface.

Figure 7: White layer with traces of polychromy on the surface of reliefs, (a) sampling location, (b) cross-section of sample showing traces of red color on the surface with a ground layer between paint and stone, (c) SEM elemental map for Phosphorous.

Conclusions

In this study, it was used a combined methodology including historical archival study and laboratory analysis, in order to identify the decay patterns and evaluate their progress in the course of time, especially during the last 100 years. Moreover, the results of this study uncovered new features about the polychrome layers on the rock reliefs, which can be used in future conservation planning in order to save as much as possible of the historical materials. Analysis of these materials together with recording of their traces and relevant materials on the maps, were carried out for the first time on the tomb of Xerxes. The results showed interesting features about the origins of the phosphorus-containing layer which is in agreement with other evidence of polychromy during the second half of the Achaemenian period.

Acknowledgements

The authors want to thank Dr. H. Fadaei, the head of Persepolis-Pasargadae scientific foundation for supporting this study.

References

Dieulafoy, M. 1885. Mission de Susiane. Note relative à la découverte sur le tombeua de Darius de sept inscriptions nouvelles. Revue Archéologique ser.3.6 : pp. 224–227.

Della Valle, P., 1843, Viaggi di Pietro Della Valle, V. II, Brighton, G. Gancia.

Fitzner B., Heinrichs K. & Kownatzki R., 1995, Weathering formsclassification and mapping, Verwitterungsformen – Klassifizierung und Kartierung. Denkmalpflege und Naturwissenschaft, Natursteinkonservierung 1. Ernst & Sohn, Berlin, pp.41–88.

Fronteau, G., Thomachot-Schneider, C., Chopin, E., Barbin, V., Mouze, D., Pascal, A., 2010. Blackcrust growth and interaction with underlying limestone microfacies. Geological Society, London, Special Publications, 333: 25–34. Kaempfer, E., 1712. Amonitatum exoticarum politico-physico-medicarum fasciculi V, quibus continentur variae relationes, observationes et descriptiones Rerum Persicarum et Ulterioris Asiae. Lemgo. Meyer.

ICOMOS-ISCS, 2008. Illustrated glossary on stone deterioration patterns, English/French version. MONUMENTS AND SITES, XV. Paris: ICOMOS.

Masetti, C., 2018. Città varie e costumi il fin prescrisse. La Persia di Pietro Della Valle (1617–1623). Franco Angeli.

Ridolfi, S., Laurenzi Tabasso, M., Askari Chaverdi, A., Callieri, P., 2018. The Finishing Technique of the Stone Monuments of Persepolis: Further Studies and New Findings Through the Use of Non’Destructive Analytical Techniques. Archeometry, 61(2): pp. 272–281.

Schmidt, E., 1970. Persepolis. Vol. III: The Royal Tombs and Other Monuments. Chicago: Oriental institute.

Sohrabi, M., Favero-Longo, S., Pérez-Ortega, S., Ascaso, C., Haghighat, Z., Talebian, M., Fadaei, H., De los Ríos, A., 2017. Lichen colonization and associated deterioration processes in Pasargadae, UNESCO world heritage site, Iran. International Biodeterioration & Biodegradation, 117: pp. 171–182.

UNI., 2006. 11187:2006 – Beni culturali – Materiali lapidei naturali ed artificiali – Pulitura con tecnologia laser. Milan: UNI – Ente Nazionale Italiano di Unificazione.

77

EFFECT OF FIRE ON THE DURABILITY OF A POROUS CALCARENITE: THE CASE STUDY OF A TOBACCO FACTORY IN TRICASE (LECCE, SOUTHERN ITALY)

Emilia Vasanelli¹, Angela Calia¹, Giovanni Quarta¹, Davide Melica², Maurizio Masieri¹, Antonio Monte¹, Francesco Micelli³

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 CNR-ISPC National Research Council – Institute of Science for Cultural Heritage, University Campus, Prov.le Lecce Monteroni, 73100, Lecce, Italy

2 Conservation Scientist via Carlo D’Angiò 31, 73043 Copertino (Lecce), Italy

3 University of Salento Dept. Innovation Engineering University Campus, Prov.le Lecce Monteroni, 73100, Lecce, Italy

Abstract

Thermal decay induced by fire produces chemical and mineralogical alterations, which are often accompanied by discoloration changing the aesthetic characteristics of the stone surfaces. It may compromise also the load-bearing capacity of the masonry elements and lead to stability problems.

Here we report on the investigation of the fire effects on a soft calcarenite used within a masonry building.

Mineralogical transformations of iron hydroxides were detected through XRD and accounted for red discolouring. They were confirmed by DSCTGA analyses and indicated a temperature around 300 °C affecting the stone. Microstructural modifications were investigated by means of optical microscopy and UPV propagation test. A quantitative determination of physical parameters such as color, bulk density, porosity and water uptake was also performed. Implications of thermal stress with the mechanical properties were assessed through uniaxial compressive tests. Thermal effects on the stone microstructure were not microscopically evident. On the contrary, UPV decreases indicated a microfissuring, but this slightly affected porosity, water uptake and compressive strenght.

Introduction

Stone materials involved in fires may be affected by a variety of phenomena, which can change their original properties and related performance in buildings. Depending on the temperature, both chemical and mineralogical trasformations may occur and lead to stone colour changes (Kompaníková et al. 2014). In addition, volume variations due to phase transitions (Calia et al, 2015), different thermal expansion of adjacent minerals (Vázquez et al., 2015) or strongly anisotropic thermal properties of some minerals (e. g. calcite) (Siegesmund et al., 2000) may affect the stone microstruture and lead to microfissuring, which may increase the stone susceptivity to weathering and compromise its load-bearing performance (Sippel et al., 2007). Thermally induced effects on the microstructure strongly depend on the inherent stone characteristics and have a high incidence on low porosity materials, due to the dense packing of crystals and grains (Yavuz et al., 2010). Thermal behaviour has been studied for a variety of stones, which mainly include compact materials (Martinho et al., 2018), while poor literature deals with heating damage on porous stones (Gomez-Heras et al., 2006; Brotóns et al., 2013; 78Franzoni et al., 2013). In this paper we present the results of a case study where the effect of a fire on highly porous calcarenites were assessed by using integrated investigation techniques.

Material

The study was carried out on the stone materials used within the masonry of the ACAIT (Azienda Cooperativa Agricola Industriale del Capo di Leuca) industrial building (Fig. 1). The building was a factory for the processing of the tobacco, in the province of Lecce (Southern Italy). It was built in the early 1900 and dismissed at the end of the 1980s. The factory is a remarkable example of the industrial archaeological heritage relating to the tobacco manufacturing, a flourishing activity in which Puglia region had a leading role on a national basis in the past century.

The original building developed on the ground floor only and it has undergone several enlargements over the years. Like all industrial buildings, it has a simple and modular layout, composed of large rooms with corner-vaulted (“volta a spigolo”) roof, typical of the local tradition, load-bearing pillars and masonry walls with regular local limestone ashlars.

Figure 1: The building of the tobacco factory ACAIT affected by a partial collapse.

Figure 2: Detail of the stone from the collapsed vaults, which shows a yellow-beige-color (Y) passing to a reddish one (R) across the thickness of the masonry unit.

In 2018, after a strong rainstorm, part of the structure collapsed (Fig. 2), involving in a first time one vault and part of the two adjacent ones and in a second time (about 7 days later) the remaining part (five vaults) of the room. The collapse evidenced the inner structure of the vaults and external walls. The latter were two leafs walls without horizontal connections, filled with incoherent material (pieces of rocks and debris).

The vault collapse revealed traces of an historic fire, which were hidden by the presence of plasters, in the form of fumes and a diffuse reddish color across the thickness of the masonry units up to some centimeters from the surface (Fig. 2).

In the framework of a diagnostic activity supporting a restoration project in view of a building reuse, the study of the fire damage on the stone was undertaken.

Methods

Collapsed blocks measuring 21x20x50 cm were taken from the site and samples were obtained from both the unaltered and altered portions, having yellow-beige (Y) and reddish colour (R), respectively (Fig. 2).

The following analyses and tests were performed.

— Thin-section samples were observed in plane-polarised and cross-polarised transmitted light by means of an optical microscope (Eclipse LW100 Nikon) at magnifications of 50x e 100x.

— X-Ray Diffraction analyses (XRD) were performed on both the whole rock and insoluble residue. The insoluble residue was separated by a chemical attack of the grinded stone with HCl-3N in order to remove the carbonates by dissolution. A Philips 1742 diffractometer (APD – 3.6j version) was used for the analyses (CuKα, 40 kV, 20 mA, 2ϑ step size of 0.02 °, counting time 1.25 s, scan interval between 3 ° and 60 °). The diffraction data 79were processed with a X’Pert software — Philips Analytical.

— Simultaneous thermal analyses by Differential Scanning Calorimetry and Thermogravimetry (DSC-TG). A Netzsch STA 449 F3 Jupiter^® was used. Both samples of the whole rock and of the insoluble residue were analyzed. Approximately 25 mg of each powder sample were heated in air from the ambient temperature to 1,000 °C, at a heating rate of 10 °/min.

— Colour changes were recorded by colorimetric measurements. These were taken by light absorption in diffuse reflection using a Konica Minolta CM700d spectrophotometer. They were carried out with a D65 illuminant and under a 10 ° standard observer. L*, a* and b* colour coordinates in the CIELab system were measured and the colour variation (ΔE*) was calculated.

— Measurements of bulk density, porosity accessible to water and water absorption amounts of the stone samples were performed through saturation and buoyancy techniques, following the ISRM recommendation [ISRM, 1981].

— Ultrasonic Pulse Velocities (UPVs) were measured on specimens (cubes 70 mm side obtained from masonry blocks coming from the site) after drying at 70 °C, according to ASTM D2845-05 (ASTM 2005). In particular, three visible unaltered (Y) and three colored specimens (R) were taken from the collapsed portion of the building. Velocities were measured by direct transmission method using a TDAS 16 (Boviar) instrument and probes with a frequency of 55 kHz. They were recorded in each direction (x, y, z) of the cubic specimens and expressed as mean values.

— Compressive strength tests were performed according to UNI EN 772-1 (UNI 2011) on the same specimens used for UPV test, after drying at 70 °C. A universal testing machine (Metrocom Engineering spa), with a load capacity of 200 kN and a speed of 0.2 mm/min, was used for the test.

Results and Discussion

The petrographic characteristics, as observed by optical microscopy under polarized transmitted light (Fig. 3a), show that the investigated stone is a medium grainstone. It is almost exclusively made of calcareous fossil remains, which mainly consist of coralline algae and, at lower extents, of benthic foraminifera, echinoids, bivalves and bryozoans. The average dimensions of the bioclasts fall between 0.3 and 0.4 mm with a maximum size of 0.6 mm. The stone contains sporadic quartz and feldspar crystals. The micrite is nearly absent and the cement is made of calcite, with a texture varying from microsparitic to sparitic type. It is in poor amount and fills only partially the interparticle porosity, which results very high. At large extents the cement is in the form of a thin level surrounding the grain borders. In some areas it is present in larger spots and exhibits a well-developed sparitic texture.

Figure 3: Microscopic features of the stone (N//). a: yellow-beige level; b: red level.

No damage in the form of microfissuring affecting the stone microstructure was observed in the red portions, compared to the yellow-beige ones (Fig. 3b). On the contrary, there was an increase of red pigmented bioclasts and iron rich agglomerations, which may be related to an effect of the high temperature on the iron rich components.

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Figure 4: XRD spectra of the whole rock (on the top) and insoluble residue (on the bottom) from the yellow (Y) and red (R) levels.

The mineralogical composition, as determined by XRD analyses of the whole stone samples coming from the levels having the different colours (Fig. 4, top) does not show any differences. In all cases, almost exclusively calcite was detected. A diffraction peak at low angles was visible, as relating to the presence of clay minerals.

To detect the presence of other minerals, masqued by the preponderant CaCO₃ in the whole stone composition, the insoluble residue, after removing all carbonates by dissolution in HCl-3N, was also analysed. The XRD powder patterns of the insoluble residue obtained from the yellow-beige and red levels in the stone after this chemical attack are reported in Fig. 4, bottom. Different mineralogical compositions were found. The presence of quartz, goethite, along with some feldspars and clay minerals was detected in the yellow-beige level. Goethite was absent in the red level, instead hematite was detected. The transformation of goethite to hematite comes from a dehydroxylation process. Such a transition takes place at temperature of 300 °C (Földvári 2011).

Results of the simultaneous TG and DSC analyses performed on the whole stone from the yellowbeige and rel levels are illustrated in Fig. 5.

TG curves well recorded the calcite decomposition between 670 °C and 840 °C with a mass loss of about 40 %.

Figure 5: TG/DSC curves of the whole rock from the yellow (Y) and red (R) levels.

Figure 6: TG, DTG and DSC curves for the insoluble residue from the yellow (Y) and red (R) levels.

Calcite decomposition in the same range of temperature is evident in the DSC curves through the 81presence of an endothermic peak. A slight endothermic peak at about 80° C is also present, along with a larger one at 550 °C. Both are better shown in the DSC curves of the insoluble residue. TG/DTG curves (Fig. 6) of the insoluble fraction from the yellow-beige levels show a first mass reduction, with a peak in the DTG curve at 84 °C. In the R sample this mass loss is shifted at 95 °C and it is less pronounced. These thermal variations are consistent with dehydration due to the evaporation of the adsorptively bound water from the specimen (Földvári 2011). A second mass loss with a peak in the DTG curve at 287 °C is observed in the sample from the yellow-beige stone, which can be attributed to the goethite dehydroxylation (Földvári 2011). This peak is absent in the DTG curve of the R sample according to the XRD findings, which did not detect goethite in this sample, but hematite as a product of its transformation. For temperatures higher than 400 °C, the pattern of the DSC curve evidences an endothermic-exothermic process. It corresponds to a solid-phase structural decomposition of organic matter and clay minerals, which is more pronounced in the Y sample compared to the R one. It is followed by a crystallization of new phases, whose evidence is given by a subsequent exothermic bump.

Colour changes, bulk density, porosity accessible to water and water absorption measured for the stone from the yellow-beige and red levels within the blocks are reported in Table 1.

A strong colour variation was recorded in the red level, which may be attributed to the transition of goethite to hematite detected through the mineralogical and thermal analyses. High porosity and water absorption, as well, were measured in the yellow level and not significant decreases of 6 % and 5 %, respectively, were measured in the red level. Also the bulk density showed a slight reduction, namely 3 %. These variations are comparable with those reported for limestones with high porosity and notably lower than the decreases recorded for compact stones (Gomez-Heras et al., Brotons et al. 2013). They were in the range of variability of the measurements, thus they could be due to the intrinsic stone heterogeneity.

However, a decrease of UPV in the samples from the red level was recorded. It was 21 % (Table 1).

UPV decrease as an effect of the heating is reported in the literature (Yavuz et al. 2010, Andriani 2014), although at entities depending on the temperature and stone structure, as well. It mainly relates to a thermal micro fissuring, which causes the reduction of the propagation velocities.

The microfissuring detected by the UPV test slightly affected the above mentioned physical parameters measured by stauration and buoyancy techniques. This finding suggests that the recorded decreases of the wave velocities may be relevant to the generation of a microporosity which has no effect on the water penetration, as reported in previous studies (Franzoni et al., 2013; Freire-Lista et al., 2016). Microfissuring recorded by UPV had a negligible effect also on the mechanical performance of the stone. Very close values of the compressive strength were measured in both yellow and red levels, corresponding to a strenght loss of 4 % in the discoloured level (Table 1). Similar entities of decrease have been recorded for porous limestones by Franzoni et al., 2013.

Conclusions

Macroscopic evidences of a fire in the calcarenites employed in an historic building were confirmed by mineralogical changes, which reflects 82on strong color changes. In particular, the change from yellow-beige to reddish color of the stone is consistent with the thermally induced transformation of goethite to hematite. This transition phase indicates that temperatures around 300 °C were reached in the red stone levels during the fire. Effects on the stone microstructure were not visible under optical microscope. Nor the measurement of physical properties showed meaningful variations in this regard. On the contrary, UPV detected a decrease of the propagation velocities, which probably denotes a stone microfissuring. Nonetheless, its entity did not compromise the mechanical resistance of the stone, which remained almost unchanged. High porosity may account for a slight microstructural damage recorded for the investigated calcarenite, where pores likely behave as free spaces for expansion of calcite grain preventing in this way an extensive damage.

Table 1: Color change (ΔE), bulk density (γb), porosity accessible to water (P), water absorption (WA), ultrasonic pulse velocity (UPV) and uniaxial compressive strength (UCS) measured for the stone from the yellow-beige and red levels.

References

Andriani G. F., Germinario L. 2014. Thermal decay of carbonate dimension stones: fabric, physical and mechanical changes. Environ Earth Science 72: 2523–2539.

ASTM D2845-05 2005. Standard test method for laboratory determination of pulse velocities and ultrasonic elastic constants of rock. American Society for Testing Materials.

Brotóns V., Tomás R., Ivorra S., Alarcón J. C. 2013. Temperature influence on the physical and mechanical properties of a porous rock: San Julian’s calcarenite. Engineering Geology 167: 117–127.

Calia A., Colangiuli D., Lettieri M., Quarta G., Masieri M. 2016. Microscopic techniques and a multi-analytical approach to study the fire damage of the painted stuccoes from the Petruzzelli Theatre (Bari, Southern Italy). Microchemical Journal. 126: 42–53.

Földvári M. 2011. Handbook of thermogravimetric system of minerals and its use in geological practice. In: Geological institute of Hungary. Budapest. isbn 978-963-671-288-4.

Franzoni E., Sassoni E., Scherer G.W, Naidu S. 2013. Artificial weathering of stone by heating. Journal of Cultural Heritage. 14: 85–93.

Freire-Lista D. M., Fort R., and Varas-Muriel M. J. 2016. Thermal stress-induced microcracking in building granite. Engineering Geology 206:83–93.

Gomez-Heras M., Alvarez de Buergo M., Fort R., Hajpál M., Török A., Varas M. J. 2006. Evolution of porosity in Hungarian building stones after simulated burning. In: Heritage Weathering and Conservation HWC-2006, Taylor & Francis, Rotterdam, 513–519.

ISRM. Rock characterization testing and monitoring. In: Brown ET, editor. ISRM suggested methods, Oxford:Pergamon Press; 1981.p. 135–40.

Kompaníková Z., Gomez-Heras M., Michňová J., Durmeková T., Vlčko J. 2014. Sandstone alterations triggered by fire-related temperatures. Environmental Earth Science 72: 2569–2581.

Martinho E., Dionísio A. 2018. Assessment Techniques for Studying the Effects of Fire on Stone Materials: A Literature Review. International Journal of Architectural Heritage. 00 1–25.

Siegesmund S., Ullemeyer K., Weiss T., Tschegg E. K. 2000. Physical weathering of marbles caused by anisotropic thermal expansion, Int. J. Earth Sci. 89: 170–182. doi:10.1007/s005310050324.

Sippel J., Siegesmund S., Weiss T., Nitsch K. H., Korzen M. 2007. In: Pikryl, R. & Smith, B. J. (eds) Building Stone Decay: From Diagnosis to Conservation. Geological Society, London, Special Publications 271: 139–151.

UNI EN 772-1 2011. Methods of test for masonry units – Part 1: Determination of compressive strength.

Yavuz H., Demirdag S., Caran S. 2010. Thermal effect on the physical properties of carbonate rocks. International Journal of Rock Mechanics & MiningSciences 47: 94–103.

Vázquez P., Shushakova V., Gómez-Heras M. 2015. Influence of mineralogy on granite decay induced by temperature increase: Experimental observations and stress simulation. Eng. Geol. 189: 58–67. doi:10.1016/j.enggeo.2015.01.026

83

VARIATIONS OF CHARACTERISTICS OF SANDSTONE SUBJECTED TO WEATHERING AND CONSERVATION INTERVENTIONS

Miloš Drdácký¹, Dita Frankeová², Zuzana Slížková²

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, Department of Heritage Science, Prosecká 76, 190 00 Praha 9, Czech Republic, drdacky@itam.cas.cz

2 Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, Department of Material Research, Prosecká 76, 190 00 Praha 9, Czech Republic

Abstract

The paper presents selected results of a comprehensive study of characteristics and behavior of seven typical sandstone types used for historic buildings in the Czech Republic, mostly in Prague. Stone characteristics were studied on materials affected by various historic environmental impacts and conditions generated by previous interventions. From seven types of sandstone were prepared nine series of test specimens which included chemically deteriorated surface layers (crust), cleaned surface layers, and virgin material from the stone core. The above-described sets were manufactured without any consolidation treatment as well as in two further sets consolidated with two agents, namely Funcosil 100 and 300, based on the silicic acid ester. The test specimens were cut from damaged sandstone blocks, which were extracted from a masonry rail of the Charles Bridge in Prague before replacement with new elements. The results supplied data for comparing the efficiency of the consolidation treatment with silicic acid ester products in relation to three pre-treatment stone conditions, as well as to the type of sandstone cementation (mostly a kaolin or silica, rarely goethite cementation). In the paper, the most important results and conclusions taken from the tests and their comparison are discussed.

Introduction

Charles bridge in Prague has been subjected to various types of deterioration or damaging actions during its nearly eight hundred years history. As a result, some parts were substantially repaired using different types of sandstone available at the given periods. The stone materials exhibit different characteristics decisive for the application of efficient conservation or maintenance technologies. Therefore, a detailed investigation programme has been launched in order to provide restorers with reliable data on material characteristics as well as the response to a selected pilot consolidation treatment.

Sandstone types and specimens

Seven types of sandstone were excavated in the past directly in Prague or in mostly close Bohemian quarries.

They are denoted by the names of the quarries and include Božanov (arkose sandstone), Žehrovice (arkose), Droba (wacke sandstone, unknown quarry), Hořice (quartz sandstone), Libná (quartz sandstone with glauconite), Praha (quartz sandstone), Petřín (quartz sandstone).

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Figure 1: Divison of the prismatic samples taken from rail stones that had to be replaced.

Prismatic samples in dimensions of 50 mm × 50 mm × approx. 200 mm were cut into two test specimens – a weathered part with the degraded surface layer and a part of the unweathered material, Figure 1. The deteriorated stone exhibited a significant variation of its characteristics along the depth profile. Therefore, the stone specimens were prepared first in the form of cubes for non-destructive US tests, Figure 2.

Then the cubes were cut into thin plates according to the methodology recommended by Drdácký & Slížková (2008). Thin plates enable to design a sequence of tests that provide first data from nondestructive tests, typically volumetric change due to hydric and temperature variations, and then from destructive tests of mechanical characteristics, Figure 3.

The procedure above was applied to the non-weathered specimens as well as on the weathered stone with both the uncleaned and cleaned deteriorated surfaces. For the cleaning, a sandblasting approach has been adopted.

Figure 2: Ultrasonic testing of material characteristics in 5 mm equidistant profiles.

Figure 3: Scheme of the cutting plan for characterization according to the depth profile of the sandstone specimens.

Sandstone consolidation

For the pilot consolidation tests, two ethyl silicate-based agents have been selected, namely non-diluted Funcosil^® Steinfestiger 100 and Funcosil^® Steinfestiger 300. They were applied in amounts of 1l per 1 m² after one-week conditioning at 20 °C/60 % RH. The agents were applied by syringe extended with the cigarette filter on stone surfaces vertically arranged in order to imitate the expected treatment situation on the rail walls, Figure 4.

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Figure 4: Treated stone specimens with marked points for US velocity measurements and clearly visible depth of penetration.

After the treatment, the specimens were conditioned for one month at 20 °C/60 % RH before starting the testing.

Material testing

During experimental work, the following characteristics were tested: ultrasonic velocity in transmission, micro-drilling resistance, water uptake, porosity, hydric dilation and thermal dilation, bending strength, modulus of elasticity and frost resistance.

Test results

Porosity and mechanical characteristics represent the most interesting data at the consolidation tests. Changes of porosity, as well as in mechanical characteristics, substantially influence the behavior and life cycle of treated historic materials. Naturally, the surface stone deterioration creates very non-homogeneous profiles along the depth in different distances from the surface. Stone material then responses in various ways to consolidation interventions (Sasse & Snethlage 1996). Figure 5 illustrates changes in US velocities in the tested sandstone after consolidation with the Funcosil 300 Steinfestiger.

Figure 5: US velocity changes after consolidation by Funcosil 300 – non-weathered stone (upper) and weathered stone (lower).

The highest increase of ultrasonic velocity was observed in samples from Praha sandstone (light blue lines), which has the highest porosity as well as the largest mean pore size. Other stones appeal improvement in USV only in case of weathered surface treated by the higher concentration of consolidant.

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Figure 6: Three-point bending tests of thin sandstone plates.

Figure 7: Changes in the bending strength of the unweathered sandstone after consolidation.

Similarly, the bending strength tests on thin plates (Figure 6), cut from the unweathered stone samples, exhibited the highest impact of consolidation on the high porosity Praha sandstone, Figure 7.

In most cases, the consolidation by Funcosil 100 caused a higher increase in strength than by Funcosil 300.

Figure 5 above clearly shows significant differences in the material characteristics of the weathered and deteriorated sandstone in the depth profiles. Due to crust formation on some stones, the surface and near-surface layers may have elevated mechanical properties – strength and the modulus of elasticity usually together with a decreased mean pore size. On the other hand, disintegrated sandstone types exhibit lower mechanical properties and higher mean pore size characteristics. As an example of both types, let us present Figure 8 showing a variation of the bending strength in the depth profile and Figure 9 comparing mean pore size variations.

Figure 8: Examples of the bending strength variations according to the dept profile of the weathered and “virgin” sandstone samples.

Figure 9: Comparison of mean pore size before and after consolidation.

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Figure 10: Consolidation impact on the stone porosity.

Figure 11: Hydric dilatation after Funcosil 300 treatment on the first two plates under the treated surface – αH in μm/m.

It is seen in Figure 8 that the bending strength of the inner layers of the weathered stone is higher than that of the virgin material. Here must be taken into account that the weathered layers might have some consolidation history, which is not exactly known but could increase the strength of the original material in the near-surface layers.

The weathering with subsurface deposits, as well as the consolidation, decreases the volume of pores especially by filling the small pores which reflect in an increase of the mean pore size value. The value of porosity changes can be studied on Figure 10.

From the other test results, the hydric dilation changes are interesting. Figure 11 shows a series of results of hydric dilation measurements on the surface and the first subsurface layers of the weathered stones. At the same time, the effect of sandblasting cleaning has been investigated.

In Figure 11 the dark blue denotes the weathered uncleaned material, the light blue the material which was sandblasted.

It is apparent that the cleaning of the stone surface significantly reduced hydric dilatation up to 5 mm depth. Probably some effect of packing during blasting may be the reason.

In depths from 5–10 mm (second plate), the hydric dilatation is more affected by a consolidation agent.

Conclusion

The tests were required by restorers before planning a rather massive conservation campaign on the Charles bridge in Prague – one of the most important stones Gothic structure. The results achieved helped to make an appropriate choice of consolidation agent, to decide about necessity and type of surface cleaning, to be prepared for a selection of an appropriate stone in cases of replacement needs and to assess intervention impacts. It enhanced the overall design of restoration interventions.

Acknowledgements

The paper is based on the results of research supported by the institutional project RVO 68378297. The authors acknowledge experimental support of E. Čechová, A. Zeman, J. Valach and professional advice of J. Novotný.

88References

Drdácký, M. F., Slížková, Z. Performance of glauconitic sandstone treated with ethylsilicate consolidation agents, In Proc. of the 11th Int. Congr. on Stone, Vol. 2. Toruń; 2008. pp.1205–1212.

Sasse, H. R., Snethlage, R. Evaluation of stone consolidation treatments. Science and Technology for Cultural Heritage. 1996;5(1):85–92.

Table 1: Annex. Sorption characteristics of the tested sandstones.

89

THERMAL BEHAVIOR OF BUILDING SANDSTONE: LABORATORY HEATING EXPERIMENTS VS. REAL FIRE EXPOSURE

Nadine Freudenberg¹, Thomas Frühwirt¹, Klaus-Jürgen Kohl², Monika Kutz², Heiner Siedel³, Jörn Wichert³

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Technische Universität Bergakademie Freiberg, Institute of Geotechnics, Gustav-Zeuner-Str. 1, 09599 Freiberg, Germany

2 Institut für Brandund Katastrophenschutz Heyrothsberge, Research Division, Biederitzer Str. 5, 39175 Biederitz, Germany

3 Technische Universität Dresden, Institute of Geotechnical Engineering, 01062 Dresden, Germany, Heiner.Siedel@tu-dresden.de

Abstract

Heat-induced short-term decay of dimension stone on buildings and monuments caused by fire is a well-known phenomenon. Most of the scientific studies about thermal behavior and thermal changes of building stones are carried out in laboratory ovens by stepwise heating of stone samples to different stages of temperature. However, real conditions of fire attack on stone elements of buildings might differ considerably from the relatively slow, even heating of small samples in ovens. Therefore, more realistic fire scenarios were designed to test the behavior of sandstone specimens such as cylinders and balusters (height 58 cm and max. diameter 19 cm). The samples comprise the Cotta and Posta type of the Cretaceous Elbe sandstone. They were exposed to a real scale fire test, based on the standard ISO 9705 (room corner test). The specimens were mounted in a fire container at a height of 170 cm above the fire source, a wood crib in accordance to DIN EN 3–7. The standard defines a known theoretical heat release rate, producing a maximum air temperature of approx. 900 °C for about 15 minutes. The temperature in the container as well as on the surface and within the stone specimens was monitored by thermocouples during the tests. The measured surface temperatures vary between 350 and 600 °C, whereas the temperatures at some 4.5–9.5 cm below surface vary only between 200 and 350 °C, depending on the shape of the samples. After the fire tests, different crack patterns were observed. In contrast, smaller specimens heated in a laboratory oven did not reveal any macroscopic cracks, although they were exposed to the same or even markedly higher temperatures (1,000 °C in the sample core). However, both treatments are needed for a better understandig of fire damages on stone buildings since the material behavior of sandstone on grain size scale (fabric and mineralogy) triggers macroscopic crack patterns such as fragmentation and scaling.

Introduction

Due to firestorms caused by heavy bombardments during the Second World War, lots of buildings and objects made of sandstone were massively damaged. These damages became probably more severe by extinguishing fire by water, leading to another short-term temperature change. The typical damage observed is fragmentation, thus reducing the load-bearing capacity of architectural elements, as shown by the examples of the Church of Our Lady (Frauenkirche) and the altar in the Church of the 90Three Kings (Dreikönigskirche) in Dresden, Germany (Fig. 1).

Figure 1: Fire damages of historical buildings in Dresden a) Church of Our Lady (Frauenkirche) 1971 (Wikimedia Commons, CC BY-SA 3.0, Lencse Zoltán) b/c) altar in the Church of the Three Kings (Dreikönigskirche).

Numerous studies (e. g. Chakrabarti et al. 1996, Hajpál & Török 2004, Gómez-Heras et al. 2006, Hager 2014, or Lintao et al. 2017) deal with methods to record material changes of different sandstones caused by high temperatures. However, most of these studies investigate the thermal behavior of small samples with laboratory heating regimes in high temperature ovens. In contrast, there are only few studies dealing with small scale real fire scenarios of sandstones, e. g. Koser & Althaus (1999), Ehling & Köhler (2000), Pohle & Jäger (2003), McCabe et al. (2007), or Smith & Pells (2008). Obviously, the damage patterns of heat-treated laboratory samples and fire-affected objects and buildings (cf. Fig. 1) are different. This study on Elbe sandstones compares the behavior of oven-heated with flame-treated samples, the latter corresponding to a more realistic fire scenario.

Materials and testing procedures

The investigated material comprises sandstone of Cotta and Posta type which are the two main varieties of the Upper Cretaceous Elbe sandstone, occurring south of Dresden (Saxony, Germany). The Cotta type is a grey to yellowish-brownish sandstone with clay-bearing, organic and ferritic flakes parallel to bedding. It is a fine-grained and siliceous quartz arenite (> 90 % quartz). In addition, K-feldspar, kaolinite and few illite, glauconite, and rare organic components occur. The color of the Posta type varies between light grey and yellowish-brownish. It is a fineto medium-grained, occasionally coarse-grained, porous and siliceous quartz arenite (quartz nearly 100 %) without organic matter and with very small amounts of kaolinite (Grunert 2007, Grunert & Szilagyi 2010).

For the laboratory heating experiments cylindrical specimens of both sandstone types with different dimensions of 50 × 25 mm and 50 × 100 mm (Fig. 2a) were used. They were orientated normal and parallel to bedding. The dimensions of the specimens for the real-scale fire exposure tests were significantly larger with heights of 58 cm and an approx. diameter of 19 cm. To imitate real shapes of architectural elements such as pillars, balusters and cylinders were carved from Cotta and Posta type sandstone blocks (Fig. 2b/c).

The small sandstone specimens (cf. Fig. 2a) were treated in a laboratory oven (Nabertherm LT24/12) at the Institute of Geotechnics, Technische Universität Bergakademie Freiberg (TU BAF) at 6 different temperature levels (400, 500, 600, 700, 800, 1,000 °C) with a heating rate of 10 K/min and a cooling rate of 1 K/min after a holding time of 6 hours at each target temperature level.

The cylinders and balusters (Fig. 2b/c) were marked for drilling boreholes to mount thermocouples (Fig. 3a/b) which monitored the temperatures on the stone surfaces and within the stones during fire exposure over time. In the cylinder samples, 5 boreholes with a diameter of 8 mm were drilled 91to a depth of 9.5 cm. In accordance to the specific shape of the balusters, 7 boreholes with a diameter of 8 mm were drilled to depths between 4.5 and 9.5 cm. Flowable mortar was used to fix the thermocouples in the boreholes and to guarantee undisturbed heat transfer.

Figure 2: Investigated specimens of Posta and Cotta type Elbe sandstone a) small cylinders (50 × 100 mm) b) cylinders (58 × 19 cm) c) balusters (58 cm in length and max. diameter of 19 cm).

Figure 3: a) Scheme of drill holes on a baluster and a cylinder specimen b) cylinders with mounted thermocouples.

For the real scale room fire tests a fire container (height: 2.40 m, width: 2.35 m, depth: 4.13 m) was used based on the standard ISO 9705 (room corner test at the Institute of Fire Protection and Disaster Control (IBK) in Heyrothsberge (Fig. 4). In the fire container, the cylinder and baluster sandstone specimens were placed at a height of 1.7 m above the fire source (Fig. 5a), achieving a direct flame treatment. The fire source consisted of a wood crib according with DIN EN 3-7 which provided a known theoretical heat release rate with a maximum temperature of approx. 900 °C for about 15 minutes. N-heptane acted as a fire accelerant which was ignited in a pan below the wood crib.

The temperatures in the container were monitored by thermocouples over time. An infrared and a video camera (Fig. 5b) recorded the heat distribution and the fire behavior in the container which could be followed in real-time on a monitor in the nearby laboratory (Fig. 5c/d).

Figure 4: Sketch of the fire container (top view) with sample and thermocouple positions; red: wood crib in accordance to the DIN 3-7 standard as fire source.

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Figure 5: a/b) Views inside the fire container b) positions of video and infrared camera c) record of video monitoring d) record of infrared monitoring.

Results and Discussion

The macroscopic results for the stone samples after heating are shown in Fig. 6a/b. There are significant differences between heating in the laboratory oven and in the fire container. In Fig. 6a the small Posta and Cotta type sandstone specimens with axis parallel (PS_P & CS_P) and normal (PS_N & CS_N) to bedding are displayed. They are arranged according to their temperature treatment levels (from left to right: 25, 400, 500, 600, 700, 800 and 1,000 °C). The specimens appear more reddish with higher temperatures. These color changes are related to mineral transformations, namely of iron-bearing minerals (cf. Hajpál & Török 2004).

Within the brownish to yellowish Elbe sandstones, mainly limonite changes to the red hematite at elevated temperatures (Fig. 6a). Slight color changes to red can be detected for all sandstone types already at 400 °C (cf. Gómez-Heras et al. 2009). In Cotta sandstone also glauconite transformations might contribute to discoloration.

The cylinders treated in the oven did not reveal any macroscopic cracks. The treated and untreated specimens were stored in plastic bags after cooling down. After moving these sample bags for further investigations, loose single sand grains, increasing in number with temperature, were detected in the bags for those samples heated above 500 °C. They indicate decreased cohesion of sand grains in the respective sandstones. In case of Cotta type sandstone this effect was less developed than for Posta type sandstone.

All baluster and cylinder specimens exposed for heating in the fire container show macroscopically visible cracks (Fig. 6b). Moreover, they reveal heavy sooting on the surfaces. Discoloration of the sandstone or crack surfaces could not be detected by the naked eye.

The temperature curves for the small sandstone specimens (50 × 25 mm) are shown in Fig. 6c. They were heated at different temperature levels of 400, 500, 600, 700, 800 and 1,000 °C in the laboratory oven at the TU BAF. The set-point temperatures (dashed lines) and the actual temperatures measured (solid lines) show a good correlation. It is recognisable that the heating experiments in the laboratory oven are precisely reproducible.

In contrast, the temperature curves in the fire container at a height of 1.8 m (solid lines) illustrate that the temperature increased very fast (after approx. 3 minutes) to max. 900 °C (Fig. 6d). After a dwell time of about 15 minutes (= the time the wood crib takes to burn through), the temperature in the fire container decreased rapidly.

If one compares the slow and even heating in the laboratory oven to the dynamic heating in the fire container, the differences between both treatments become obvious. According to the temperature curves measured by the thermocouples in the experiment displayed in Fig. 6b, on the stone surfaces (dashed lines), even in one and the same experiment, the temperatures range between 400 and 600 °C (Fig. 6d). The maximum temperature of about 600 °C is reached after approx. 14 minutes. The heating of the air in the container is faster and reaches higher maximum temperatures than the stone surfaces. However, the stone surface is cooling down much slower than the 93surrounding air. The upper right diagram in Fig. 6d shows remarkable lower temperatures within the stone compared to the stone surfaces. The maximum temperature of about 230 °C is reached only after approx. 55 minutes, i. e. long after the rapid decrease of the temperature of the surrounding air. Although the absolute temperatures measured may differ between single experiments, the general patterns of temperature development in the air, on the stone surfaces, and within the stone are similar. That means that the direct fire impact results in very unequal spatial and temporal distribution of temperature in the specimen within a short time of heating. These differences in temperature may lead to material tension caused by different thermic dilatation between the outer and the inner parts of the objects, resulting in cracks (Gómez-Heras et al. 2009). Many authors refer to the transformation of α-quartz to β-quartz at around 573 °C and the related volume increase to explain deterioration and damage of quartz-rich building stones (e. g. Chakrabarti et al. 1996, Hajpál & Török 2004). In the presented example, heavy damages (cracks) occur, although this temperature is hardly reached on the sandstone surface (see Fig. 6d).

Figure 6: a) Small specimens (50 × 25 mm) of Posta and Cotta sandstone parallel (PS_P & CS_P) and normal (PS_N & CS_N) to bedding after heating at different temperature levels in the laboratory oven at the TU BAF, from left to right: 25, 400, 500, 600, 700, 800 and 1,000 °C b) significant cracks and heavy sooting on the Posta type sandstone cylinder after the fire test at the IBK in Heyrothsberge c) temperature curves of the small sandstone specimens (50 × 25 mm) heated in the laboratory oven at the TU BAF for different temperature levels d) temperature curves of the thermocouples in the fire container at a height of 1.8 m (solid lines) and at the stone surfaces (dashed lines). The small diagram shows the temperature curves of the thermocouples inside the Posta type sandstone cylinder.

In the oven-heated smaller specimens, tension due to temperature gradients does not occur due to 94slower, even heating. From this point of view, this kind of experiment does not reflect real, short-term fire scenarios on buildings. However, these testings give insight into effects of heat on mineral grains and intergranular matrix. In case of long-lasting fire events, these effects may additionally affect building stones and their material properties.

Conclusions

This study compares two different heat scenarios which are both necessary to investigate fire damages on sandstone objects or monuments.

The realistic fire scenario with the exposure of architectural sandstone elements to a burning wood crib within a fire container for a short time results in damages comparable to those observed on monuments which suffered from fire attack. The temperatures measured on stone surfaces and within the inner core of the objects indicate high gradients, resulting in material tension and subsequent cracking.

In contrast, smaller specimens of the same sandstone materials reveal no cracking even at higher temperatures when gradually heated in a laboratory oven. However, such tests and the respective test specimens which will be further investigated, may also be useful for enlightening the change of petrographic and material properties during heating.

Acknowledgements

The authors thank the Free State of Saxony (Sächsische Aufbaubank – Förderbank – SAB) for funding the work within the project WI631.

References

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Gómez-Heras M., Álvarez de Buergo M., Fort R., Hajpál M., Török Á., Varas M. J. 2006. Evolution of porosity in Hungarian building stones after simulated burning. In: Fort R., Álvarez de Buergo M., Gómez-Heras M., Vazquez-Calvo C. (eds) Heritage, Weathering and Conservation. Taylor & Francis, London, 513–519.

Gómez-Heras M., McCabe S., Smith B. J., Fort R. 2009. Impacts of Fire on Stone-Built Heritage. Journal of Architectural Conservation 15(2):47–58.

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Grunert S., Szilaghy J. 2010. Petrophysikalische Eigenschaften einer Auswahl von Baugesteinen aus Deutschland und ihr Bezug zur Petrographie dieser Gesteine. Geologica Saxonica – Journal of Central European Geology 56(1):39–82.

Hager I. 2014. Sandstone colour change due to the high temperature exposure. Advanced Materials Research 875-877:411–415.

Hajpál M., Török Á. 2004. Mineralogical and colour changes of quartz sandstones by heat. Environmental Geology 46:311–322.

Koser E., Althaus E. 1999. Brandschäden an Bauwerken aus Naturstein – Hohenrechberg und andere Objekte im Laborexperiment. Internationale Tagung des SFB 315 Heft 16/1999.

Lintao Y., Marshall A. M., Wanatowski D., Stace R., Ekneligoda T. 2017. Effect of high temperatures on sandstone – a computed tomography scan study. International Journal of Physical Modelling in Geotechnics 17(2):75–90.

McCabe S., Smith B. J., Warke P. A. 2007. Sandstone response to salt weathering following simulated fire damage: a comparison of the effects of furnance heating and fire. Earth Surface Processes and Landforms 32:1874–1883.

Pohle F., Jäger W. 2003. Material properties of historical masonry of the Frauenkirche and the masonry guideline for reconstruction. Construction and Building Materials 17:651–667.

Smith A. G., Pells P. J. N. 2008. Impact of fire on tunnels in Hawkesbury sandstone. Tunnelling and Underground Space Technology 23:65–74.

95

EXPLORING MICROBIAL COMMUNITIES INHABITING GYPSUM CRUSTS OF WEATHERED NATURAL BUILDING STONES

Laurenz Schröer^{1, 2}, Tim De Kock^{1, 3}, Nico Boon², Veerle Cnudde^{1, 4}

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Ghent University, PProGRess-UGCT, Department of Geology, Ghent, Belgium

2 Ghent University, Center for Microbial Ecology and Technology (CMET), Ghent, Belgium

3 Antwerp Cultural Heritage Sciences (ARCHES), University of Antwerp, Antwerp, Belgium

4 Environmental hydrogeology, Utrecht University, Department of Earth Sciences, Utrecht, The Netherlands

Abstract

Microbes thrive in almost every possible environment, including natural building stones. Microbial communities affect these materials which can lead to biodeterioration. Among others, air pollution, especially SO₂ and NO_x, is an important actor for stone degradation. This leads to crust formation and in limestone typically to gypsum crusts. There are questions if and how sulphur-oxidizing prokaryotes can play a role in crust formation, oxidizing sulphur dioxide to sulphuric acid. This study explores the microbial community inside and underneath gypsum crusts to find out who is there and how they relate to the weathering and gypsum crust formation.

We studied Lede stone, a sandy limestone or calcareous sandstone, used in many historical buildings in north western Belgium. This stone is prone to weathering and gypsum crust formation. Two historic monuments have been sampled both in the urban environment (City hall, Ghent, Belgium) and in the countryside (Castle of Berlare, Belgium). These monuments consisted of severely weathered Lede stone: the City Hall in Ghent contained very thick botryoidal gypsum crusts while the crusts in Berlare were more superficial. Stone material was collected with a flame sterilized chisel and drill chuck and was used to isolate bacteria. To desribe the prokaryotic community: DNA was extracted, 16S rRNA genes were amplified and sequenced by Illumina Mi-Seq Next Generation Sequencing (NGS). The isolation campaign gave more information on the genus and species level of the bacterial community and makes it possible to test their abilities. The isolates from the two localities differ significantly and include diverse pigmented bacteria (orange, red, pink, yellow). The pigmented bacteria might contribute to the overall rock discoloration. The impact of the prokaryotic colonization on potential crust formation will be discussed.

Introduction

Our historic built heritage consists mainly out of natural building stones. As these stones interact with the environment, their properties slowly alter and the stones degrade. On limestones, gypsum crusts are among the most abundant deterioration features. These are sulphate encrustations that incorporate airborne dust, giving them a black appearance (Camuffo, Del Monte and Sabbioni, 1983). There is a strong correlation with pollution, especially with SO₂ and NO_x. It has been regarded that atmospheric SO₂ oxidizes and forms H₂SO₄, 96which will transform CaCO₃ to gypsum. NO_x acts as a catalyst (Bai, Thompson and Martinez-Ramirez, 2006).

Besides air pollution, biodeterioration by lichens, algae, fungi, archaea and bacteria can alter building stones significantly. Several groups of prokaryotes produce acids, chelating agents and pigments leading to dissolution and discolouration. (Doehne and Price, 2010). Some autotrophic prokaryotes can oxidize sulphur or nitrogen compounds and produce respectively H₂SO₄ or HNO₃. By other means they can turn air pollutants such as NO_x and SO₂ into nitrates and sulphates and can play a role in gypsum crust formation (Doehne and Price, 2010). Those prokaryotes have been isolated and sequenced from buildings and a correlation between air pollution and their occurrence has been indicated (Mansch and Bock, 1998; Villa et al., 2015; Li et al., 2016) Laboratory tests by Mansch and Bock (1996) show an increase of gypsum formation in the presence of nitrifying bacteria. The role of nitrogen and sulphur oxidizers in gypsum crust formation is still unclear and for this reason we sampled specifically gypsum crusts of Lede stone at the city and the countryside (De Kock et al., 2015), to characterize the prokaryotic community and to test if they can alter natural building stones.

This manuscript will focus on sulphur and nitrogen oxidizing prokaryotes and the discolouration potential of one of the isolates. A more in-depth description of the whole microbial community can be found (Schröer et al., 2020).

Material and Methods

The samples were retrieved at the beginning of April 2019 from two monuments in north western Belgium: six samples (G1–G6) from the City Hall of Ghent and seven (B1–B7) from the Castle of Berlare, representing respectively an urban and rural environment. Data from the Flemish environmental agency (VMM) indicates both a higher concentration of NO_x and SO₂ in the city centre of Ghent, compared to the area of Berlare. However, overall SO₂ concentrations declined around 90 % since the eighties resulting today in a minor difference between city and countryside. NO₂ emission decreased significantly as well, but here a bigger difference remains between urban (+− 30 μg/m³) and rural environment (+− 15 μg/m³) (Vlaamse Milieumaatschappij, 2019). Both monuments contain deteriorated Lede stone with gypsum crusts. Lede stone is a sandy limestone from north western Belgium out of the Lutetian (Eocene) (De Kock et al., 2015). The material for amplicon sequencing has been collected using a small flame sterilized drill, while around the drill hole, crust and underlying rock has been collected with a flame sterilised chisel to perform the isolations and soluble salt measurements.

Figure 1: A) Gypsum crust in Ghent and B) Berlare.

DNA was extracted out of the drill powder using the DNeasy PowerSoil Kit (Qiagen, Venlo, Netherlands), following the manufactures instructions. DNA extract was sent out to LGC genomics GmbH (Berlin, Germany) for 16S rRNA gene sequencing on an Illumina MiSeq platform and library preparation. For the bacteria it followed the same procedure as De Paepe et al. (2017) with 35 PCR cycles. Additionally, the archaea were determined, on three samples of each location, using a nested approach (De Vrieze et al., 2018).

Furthermore, one powdered sample of each location was used as inoculum for isolations: R2A agar for heterotrophic bacteria and thiosulphate plates for sulphur oxidizers containing (per Litre) 980 mL fresh water basal mineral medium, 9.7 g Na₂SO₄, 6 g Na₂S₂O_3, 10 g agar, 0.02 g bromothymol blue, 10 mL 971 M MOPS buffer, 10 mL 1 M NaHCO₃, 1 mL SL-10 trace element solution and 1 mL 7-vitamin solution The plates were incubated between two and six weeks at room temperature. For anaerobic isolations 2 g/L NaNO₃ was added. The isolates have been characterized by Sanger sequencing (LGC Genomics GMbH, Berlin, Germany) using 27 F and 1492R LGC primers. The resulting sequences were blasted with NCBI’s BLAST and with RDP Seqmatch. The gypsum crusts were characterized by their soluble ions. These have been extracted from powdered crust and underlying rock with Milli-Q water with a 1 : 5 ratio. The cations Na⁺, K⁺, Ca²⁺, Mg²⁺ and the anions Cl^–, NO₃^–, NO₂^–, SO₄^2–, PO₄^3– have been quantified on a 930 Compact Ion Chromatograph Flex (Methrohm, Switzerland) with a conductivity detector. From the measured concentrations, the amount of soluble ions was calculated. RUNSALT was used to model the phases of the salt mixture in function of relative humidity (Price, 2000; Bionda, 2005).

The potential discolouration of Arthrobacter agilis, one of the isolates was tested on the French oolitic Savonnières limestone. This stone is extensively used as a replacement for Lede stone (Dewanckele et al., 2014) This was tested by a water run-off test with a similar setup as De Muynck et al. (2009). Arthrobacter agilis was grown in R2A broth at 20 °C, after which it was sprinkled during two hours over six slaps of Savonnières limestone and dried during 24 hours. This was repeated two times more and in between its colour at two spots on the rock was examined with a point measurement using the CM-2600d spectrophotometer.

Results and Discussion

Gypsum crusts were present both in the urban and rural environment. These were thick botryoidal and black in Ghent, while in Berlare those were thinner and laminar with a rusty colour. The occurrence of the gypsum crusts has also been confirmed by the soluble salt analysis (see Table 1 for the anions). In every sample, there was a high concentration

of soluble SO₄^2– ranging between 1 mg/g_rock and 10 mg/g_rock. Although highly variable between the samples, overall, the soluble salt content was higher in the City Hall of Ghent compared to the samples of Berlare. This was also the case for the Cl^– and NO₃^–. Sample B6 was the only sample of Berlare with more Cl^– and NO₃^–, because of its sheltered position underneath a windowsill. Nitrite and phosphate were not found or only available in a very low amount. The compatible cations were primarily calcium (0.6−5 mg/g_rock) and in a lesser extend also potassium and magnesium. Sodium was most of the times slightly present, but could reach a high concentration (about 5 mg/g_rock for G4) combined with a high amount of soluble chloride. Based on the modelling performed using RUNSALT the accompanying salts (except gypsum), for the samples with high soluble salts content, at 20 °C and low RH were among others carnallite (KCl·MgCl₂.6H₂O), niter (KNO₃), nitromagnesite, (Mg(NO₃)₂.6H₂O), halite (NaCl), nitratine (NaNO₃) and Ca(NO₃)₂. These salts would be in solution at a relative humidity of about 60 % and more. For the other samples, the result of the model was more complex.

Within and underneath the gypsum crusts, 16S rRNA gene sequencing successfully identified the prokaryotic community on the City hall of Ghent and the Castle of Berlare, except for one sample (G1). Sulphur oxidizing, sulphur reducing and nitrogen oxidizers have been identified in some of the samples. Genera of nitrifying bacteria from different taxa were present: Nitrolancea (0.03 % 98in G2), Nitrospira (0.06 % in G2), Nitrosomonadaceae with Nitrosospira (0.002 % in G4, 0.7 % G6 and 0.0003 % in B1) and Nitrosomonas (0.002 % in G6). Archaea have been found in only one sample (G6) belonging to the nitrifying Nitrososphaeraceae.

Table 1: Soluble anions of the different samples in the urban and rural environment (mg/grock).

Sample	Cl–	NO3–	SO42–
G1	0.351	1.626	6.795
G2	1.014	5.404	6.858
G3	0.103	0.261	7.337
G4	5.348	12.537	9.651
G5	0.112	0.238	6.948
G6	0.087	0.306	6.884
B1	0.004	0.006	4.000
B2	0.207	0.129	7.156
B3	0.005	0.004	4.952
B4	0.004	0.003	1.261
B5	0.005	0.005	4.632
B6	0.801	1.150	7.221
B7	0.008	0.005	2.851

Some sulphur oxidizing bacteria were found as well, belonging to the “purple non-sulphur” bacteria (PNS). Despite their name, members of this group can oxidize low concentrations of sulphur (Hunter et al., 2009). PNS were abundant in G5 with 16 % of Rhodoplanes. Furthermore, there might be more PNS present in the samples as the order Rhodospirillales has been detected in G4 and G6 with an unclassified chemolithoautotrophic Magnetospira (0.2 % in G4) and with an unclassified genus of the family Rhodospirillaceae (0.3 % in G4). Furthermore, potential PNS in Rhodobacteraceae were represented as well, however again unclassified (0.5 % in G2, 0.002 % in G4, 0.4 % in G6 and 0.0003 % in B1) (Hunter et al., 2009; Williams et al., 2012). Beside sulphur oxidation, some identified bacteria belonged to the sulphur/sulphate reducing bacteria such as Desulfuromonadales (0.005 % in G3) and 4 % Geoalkalibacter in G5, but also Desulfobacteraceae, with Desulfofrigus (0.008 % in G5 and 0.0006 % in B4), Desulforhopalus (0.0006 % in B4) and unclassified genera (0.0009 % in G6, 0.003 % in B4 and 0.003 % in B6) (Greene, Patel and Yacob, 2009; Rosenberg et al., 2013).

There is a strong link between the location and the occurrence of the nitrogen oxidizing and sulphur bacteria. Both are present in selected samples across both locations, but mainly in the City Hall of Ghent, where in one sample they even reach high abundance. There is also a link between the occurrence of the nitrifying and sulphur prokaryotes and the higher soluble salt content of respectively nitrate or sulphate. However, also here there are some exceptions. The occurrence of sulphate reducing bacteria followed the same trend. These bacteria can cause further deterioration (Krumbein and Gorbushina, 2009) but besides inducing gypsum crust formation, they can reduce the sulphate and precipitate calcium carbonate. This process also produces sulphur and H₂S (Castanier, Le Métayer-Levrel and Perthuisot, 1999). As one sample in the city (G5) contains both a high concentration of PNS and sulphate reducing bacteria, it is not impossible that both groups are strongly linked together.

These results combine some of the findings of previous research by Mansch and Bock (1998) & Villa et al. (2015). The former found more nitrifying bacteria in the urban environment and the latter more sulphur oxidizing bacteria. It also confirms the findings of Li et al. (2016). However, members of sulphur compound oxidizing genera of which it is known that they deteriorate building stones, such as Thiobacillus (and related genera) have not been found (Krumbein and Gorbushina, 2009). This contrasts with the earlier findings of Villa et al. (2015). Furthermore, it is unclear if the PNS bacteria would affect natural building stones the same way as Thiobacillus. The presence of sulphur bacteria that only tolerate lower amounts of sulphur can be related to the decreasing SO₂ concentrations in the atmosphere. The low abundance of sulphur and nitrogen oxidizers, combined with the absence of those groups in several samples, indicates that other chemical factors are most likely still dominating gypsum crust formation. Although those gypsum crust have been formed since decades, when the SO₂ and NO_x concentrations were higher, it cannot be excluded that back then more sulphur and nitrogen oxidizers were present affecting crust formation.

Besides the metagenomic approach, also an isolation campaign has been performed. This did not succeed to retrieve chemolithoautotrophic prokaryotes. However, 20 genera belonging to 31 species of heterotrophic bacteria were successfully isolated and identified. Growth occurred mainly aerobic and denitrification occurred barely. Many isolates contained a red, orange, pink or yellow pigment and one of them, Arthrobacter agilis, has been successfully applied on the water run-off test (Figure 2). After one cycle of two hours, the rocks became significantly red and this has been confirmed by the spectrophotometric data. There was a dip between 450 and 560 nm in the reflectance, leading to an increase visibility of the complementary 99red/orange colours. During this test, we applied bacterial concentrations that do not occur in nature. However, it shows how easy it can discolour a natural building stone and the potential of some of the bacteria to cause aesthetic changes. Besides natural oxidation of iron-bearing minerals, these bacteria could also contribute to the rusty/red colour on the Castle of Berlare.

Figure 2: A) Spectral data showing the initial state of the Savonnières limestone (blank) and the progress of the colour change after adding Arthrobacter agilis during the different cycles (Cycle 1–3). B) Savonnières stone after three cycles showing the discolouration compared to the initial state at the top section.

Conclusion

Lede stone at Ghent and Berlare were both deteriorated and contained gypsum crusts. The crusts were more elaborate in Ghent with overall a higher content of soluble salts. 16S rRNA gene amplicon sequencing revealed potential stone deteriorating nitrogen and sulphur prokaryotes, especially on the City Hall. This suggests a positive correlation with air pollution. However, they were only present with a low concentration in some samples and absent in the isolates. Currently, a strong effect of prokaryotes on crust formation is not expected. Furthermore with decreasing sulphur pollutions, the gypsum is mainly a result of historical accumulation. on Lede stone, prokaryotes can have an impact on the appearance of the monuments as Arthrobacter agilis successfully discoloured Savonnières limestone.

Acknowledgements

This work was funded by Research Foundation Flanders (Research grant number 11D4518N). We would like to thank Sarah Williamson and the lab technicians of CMET for their help with the experiments.

References

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Bionda, D. (2005) RUNSALT – A graphical user interface to the ECOS thermodynamic model for the prediction of the behaviour of salt mixtures under changing climate conditions. Available at: http://science.sdf-eu.org/runsalt.

Camuffo, D., Del Monte, M. and Sabbioni, C. (1983) ‘Origin and growth mechanisms of the sulfated crusts on urban limestone’, Water, Air, and Soil Pollution. Kluwer Academic Publishers, 19(4), pp. 351–359. doi: 10.1007/BF00159596.

Castanier, S., Le Métayer-Levrel, G. and Perthuisot, J.-P. (1999) ‘Ca-carbonates precipitation and limestone genesis — the microbiogeologist point of view’, Sedimentary Geology, 126(1–4), pp. 9–23. doi: 10.1016/S0 037-0738(99)00028-7.

Dewanckele, J. et al. (2014) ‘Neutron radiography and X-ray computed tomography for quantifying weathering and water uptake processes inside porous limestone used as building material’, 100Materials Characterization. Elsevier Inc., 88, pp. 86–99. doi: 10.1016/j.matchar.2013.12.007.

Doehne, E. and Price, C. A. (2010) Stone Conservation, An Overview of Current Research. 2nd ed. Re. Los Angeles, CA: Getty Conservation Institute.

Greene, A. C., Patel, B. K. C. and Yacob, S. (2009) ‘Geoalkalibacter subterraneus sp. nov., an anaerobic Fe(III)- and Mn(IV)-reducing bacterium from a petroleum reservoir, and emended descriptions of the family Desulfuromonadaceae and the genus Geoalkalibacter’, INTERNATIONAL JOURNAL OF SYSTEMATIC AND EVOLUTIONARY MICROBIOLOGY, 59(4), pp. 781–785. doi: 10.1099/ijs.0.001537-0.

Hunter, C. N. et al. (eds) (2009) The Purple Phototrophic Bacteria, Photosynthetica. Dordrecht: Springer Netherlands (Advances in Photosynthesis and Respiration). doi: 10.1007/978-1-4020-8815-5.

De Kock, T. et al. (2015) ‘Lede Stone: A potential “Global Heritage Stone Resource” from Belgium’, Episodes, 38(2), pp. 91–96. doi: 10.18814/epiiugs/2015/v38i2/004.

Krumbein, W. and Gorbushina, A. (2009) ‘Some Aspects of Biological Weathering and Air Pollution Some Aspects of Biological Weathering and Air Pollution’, in The Effects of Air Pollution on Cultural Heritage. Boston, MA: Springer US, pp. 127–145. doi: 10.1007/978-0-387-84893-8_5.

Li, Q. et al. (2016) ‘Distribution and diversity of bacteria and fungi colonization in stone monuments analyzed by high-throughput sequencing’, PLoS ONE, 11(9), pp. 1–17. doi: 10.1371/journal.pone.0163287.

Mansch, R. and Bock, E. (1996) ‘Simulation of Microbial Attack on Natural and Artificial Stone’, in Microbially Influenced Corrosion of Materials. Berlin, Heidelberg: Springer Berlin Heidelberg, pp. 167–186. doi: 10.1007/978-3-642-80017-7_13.

Mansch, R. and Bock, E. (1998) ‘Biodeterioration of natural stone with special reference to nitrifying bacteria’, Biodegradation, 9(1), pp. 47–64. doi: 10.1023/A:1008381525192.

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101

STONE CULTURAL HERITAGE IN MONGOLIA – MODEL-LIKE STUDY AND CONDITION ASSESSMENT OF THE SITE OF IKH KHÖSHÖÖT

Martina Haselberger¹, Marija Milchin^1*, Katharina Fuchs¹, Galbadrakh Enkhbat², Tserendorj Tsolmon², Johannes Weber³, Gabriela Krist¹

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Institute of Conservation, University of Applied Arts Vienna, Austria

2 National Center for Cultural Heritage, Ulaanbaatar, Mongolia

3 Institute of Art & Technology, University of Applied Arts Vienna, Austria *corresponding author

Abstract

The monument of Turegs Kul’Chur at Ikh Khöshööt is among the numerous funerary monuments spread over the Mongolian countryside dating back to the Turkic period. It comprises steles with inscriptions, humanand animal-shaped sculptures and remnants of a sarcophagus all made of different varieties of local granitoids. Since 2016, Austrian-Mongolian conservation training workshops have been held there. The aim was to preserve the site and achieve a better understanding of physical and chemical properties of the used material, weathering and decay processes and the impact of the climatic conditions and environment to the site, in-depth research that is only at the beginning in Mongolia.

Two decay patterns common for granites, delamination and disintegration, pose a considerable threat to the original substance. It is supposed that thermal loads, temperature gradient, stone-intrinsic properties including stress release (Winkler 2013) and quarrying may have caused and accelerated the decay.

Visual inspections in situ were combined with petrographic studies of samples using instrumental analysis to determine composition, structure and texture of the stone and to describe decay patterns. The combined use and evaluation of three non-destructive field tests – two tests on water absorption and ultrasonic pulse velocity measurement – enabled to characterize the degree of weathering and decay of the different stone elements. Results were put in context with surface temperature measurements to characterize heating and cooling behavior and meteorological and climate data. Thus, the hypotheses on environmental and material intrinsic factors that accelerate and contribute to decay could be partly supported. Based on the condition survey underpinned with extensive analyses, a conservation treatment concept was developed and carried out.

The Complex

Over centuries the Mongolian nomads had influenced art, culture and religion in Central Asia significantly and left a remarkable legacy including numerous funerary monuments. They are spread over the area of today’s Mongolia, often remote and far away from settlements. Among them is the monument of Ikh Khöshööt, situated in the Delgerkhaan soum (sub-district) of Tov Province in 102Eastern Mongolia. It is acknowledged as tomb for a Turkic military leader from the beginning of the 8th century. The complex comprises 15 main stone sculptures arranged over an area of many square metres. They include two rectangular steles, whereby the larger shows Turkic runic inscriptions on three sides. The other animal- and human-shaped sculptures are present in pairs, as already observed by Kotwicz and Samoïovitch (1928: 69). While the animal-like ones probably depict lions and sheeps (Kotwicz and Samoïovitch 1928), the human-likes appear as simple torsos.

In addition fragments of a former sarcophagus are lying dispersed on the West side of the complex. The original arrangement and positioning of the single objects is no longer comprehensible as the objects have been repeatedly dislocated in the framework of expeditions and site visits. Nevertheless, a linear arrangement is assumed following an axis running along the former sarcophagus and the steles with the remaining sculptures arranged alongside this straight. Research on the complex site was pursued since the beginning of the 20th century, whereby emphasis was placed on documentation (visual and descriptive) and transcription of the inscriptions. Conservation scientific research on the site has started in 2016 in the framework of the collaboration between the University of Applied Arts Vienna and the National Center for Cultural Heritage (NCCH) in Mongolia. On site campaigns in 2016, 2018 and 2019 co-funded by the Eurasia-Pacific Uninet and the NCCH were dedicated to the documentation, the condition survey, non-invasive tests, the implementation of conservation measures and monitoring as well as to the training of Mongolian colleagues. In parallel, analyses of taken samples were carried out in Vienna.

Condition, Damage and Decay Patterns

The sculptures comprising the complex monument are made of three varieties of granitoids, which were identified on the basis of petrographical studies of samples using thin-section microscopy (transmitted and reflected light) and scanning electron microscopy (SEM-BSE): a fine-grained lithotype A1 (c. 1.5 mm grain size), a medium-grained lithotype A2 (c. 4 mm grain size) and a coarse-grained lithotype A3 (c. 5 mm grain size). Beside deviations in grain size, they also slightly differ in colour and accessory mineral content. Two of them (A1 and A3) show a more distinctive layering (stratification) and were both used for the two steles at the site. This choice is probably based on the stone intrinsic property and the resultant sheeting of the rock deposit, which facilitated the quarrying in the form of relatively thin layers. Another possibility is that the sheets were already available as separate boulders. The other variety (A2) is more bulky and was used for the human- and animal-shaped sculptures. All sculptures were probably processed and shaped by use of different chisels, whereby only some remnants of tool marks could be detected on the bottom side of one of a torso.

Figure 1: Monument of Ikh Khöshööt, 2019. © Institute of Conservation, University of Applied Arts Vienna.

Considering the age of the monument (almost 1,300 years), the harsh environment as well as the absence of previous large-scale conservation treatments and regular maintenance, the condition of the monument can be described as relatively good. The engraved inscription on the main stele, with a presumable original depth of some millimetres, is still visible in large parts, which indicates a comparably slow weathering and erosion of the upper surface layers.

Nevertheless, several damage and decay patterns are evident ranging from fractures to heterogeneous deposits comprising remnants of animal rubbings and bird droppings, which have partly converted to compact, thick, hardly removable crusts. Among 103them particularly two decay patterns common for granites, delamination and disintegration (Rodrigues 1980), attracted the attention and concern of the conservators and conservation scientists.

Characterizing the Decay – Non-Destructive Tests and Investigations

Already at the first inspection it was noted that the individual sculptures are affected to a varying degree by delamination and disintegration.

As a first step, level and extent of both decay patterns were thus assessed visually and sensorily as well as recorded in descriptive form and visualized in mappings. It revealed that the two steles are mainly affected by delamination while the human- and animal-shaped sculptures suffer from disintegration. This already suggests a link and dependence between the stone variety and the main decay pattern. Further, the survey showed that particularly the upper third of each of the steles is affected by delamination.

On the main stele the east face shows a more extensive scaling parallel to the surface than the opposite. The human- and animal-shaped sculptures show an even roughness on all sides. Surfaces are hardly sanding, which is likely due to a past consolidation treatment as communicated by NCCH.

Figure 2: Detail of the delamination in the upper third of the main stele, 2018. © Amarsanaa.

In a second step, a set of non-destructive field tests were carried out in order to further characterize and evaluate the degree of decay and weathering. Multiple ultrasonic pulse velocity measurements using the PUNDIT^® PL-200 (Proceq) with p-wave transducers 250 kHz (and 350-fold receiver gain) were done at all objects in different directions to detect cracks, voids and cavities within the materials more precisely. Further, inhomogeneity or disintegrated areas, which are an indication of enhanced degradation, were determined. Figure 3 illustrates the obtained measuring results at the main stele. In the upper two thirds a signal was received only very sporadically. The measurements perpendicular to the layering (i. e. from east to west) showed an average pulse velocity of 1,769 m/s in the lower third. Parallel to the layering an average pulse velocity of 2,050 m/s was measured in the upper third.

At the small stele signal were received only sporadically perpendicular to the layering (i. e. from north to south) with an average pulse velocity of 949 m/s. Parallel to the layering an average pulse velocity of 3,116 m/s was measured whereby signals were detected over the whole height. At four of the animal- and human shaped sculptures (no. 2, 6, 7, 11) no signal was received regardless of the direction. At the others a transmission of the pulse was partly possible. Altogether the average pulse velocities were lower than those at the two steles. The Karsten tube and the contact-sponge were used to assess the Water Absorption Behaviour (WAB) of the stone materials in order to further describe the porosity and degree of weathering (Svahn 2006: 21; Vandevoorde et al. 2009; Vandevoorde et al. 2011). Of particular interest were striking deviations in the WAB among the individual objects.

The contact-sponge method was applied on all objects. Results revealed that the average water absorption (WA) of the stone varieties with layering is 0.76 g/ m²s (A1, main stele) and 0.59 g/ m²s (A3, small stele) respectively. Only a roughened surface 104on top of the west face of the main stele showed a maximum WA of 5.38 g/ m²s. The third stone variety (A2, human- and animal-shaped sculptures) shows a slightly higher average WA of 1.12 g/ m²s, whereby four sculptures (no. 6, 9, 11 and 14) stand out due to a noticeably higher WA.

Figure 3: Mappings of the ultrasonic pulse velocity at the main stele (no. 1); measurements at the front perpendicular to the layering (left) and at the side parallel to the layering (right).

Due to time constraints only the two steles and five human- and animal-shaped sculptures (No. 2, 4, 7, 9, 11) could be exemplarily tested with the Karsten tube method. Results obtained from this field test widely correspond to the results of the contact-sponge tests: the two steles showed very low to no WA over a period of 45 minutes. Among the animal- and human-shaped sculptures no. 9 and 11 had the highest WA, equivalent to the results given by the contact-sponge tests. The others showed a constant but slower water uptake.

Interpretation of Results

In general the measured ultrasonic velocities were rather slow compared to the other values of granitoids given in literature (Hoffmann 2006: 80; Vasconcelos et al. 2008: 27), which indicate an advanced degree of deterioration (Svahn 2006: 21–23). This assumption was supported by the WA measured with the contact-sponge and Karsten tube method. Although it was not very high (compare Vandevoorde et al. 2009), it proves a moderate weathering of the respective material.

The lack of signals in the upper third of the main stele (in right angle to the layering) confirmed the visually determined heavy decay. It showed that delamination is advanced and cracks and cavities run through the entire material. Also the smaller stele seems to suffer from advanced delamination as the pulse was only partly transmitted from bottom to top (in right angle to the layering). As the steles absorbed little to no water it can be assumed that they are hardly affected by disintegration or fissures.

Figure 4: Water absorption measured with Karsten tube (4 ml, contact time up to 60 min) at the two steles (no. 1 and 10) and the sculptures (no. 2, 4, 7, 9, 11).

The failed or partial transmission of the pulse at all of the animal- and human shaped sculptures 105indicated that the material suffered from advanced disintegration. This assumption was confirmed by the water absorption behaviour of the stones. Interestingly, this finding contradicted the macroscopic examination, which indicated a rather good condition (hardly cracks and no sanding). Particularly the sculptures no. 9 and 11 with the highest water absorption (contact-sponge and Karsten tube) showed a rather slow or no signal during ultrasonic measurements. In general, sculptures with a comparatively higher water uptake also had a slower or no transmission of the pulse.

The combined use of the three non-destructive fields test and the comparative analyses of results helped to gain a better understanding on the degree of decay among the individual sculptures. Results mutually support and confirm each other. Thus a clearer picture of the state of condition could be gained.

Reflections on Decay Patterns

The reflection on probable material intrinsic and environmental factors that might cause, contribute or accelerate the two predominant decay patterns was important in order to gain a better understanding on their damaging potential and the rates of degradation.

It was assumed that on one hand, stone-intrinsic properties and style of quarrying have caused and accelerated these kinds of decay. The distinctive stratification of the stone varieties A1 and A3 combined with the unsuitable positioning of the steles – the layers are orientated at a right angle to the ground – might have favoured delamination. The vertically aligned layers can be a wear point and initiate cracking and fissuring.

It is also likely that the granite quarried was not totally sound. The hardness of the material makes it very difficult to quarry, especially when structural defects are lacking. So it is possible that pre-weathered stone slabs or boulders from the surrounding were used in first place for the erection of the complex monument. This was also underpinned by the inspection of the surrounding area which indicated no traces of possibilities for mining or quarrying. The pre-weathering in turn can adversely affect the subsequent degradation rate of the stone (Rodrigues 1980).

On the other hand, the hypothesis is proposed that thermal loads and temperature gradient play a significant part in the decay process.

The evaluation of meteorological and climate data from a weather station approx. 150 km south of the respective site (Saikhan-Ovoo, Dundgovi) showed that in summer maximum temperatures reach around 35 °C, while in winter temperatures can drop to minus 25 °C. Throughout the year the temperature difference between day and night is approximately 15 °C. The area counts almost 250 sunny days a year, often with clear cloudless skies. Combined with the lack of trees and therefore shadow in the steppe, this direct sunlight leads to a quick heating up of the stone. This in turn is assumed to result in a considerable temperature gradient within the stone and thermal stress due to a differential expansion of the rock minerals. Both factors might accelerate physical weathering and delamination (Rodrigues 1980).

In order to examine this assumption, the surface temperature on the top and the bottom of all faces of the main stele was measured in 2019 hourly with an infrared thermometer (Bosch Thermodetektor PTD 1) from sunrise to sunset. It is notable that particularly after sunrise (between 6:30 and 7:30) the surface of the east face heated up verifiably faster than the shaded west face. While 32 °C were measured on the top of the east face, the top of the west face had only 11.5 °C. Further, a perceptible difference of up to 4.1 °C could be measured from 8:30 to 14:30 between the top and the bottom of the East and South face. Over the measuring period the surface temperature varied from the minimum of 7.7 °C to the maximum of 44.6 °C.

Although the measurements only reflect heating and cooling during an average summer day, it is obvious that there have been thermal loads and a measurable temperature gradient between the front and back face of the stele since its erection 1,300 years ago. Particularly the gradient on the top might play a role in the delamination which is particularly distinctive in this area.

106Freeze-thaw-cylces, which considerably contribute to stone deterioration at our (European) latitudes, could be widely excluded as cause of damage in Mongolia. From September to November and middle of February to middle of April successive temperature transitions over the freeze point have to be expected in the Mongolian steppe. As there is hardly precipitation in these periods (less than 5 mm), damage caused by freeze-thaw-cycles can be neglected (Ruedrich, Kirchner and Siegesmund 2011).

Summary

The project provided the unique opportunity for the Austrian and Mongolian conservators and conservation scientists to study a stone monument from the 8th century, which has been continuously exposed to weathering.

Condition surveys together with extensive analyses of taken samples and three non-destructive field tests enabled to characterize the extent and type of decay. The findings showed a clear correlation between material instrinsic properties and prevalent decay patterns: while the stone varieties with layering mainly suffered from delamination, those without layering were rather affected by disintegration. They further revealed that particularly the human- and animal-shaped sculptures are, different to what was expected, heavily decayed and suffer from advanced disintegration.

Surface temperature measurements and the evaluation of meterological and climate data suggested that thermal loads and temperature gradients rather than freeze-thaw-cyles enhance decay and weathering of the site.

Based on the findings, the conservation treatment concept was developed and implemented. While the emphasis at the animal- and human-shaped sculptures was on the injection of fissures and partial consolidation, backfilling and micro pointing were carried out at the two steles.

Monitoring, survey, assessment and conservation of the heritage site have been carried out in a collaborative model-like approach to be utilized as a best practice example for future preservation efforts. In addition, the field tests, which were carried out for the first time at the respective site, provided important measurement data on pulse velocities and water absorptions of local stone varities to be used for comparative studies at similar sites in Mongolia.

References

Hoffmann A. 2006. Naturwerksteine Thailands: Lagerstättenerkundung und Bewertung. Dissertation Georg-August-Universität, Göttingen.

Kotwicz W., Samoïovitch A. 1928. Le monument turc d’Ikhe-khuchotu en Mongolie centrale. Rocznik Orientalistyzny, vol. 4, pp 60–107.

Rodrigues J. D. 1980. Decay of granite, Advanced study course, Science and technology of the environment for sustainable protection of cultural heritage. Relim.

Ruedrich J., Kirchner D., Siegesmund S. 2011. Physical weathering of building stones induced by freeze-thaw action: a laboratory long-term study. Environmental Earth Sciences, Volume 63, Issue 7-7, pp 1573–1586.

Svahn H. 2006. Non-Destructive Field Tests in Stone Conservation, Field and Laboratory Tests. Rapport från Riksantikvarieämbetet, Stockholm.

Vandevoorde D., Pamplona M., Schalma O., Vanhellemont Y., Cnudded V., Verhaeven E. 2009. Contact sponge method: Performance of a promising tool for measuring the initial water absorption. Journal of Cultural Heritage, Volume 10, Issue 1, pp 41–47.

Vandevoorde D., Cnudde V., Dewanckele J., Boone M. N., Verhaeven E. 2011. Contact sponge method: Performance compared with Capillary Rise, Karsten Tube and Mirowski Pipe. European Workshop on Cultural Heritage Preservation, EWCHP.

Vasconcelos G., Lourenco P. B., Alves C. A. S., Pamplona J. 2008. Ultrasonic evaluation of the physical and mechanical properties of granites. Ultrasonics 48(5):453–66.

Winkler E. M. 2013. Stone in architecture: properties, durability. Springer, Berlin-Heidelberg.

107

CHARACTERIZATION AND DETERIORATION ASSESSMENT OF KÖSSEINE GRANITE IN PALACIO SALVO, MONTEVIDEO, URUGUAY

Manuela Morales Demarco, Eloísa Cocorel

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

Instituto de Ciencias Geológicas, Facultad de Ciencias, Universidad de la República, Uruguay

Abstract

In Uruguay, one of the most important buildings is Palacio Salvo. The present research is part of a diagnosis for the definition of an interdisciplinary project of intervention for the revalorization and conservation of its façades.

These activities started with the identification of the rock used as Kösseine Granite, of Wunsiedel, Germany. This rock is applied as polished pieces and most of them show, as deterioration features, rounding, missing parts, perforations with or without metallic inserts, often surrounded by oxide staining, deposits and soiling. Some plates eventually show up to 20 cm cracks in the edges related to splintering, chipping and two large plates are crossed by 80 cm fractures. Pieces used as steps show abrasion, rounding, and fractures in the edges causing missing parts.

Exterior and paving are among the most demanding applications of dimensional stones. Nevertheless, Kösseine Granite presents a good performance and its state of deterioration is low.

Keywords: Historical and cultural heritage, Kösseine Granite, Uruguay

Introduction

One of the most iconic buildings in Montevideo is Palacio Salvo, in the heart of the city: 18 de Julio Avenue and Plaza Independencia. Built between 1922 and 1928 by Mario Palanti, an Italian architect, it was commissioned by Salvo brothers.

In the declaration of Palacio Salvo as National Historical Monument, in 1996, it is clearly stated the relevance of its preservation and recovery (Presidential Resolution N° 1144/996, Uruguay). In this sense, an agreement was signed between the company owner of the building, Palacio Salvo S. A., and FADU (Facultad de Arquitectura, Diseño y Urbanismo – Universidad de la República) for a study for the conservation of its façades (IC-FADU, 2019). The ICG team (Instituto de Ciencias Geológicas, Facultad de Ciencias) was assigned a 3-month period the deterioration assessment of the granitic external façade of the ground floor.

Identifying the granitic façade

For the general registration of the rock applied and its deterioration assessment, the recommendations of the working group “Natural Stones and Weathering” (2019) of Aachen University were followed. It 108was not possible to map the entire external façade due to time constrains of the study, therefore only four sections of the building were mapped.

There was some controversy about the origin of the rock applied. In the Uruguayan small geological community, it was believed it was a facies of Pan de Azúcar Pluton. This pluton, studied in detail by Oyhantçabal (2005), is a km-size syenitic, quartz-syenitic and granitic circular intrusion, historically exploited with ornamental purpose (Morales Demarco, 2012). All efforts to localize the variety supposdly applied in Palacio Salvo in the area of the pluton were unsuccessful.

At the beginning of this study, 15 blueprints of the entire granitic façade were found by FADU in the General Archive of the Nation (Archivo General de la Nación). These blueprints have the localization and dimensions of around 1,000 pieces of the granitic façade of Palacio Salvo (the external façade and the façade of the passage, not studied in the present work), and some inscriptions written in German: Kösseine Granit Poliert, Grasyma Wunsiedel, Januar 1925.

The ICG team compare the dimensions of the pieces applied in Palacio Salvo, with those stated in the blueprints. The match is perfectly accurate. Simultaneously, a communication with the German company allowed the confirmation as the providers of the rock and the undoubtedly identification as Kösseine Granite (Figure 1).

Characterization of Kösseine Granite
Petrography

Due to the impossibility of sampling the rock in the buidling, as it is a National Historical Monument, a preliminary petrographic characterization was performed in situ. It is a very coarsed-grained leucocratic rock, yellowish white, to beige and greyish light brown. This is not the typical color of Kösseine Granite, but a yellowish variety that is also widely used in Germany.

The rock shows a porphyritic texture, due to the presence of alkali feldspar up to 3.5 cm, most around 2 cm, subhedral, frequently rounded. Some show zoning with a light brown core, sometimes euhedral, surrounded by a greyish white mantle. Most are grey. Inclusions of biotite (Strohmeyer, 2003) are very frequent, often evidencing growth rims. Occasionally, a translucent mineral can be observed, that gives the rock a pearl luster, this is probably plagioclase.

Figure 1: Right: Brochure of Palacio Salvo in construction with promotional poster of Grasyma (courtesy of Claus Wölfel).

Quartz is interstitial between feldspar and biotite, with light brown to beige color. Biotite shows sometimes hexagonal sections with dark reddish to coppery color. Muscovite is silver grey sometimes with greenish hues. Both minerals frequently form nests, and the biotitic sometimes show orange oxidation rims.

Garnet is present as an accessory mineral in mm euhedral to subhedral red crystals with vitreous luster. They occur occassionally as inclusions in alkali felspars, sometimes in matrix minerals.

Table 1: Mineralogical composition of Kösseine Granite.

Mineral (%)	Grimm (2018)	Strohmeyer (2003)
Alkali feldspar	41	32
Quartz	36	26
Plagioclase	13	30
Biotite	6	10
Muscovite y chlorite	2	1
Accessory	2	< 1%

The rock structure is characterized by the presences of elongated mafic enclaves, up to 20 cm with aspect ratio up to 1 : 20, some banded (mafic and felsic bands). Small rounded mafic enclaves are also present (2–3 cm). Felsic enclaves up to 40 cm 109ocurr either as fine to very fine-grained or as very coarsed-grained. All these structural elements disrupt the rocks décor. The rock is classified according to its mineral composition (Table 1) as a granite (Strohmeyer, 2003; Grimm, 2018).

Table 2: Physical properties of Kösseine Granite.

Physical properties (units)		Grimm (2018)		Strohmeyer (2003)
Bulk density (g/cm3)		2.67		2.67
Matrix density (g/cm3)		2.69		2.68
Porosity (% vol)		0.71		0.11
Average pore radii (µm)				0.02
Volume of capillary pores (% vol)				0.02
Water absorption (wt%)
Water absorption (atm) (wt%)		0.21
Water absorption (vac) (wt%)		0.27
Saturation coefficient S		0.79
Water absorption coefficient [kg/m2*√t]				0.003
Permeability (mD)		0.002
Specific surface (m2/g)		0.22
Thermal expansion coefficient (10–6K–1)				9.22

Table 3: Mechanical properties of Kösseine Granite.

Mechanical properties (units)		Strohmeyer (2003)
		x-direction	y-direction	z-direction	Average
UCS (MPa)		194 ± 11	194 ± 10	199 ± 8	195 ± 17
Indirect Tensile Strength (MPa)		12.2 ± 1.2	10.9 ± 1.3	12.5 ± 1.5	11.9 ± 2.3
Flexural Strength (MPa)		18.0 ± 1.1	19.7 ± 1.1	22.6 ± 0.8	20.1 ± 1.7
Abrasion Strength (cm3/50 cm2)		5.8 ± 0.2	5.7 ± 0.1	5.6 ± 0.1	5.7 ± 0.2

Physical and mechanical properties

No physical or mechanical tests were performed, nevertheless, results of Strohmeyer (2003) and Grimm (2018) were considered (Tables 2 and 3).

Kösseine Granite shows a very good behavior against atmospheric agents, directly related to its physical properties: very low porosity, low water absorption, low water uptake coefficient and permeability. These properties show that the rock is practically impermeable, and consequently extremely stable to weathering conditions.

Mechanical properties for Kösseine Granite are typical of granitic rocks, even slightly higher. Comparing its UCS (uniaxial compressive strength) values to those of the statistical study of Mosch (2008), it can be observed that they are above the median. Flexural strength is also above the median of Mosch (2008), but the indirect tension is slightly below this parameter.

Abrasion resistance values are slightly higher than those reported by Morales Demarco (2012) for Uruguayan granitic rocks, but in the range of values given by Siegesmund & Snethlage (2011).

Figure 2: Ground floor plan (modified from FADU, 2019).

Figure 3: a. Perforation with metallic insert, oxide staining and star crack; b. Missing gap presumably a product of bursting or removal of metallic insert.

110Deterioration assessment

For this activity, the building was divided in four main areas, the façades of Andes Street, 18 de Julio Avenue, Plaza Independencia and the Gallery (Figure 2). A total of 675 pieces were analysed. The basis used for this diagnosis is the classification proposed by ICOMOS (2011).

Table 4: Deterioration features present in Pilasters and Columns of Palacio Salvo (in brackets number of pieces with feature repaired).

Deterioration	Andes	18 de Julio	Gallery	Plaza Independencia
Crack	10	6	8	14
Star crack	2	1	0	3
Blistering	0	0	0	1
Bursting	1	0	0	0
Splintering	23	11	0	0
Chipping	8	8	3	8
Spalling	0	0	0	2
Differential erosion	0	0	0	3
Rounding	98	20	59	64
Impact damage	0	0	0	1
Cut	5	0	0	1
Scratch	0	0	1	1
Missing part	22 (18)	30 (19)	21 (4)	52 (4)
Gap	1 (12)	0	0	0
Perforation	3 (35)	4 (18)	7 (20)	2 (20)
Perforation with plastic	7	1	0	2
Pitting	0	0	0	4
Metallic insert	55 (2)	10 (4)	42 (14)	44 (12)
Strange Element	3	12	27	31
Dust Deposit	4	0	0	0
Paint	97	43	130	94
Stick	42	15	73	92
Cement	8	2	18	14
Grease	22	21	7	0
Pigeon Dropping	5	6	8	72
Bleaching	0	0	0	2
Oxide staining	18	0	32	49
Film	10	0	0	0
Graffiti	11	1	4	0
Patina	14	9	19	28
Soiling	6	21	74	103
Spider net	1	2	2	4
Pieces analysed	153	67	171	218

The five deterioration families of this framework are present in Palacio Salvo (Tables 4 and 5). The Family Crack and Deformation is represented mainly by cracks and star cracks (Figure 3). Cracks occur very seldom in pilasters and columns, and only one is present in a step (Tables 4 and 5).

Splintering is the most frequent deterioration feature of the Family Detachment observed in Andes Street and 18 de Julio Avenue, followed by Chipping, which affects pilasters and columns of the four areas studied (Figure 4). Due to the location and characteristics of the latter features, they are a product of differential movement of the structure or the masonry, and not originated from low resistance of the rock. They are absent in steps.

Fragmentation is only observed in two panels of the Gallery. Blistering and spalling are related with features of other families and will be discussed at the end of the section.

Table 5: Deterioration features present in steps of Palacio Salvo (in brackets number of pieces with feature repaired).

111

Figure 4: a. Splintering, chipping, paint film, perforation and perforation with metallic insert in pilaster of Andes Street, b. Splintering with development of two parallel fractures.

Figure 5: Soiling in inner faces of Plaza Independencia columns: a. Partially “washed out”, b. Rock without deterioration beneath partially eroded soiling.

Features induced by material loss is the second most represented family, being rounding, perforation and missing part the ones which affect the higher number of pilasters and columns. Many perforations present metallic inserts, which causes very frequently oxide staining leading eventually to bursting (Figure 3). In the steps, rounding affects almost all pieces, abrasion and missing part a quarter of them.

The most represented family is Discolouration and deposit. Six subtypes of deposits are very frequently observed (dust deposit, paint, stick, cement, grease and pigeon droppings) in pilasters, columns and steps. Patina is frequent in small areas of the pieces. Soiling is very frequent in inner areas of pilasters and columns of the Gallery and Plaza Independencia, sheltered from rain and wind (Figures 5 and 6).

A deterioration association is observed in two felsic enclaves up to 40 cm in two faces of a highly exposed column of Plaza Independencia. The association includes bleaching, pitting, blistering, spalling and differential erosion, and is more advanced in the enclave which is frequently exposed to dog urine. This association is presumably related to the action of SO₂ product of air pollution, that causes feldspar kaolinsation and possible salt formation, as described by Schiavon (2007). Family Biological colonization is only represented by very seldom spider nets.

Other alterations are modification of the original basement ventilation elements, in Andes Street, 18 de Julio Avenue and Gallery. Most of them were transformed to access steps to business, which originally had no exterior access. These steps are made up of other rocks (Pan de Azúcar Syenite due to the similarity in décor) or monolithic (Table 5). Other category is “Strange elements”, including commercial posters, door numbers, etc.

A map of a column of Plaza Independencia is presented in figure 6. View I faces Plaza Independencia and is exposed, and View III faces the Gallery and is sheltered from rain and wind.

Figure 6: Deterioration map of Column 6 of Plaza Independencia (modified from FADU, 2019).

112Discussion and recommendations

There were no conservation programs developed in Palacio Salvo. Only minor reparations can be observed. In this study it is strongly recommended the design and implementation of a conservation program. Alternative nonaggressive measures are proposed if this program can not be developed.

Cracks related to splintering and chipping should be studied in detail to determine their cause. In general, cracks, fractures (with or without fragmentation), can be consolidated and mechanically repaired after cleaning and removal of loose fragments (Matero & Tagle, 1995). As consolidant Siegesmund & Snethlage (2011) recommend TEOS. For the repair of cuts, perforation, gaps and missing parts it is recommended to prepare patches of different colors, textures and materials, that can replicate the décor of Kösseine Granite, including the different enclaves observed. The metallic inserts should be removed by gentle mechanical means or by using a drill of small diameter to prevent damaging the rock around it.

Deposits weakly attached and spider nets should be removed by mechanical means (soft brush, plastic spatula). Soiling and deposits strongly attached could be removed by steam jet, as described in Siegesmund & Snethlage (2011). Oxide staining could be mobilized by chemical complexing or sequestering agents that can be soaked up or flushed away (Matero & Tagle, 1995). Following these authors, poultices with different chemical compounds could be used to remove oxide staining, grafitti, paint films, and eventually deposits of cement, and others that could not be removed by the previous methods.

Conclusions

The present study is the first documented case of a deterioration assessment of a dimensional stone façade in Uruguay performed by a geological team. The rock applied is identified as Kösseine Granite, of Wunsiedel, Germany.

No conservation program was developed prior to this study in Palacio Salvo, although minor reparations have been recognized. Using ICOMOS (2011) deterioration glossary, five deterioration families were recognized, being Features induced by material loss and Discoloration and deposit the most represented. Of the first family rounding, perforation (with and without metallic insert), and missing part are the most frequent, both in pilasters, columns and steps, being abrasion very frequent in the latter. They are all related with anthropogenic activities and not a consequence of poor rock quality. Of the second family, the most frequent deteriorations are deposits and soiling.

Other relevant alterations are replacement of original pieces in steps, mostly by monolithic and Pan de Azúcar Syenite, that matches the décor of Kösseine Granite. The good performance of Kösseine Granite in Palacio Salvo after 92 years of exposure is related to its excellent physical and mechanical properties.

References

IC-FADU (Instituto de Construcción, Facultad de Arquitectura, Diseño y Urbanismo) (2019) Propuesta de intervención para la conservación de las fachadas del Palacio Salvo.

Grimm WD (2018) Bildatlas wichtiger Denkmalgesteine der Bundesrepublik Deutschland. Ebner Verlag GmbH & Co.

Matero FG, Tagle AA (1995) Cleaning, Iron Stain Removal, and Surface Repair of Architectural Marble and Crystalline Limestone: The Metropolitan Club. J. Am. Inst. Conserv. Vol.34 I.1.

Morales Demarco M (2012) Mineralogical, Petrophysical and Economical Characterization of the Dimensional Stones of Uruguay; Implications for Deposit Exploration. Diss Univ Göttingen.

Mosch S (2008) Optimierung der Exploration, Gewinnung und Materialcharakterisierung von Naturwerksteinen. Diss Univ Göttingen.

Oyhantçabal P (2005) The Sierra Ballena Shear zone: kinematics, timing and its significance for the geotectonic evolution of southeast Uruguay. Diss Univ Göttingen.

113

2000 YEARS OF USE OF PLASTER AS BUILDING AND RESTORATION MATERIAL FOR THE „TAKHT-E SOLEYMAN“ IN WESTERN AZERBAIJAN, IRAN

Angela Eckart¹, Robert Sobott², Holger Kletti³, Toralf Burkert¹, Wolfram Jäger⁴

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Jäger Ingenieure GmbH, Paul-Schneider-Straße 17, 99423 Weimar, Germany

2 Labor für Baudenkmalpflege Naumburg, Rudelsburgpromenade 20C, 06628 Naumburg, Germany

3 Bauhaus Universität Weimar, Fakultät Bauingenieurwesen, F. A.Finger-Institut für Baustoffkunde, Professur Werkstoffe des Bauens, Coudraystr. 11B, 99423 Weimar, Germany

4 Jäger Ingenieure GmbH, Wichernstraße 12, 01445 Radebeul, Germany

Abstract

The preservation of the UNESCO World Heritage Site Takht-e Soleyman is supported by German experts who are working on the 114reinforcement of the badly cracked north wall of the west iwan (large audience hall). The building material gypsum which was used in the past is now also used for up-to-date repair work such as grouting and filling cracks and voids in the historic masonry. Locally quarried gypsum was used to produce a high-fired gypsum mortar in the traditional way. On the basis of laboratory experiments the activation of the hydration of anhydrite and the flow behavior of the mortar suspension was optimized.

Kewords: Ilkhanid palace, calcium sulfate phases, gypsum calcination, grouting, static safeguarding of masonry

Introduction

The UNESCO World Heritage Site Takht-e Soleymann (“Throne of Solomon”) is situated in the West Azerbaijan Province of Iran. The site lies 2200 m above the sea level and has a dry continental climate. In the 6th century AD under Sasanian rule a Zorastrian fire temple and palace complex was built at an artesian lake over the ruins of Achaemenid and Parthian buildings. After the end of the Sasanian dynasty in the 7th century AD the complex fell derelict. In 1221 the Mongols started the invasion of the Islamic world and about 1275 the site was used once again for the construction of an Ilkhanid summer palace. The greatest building within the palace complex was the west iwan, a vaulted audience hall, 11,5 m wide and 27 m long and closed on three sides (Naumann 1977, Huff 2006) which was lavishly decorated inside. With the decline of the Ilkhanid empire the buildings were abandoned and fell into ruin again. Beginning in 1956, the area was investigated by archaeologists and a restoration program was initiated with German support. A huge scaffold was erected on the east side of the west iwan in order to stabilize the ruinous north wall and stop the progressive decay. What was meant as a temporary measure is still in place and actually became an iconic landmark (Figure 1).

Since 2016 the preservation of the remains of the west iwan is part of a restoration project funded by the Cultural Preservation Program of the Federal Foreign Office. Experts are working on the reinforcement of the badly cracked north wall by strengthening the historic masonry with injection anchors and filling voids and cracks in the wall structure by mortar injections (Fucke, Hansen 2012, Bräunel 2016, Burkert et al. 2019). Gypsum mortar was used for the repair work because plaster is present in Sasanian as well as in Ilkhanid wall structures and its reaction with hydraulic cement mortar may cause the problem of sulphate attack. The project is aiming to strengthen of the wall so that it can withstand earthquakes in a seismically active region and the dismantling of the scaffold which is then no longer required.

Sasanian and Ilkhanid plaster

The historic multi-leaf masonry has a filling of stone rubble and mortar. The external leaf was built of travertine worked stones, rubblestones and bricks. Plaster was the binder for the mortar of all building periods and masonry types. Chemical and phase analyses of Sasanian and Ilkhanid plaster samples yielded fairly pure gypsum mortars. Maximum silicon dioxide impurities, mainly as quartz, did not exceed 8.3 mass%.

The thin section of a Sasanian gypsum mortar revealed a secondray very porous structure (Figure 2). The firing products are more or less dissolved, creating pore space which is surrounded by a gypsum matrix. Brown inclusions in the leached remnants of the fired gypsum must be regarded as impurities in the raw material.

Figure 1: North wall of the west iwan in the centre, Takht-e Soleyman.

Figure 2: Sasanian mortar with very porous structure and fragment of gypsum rock (plane polarized light).

Figure 3: Ilkhanid render with porous structure. The grains in the upper part of photograph exhibit distinct reaction rims (arrows); plain polarized light.

Embedded in the gypsum matrix are low quantities of gypsum rock fragments and brownish aggregate material with a maximum grain size of about 6 mm. Both may have entered the mortar as contaminants in the furnace or on the construction site. However, an intentional admixture as aggregates cannot be ruled out. Unreacted anhydrite was not detected.As the polarized light microscopy of thin sections showed, the Ilkhanid plaster is quite different from a Sasanian mortar. It contains still a lot of unreacted firing products with a maximum grain size up to 8 mm. Occasionally distinct reaction rims around the grains are visible (Figure 3). Both in the firing products and in the mortar matrix fine clayey (ceramic) particles occur. They most probably derive from impure raw material and not from the intentional admixture of crushed bricks as pozzolanic material. Like the Sasanian plaster the 115Ilkhanid plaster contains gypsum rock fragments with a grain size up to 8 mm. It cannot be decided whether they were added as aggregate or derive from insufficiently fired raw material. Overall, the matrix fabric of the Ilkhanid plaster appears to be denser than that of the Sasanian plaster due to the abundance of unreacted components.

Figure 4: Furnace for plaster production. Cross section (left) and view into the shaft with installation of temperature measuring device (thermocouple 1).

Traditional production of gypsum mortar

For the restoration work locally available material is used. The raw material for the plaster production comes from a quarry some 17 km WNW off the Takht-e Soleyman. Two slightly different qualities of gypsum with respect to the contents of non sulfate accessory minerals are mined. The gypsum rock contains dolomite (ankerite), calcite, quartz, feldspar, clay minerals and celestite as accessory minerals which can sum up to 8–10 mass%.

The firing of the raw material is done in the traditional way as it was done many centuries ago (Soleymani, Pirak 2012, Sobott 2018). A shaft furnace built of bricks with a diametre of 1.90 m was sunk 2.90 m deep in the ground. The quarried gypsum lumps are piled up in the furnace in such a way that something like a corbeled vault is formed. Large pieces of gypsum rock are on the inside of the construction facing the firing chamber and small pieces are used to fill the space between the furnace wall and the rock pile. The apex of this artful cone-shaped construction surmounts the upper end of the furnace (Figure 4). A mix of combustible material, mostly wood, is piled up inside the vault.

Once the fire is lit the uncontrolled firing process lasts about 8 hours. Due to the construction of the furnace the temperature distribution in the gypsum filling is extremely variable. Thermocouples installed at different positions in the gypsum pile showed that the temperature difference between the central part and the margin may be as great as 800 °C so that the gypsum lumps are exposed to temperatures ranging from 200 and 1,000 °C (Jäger 2017, Jafarpanah 2017). The cooling period after the extinction of the fire lasts about 24 hours. Then the fired material is removed from the furnace and crushed with large hammers by hand at which high fired gypsum lumps disintegrate easily into powder while low fired lumps break into smaller pieces. The crushed material is sieved and filled into plastic bags. For use at the construction site the material was sieved to discard grains larger than 2 mm. The phase composition of the fired product is variable and depends mainly on the grain size and composition of the raw material, the firing temperature and time, and the resulting partial water vapour pressure on the surface of the lumps. For the assessment of the firing results, 116gypsum pieces adjacent to thermocouples were sampled and studied in situ by colouring tests and in the laboratory by polarized light microscopy of thin sections, X-ray diffraction and thermoanalysis (DTA, DTG). In order to improve the data interpretation of the field samples, experiments with raw material from the gypsum quarry were carried out under controlled temperature and time conditions in the laboratory and the resulting samples analyzed in the same manner.

Phase composition of the calcined raw material

Gypsum lumps from the furnace exhibited a temperature dependent zonation with respect to the occurrence of phases and phase morphologies (Lenz, Sobott 2008) . Low fired gypsum lumps typically consist of three zones. A thin surface layer of rehydrated anhydrite III (dehydrated hemihydrate), is followed by a layer of fibrous hemihydrate, and the central part is made up of a mixture of hemihydrate and dihydrate (unconverted gypsum). Thermoanalytic measurements prove anhydrite III to be stable up to 370 °C. It forms fibrous crystals, partly pseudomorph after gypsum. With increasing firing temperature hemihydrate and anhydrite III are converted to anhydrite II (natural anhydrite). Anhydrite II formed in the temperature range between 370 and 800 °C shows elongated fibrous crystals and at temperatures above 800 °C the crystals tend to be short prismatic (Figure 5).

Figure 5: left: short prismatic anhydrite II crystals in samples fired at 1,100 °C for 5 hours; right: prismatic anhydrite crystals in samples fired at 800 °C for 5 hours; crossed polarized light.

In a laboratory sample which was fired at 600 °C for 5 hours radial fibrous anhydrite crystals were observed. Obviously the shape of anhydrite II crystals and aggregates in fired samples depends largely on how the heat treatment was executed.

In thin sections of gypsum samples which were exposed to temperatures above 600 °C isotropic crystals with a hexagon outline were observed.

They were identified by EDXRF measurements as periclase MgO (Figure 6).

Müller et al. (2009) described the formation of periclase by a contact metamorphic reaction in dolomite marble at P = 1 kbar and T > 605 °C

CaMg(CO₃)₂ → MgO + CaCO₃ + CO₂ dolomite periclase calcite

Since idiomorphic to hypidiomorphic dolomite crystals up to 400µm in size were observed in thin sections of the raw material from the gypsum quarry near the Takht-e-Soleyman, it is reasonable to assume that the dolomite disintegrated and was converted to periclase according to the above mentioned chemical reaction. Therefore the appearance of periclase in samples can be regarded as an intrinsic temperature indicator if the pressure dependence of the reaction is known.

Figure 6: Periclase crystals in anhydrite II matrix; sample from furnace, adjacent to thermocouple that recorded a maximum temperature of 800 °C; plane polarized light.

117Reactivity of the new gypsum mortar

Variable amounts of dehydrated or partly dehydrated calcium sulfate phases in the dry mortar and different crystal habits of the phases are the reasons for a variable reactivity of the mixture. For example, the low degree of hydration of the sample fired for 5 hours at 600 °C is possibly due to the radial fibrous structure of anhydrite crystals. The degree of hydration after 3 days and 1 year was determined gravimetrically and by X-Ray diffraction (Rietveld method), respectively, for samples fired at different temperatures and for different times. The results are summarized in Table 1.

As stated above the fired material is a variable mixture of components with different hydration reaction rates (Glasenapp 1910). Unlike α- and β-hemihydrate, anhydrite III and low fired anhydrite II which hydrate very quickly, high fired anhydrite II reacts very slowly with water. This retarded reaction of anhydrite II can produce cracks in the plaster if the mortar fabric is very dense and cannot compensate the volume increase of the hydrated calcium sulfate phase. Therefore the hydration of high fired anhydrite II has to be activated by a suitable chemical such as citric acid or potassium sulfate. Experiments and analyses proved superfine calcium hydoxide Ca(OH)₂ to be a suitable activator to prevent a retarded hydration of high fired anhydrite II. The linear expansion coefficient of the high fired gypsum injection mortar measured after 280 days was reduced to less than 2 mm/m. The compressive strength of the set mortar measured after 28 days was 6.1 MPa and decreased to 2.9 MPa after storage of the samples under water. No cracking was observed in test walls which were built in 2016 and injected with this mortar. Obviously its properties are favourable for the repair and reinforcement of the historic masonry and it is now successfully applied since two years.

Table 1: Degree of rehydration of fired gypsum as a function of firing temperature and time.

Run No.	Firing temperature	Firing time	Reaction time	Degree of hydration
1	200 °C	–	3 days	85 %
2	200 °C	–	365 days	~100 %
3	600 °C	5 hours	3 days	4 %
4	600 °C	24 hours	3 days	15 %
5	600 °C	–	365 days	94 %
6	800 °C	5 hours	3 days	13 %
7	≥ 800 °C	> 5 hours	3 days	1 %
8	1,100 °C	5 hours	365 days	72 %

Static safeguarding of the west iwan

In a first step of the strengthening of the west iwan the masonry was grouted with a gypsum suspension developed by University of Technology Dresden (TUD) and Jäger Consulting Engineers Ltd. The worm pump SP-20 from Desoi was absolutely necessary for the grouting of large caverns, cavities and crack systems in the north wall of the west iwan. The water-to-gypsum ratio of the aggregate-free suspension was 0.63. The chemical Retardan 200 P by SIKA Company was added as a retarder. Tests with ViscoCrete by SIKA Company showed that the addition of a plasticizer to the suspension was not necessary to guarantee sufficient flowability. The gypsum suspension could easily be mixed and processed on-site. Observation of the injection process confirmed the very good flow behaviour of the suspension which ensured the closing of the cracks and cavities in the masonry. The grout remained flowable long enough and the quantity of the water taken from the surroundings were within limits. The masonry showed heavy vertical cracks and numerous holes and cavities in the interior, which had to be grouted from the bottom to the top of the wall with the developed gypsum suspension. First holes were drilled at a spacing of 30 to 50 cm along the course of the crack to grout the masonry. The holes had a diameter of 24 mm and depths of 30 to 80 cm. After completion of drilling, the entire masonry including the cracks was prepared for the subsequent rising grouting. For this purpose the cracks were cleaned of dust and loose objects and the parts of the masonry facade near the masonry facade were secured with loam mortar. This prevented contamination of the masonry during the grouting work and the preceding filling of the cracks. Then grouting was started from bottom to top (Figure 7).

118

Figure 7: Gypsum mortar injection into the cracked north wall of the west iwan. Right side: Bottom to top injection in sealed crack.

It was frequently the case that the masonry could be grouted up to 1 m of height from one hole. This demonstrated an effective internal transport of material, with continuous grouting of the entire masonry being guaranteed. At each level, grout escaped at the various holes drilled round the buttress. In total, 7 tons of gypsum suspension were grouted to the east part of the north wall of the west iwan.

References

Bräunel, M. (2016). Takht-e Soleyman – vertiefende Bestandsaufnahme der Ruinenteile des westlichen Iwans in Vorbereitung der notwendigen statisch-konstruktiven Sicherung. Diplomarbeit an der Technischen Universität Dresden, Lehrstuhl für Tragwerksplanung, Fakultät Architektur, 2016, unpublished.

Burkert, T., Fuchs, C., Sobott, R. (2019). Statisch-konstruktive Sicherungsarbeiten am westlichen Iwan der UNESCO-Welterbestätte Takht-e Soleyman, Iran. in Mauerwerk Kalender 2019, Ernst und Sohn, Berlin, 295–331.

Glasenapp, M. (1910). Plaster, Overburnt Gypsum and Hydraulic Gypsum. – Cement & Engineering News, Chicago, Illinois, 47 pp.

Fucke, D., Hansen, M. (2012). Takht-e Soleyman – vorbereitende Untersuchungen und Varianten zur Sicherung der Ruinenteile des westlichen Iwans. Diplomarbeit an der Technischen Universität Dresden, Lehrstuhl für Tragwerksplanung, Fakultät Architektur, 2012, unpublished.

Huff, D. (2006). The Ilkhanid Palace at Takht-i Sulayman. Excavation Results, in: Komaroff, Linda (Ed.): Beyond the Legacy of Genghis Khan. Brill, Leiden 2006, 94–110.

Jäger, W. (2017). Burning process of gypsum in the kiln in Tahkt-e Soleyman 20.05.2016. – Technische Universität Dresden, Faculty of Architecture, Chair of Structural Design, 12 pp.

Jafarpanah, M. (2017). Burning process of gypsum in the kiln in Tahkt-e Soleyman 13.08.2017. – Takht-e Soleyman – UNESCO World Heritage Site, 13 pp.

Lenz, R., Sobott, R. (2008). Beobachtungen zu Gefügen historischer Gipsmörtel. In: Gipsmörtel im historischen Mauerwerk und an den Fassaden. – Hrsg. von M. Auras und H.-W. Zier, WTA Schriftenreihe, Heft 30, 23–34.

Lucas, H. G. (1992). Gips als historischer Außenbaustoff in der Windsheimer Bucht. Dissertation, Fakultät für Bergbau, Hüttenwesen und Geowissenschaften der RWTH Aachen.

Müller, T., Baumgartner, L. P., Foster, C. T. Bowman, J. R. (2009). Crystal Size Distribution of Periclase in Contact Metamorphic Dolomite Marbles from Southern Adamello Massif, Italy. – Journal of Petrology, Vol. 50/3, 451–465.

Naumann, R. (1977). Die Ruinen von Tacht-e-Suleiman und Zendan-e-Suleiman und Umgebung.

Dietrich Reimer Verlag, Berlin, 126 pp.

Sobott, R. (2018). Historic and modern gypsum mortar application at the Takht-e Soleyman, Iran. Report about on-site studies and results of sample analyses. unpublished, 25 pp.

Soleymani, A., Pirak, M. (2012). Nachstellung von halbgebranntem und halbzerstoßenem Gips. (Transkribiert aus dem Iranischen) Quarterly Research Review of Razavi Architecture, Vol. 1, No. 1, 61–71.

119

EFFECTS OF ZEOLITES AND SWELLABLE CLAY MINERALS ON WATER-RELATED PROPERTIES AND THERMAL DILATATION IN VOLCANIC TUFF ROCKS

Alexandra Kück¹, Christopher Pötzl¹, Rubén López-Doncel², Reiner Dohrmann³, Siegfried Siegesmund¹

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Geoscience Centre of the Georg August University Göttingen, Germany

2 Geological Institute of the Autonomous University of San Luis Potosí, Mexico

3 Federal Institute for Geosciences and Natural Resources (BGR), Hanover, Germany

Abstract

The weathering of natural building stones in the South of Mexico is mostly controlled by the influence of atmospheric and meteoric water, thermal stress and the input of salts from the environment. Twelve varieties of volcanic tuff rocks from South and Central Mexico were analyzed regarding their petrographical and petrophysical properties, as well as their weathering behavior. The tuffs show a broad range of properties and differential weathering behavior. Moisture properties like water uptake, water vapor diffusion and hygroscopic water sorption as well as hydric expansion and salt crystallization show great dependence on the pore space properties and the content of swellable clay minerals or zeolites. The deterioration during salt bursting tests is controlled by both salt crystallization pressure and hydric expansion. Especially zeolite-rich samples show intense water-related weathering. Their sorption values and hydric expansion are high, while capillary water uptake is comparably low. The clay and zeolite-rich tuffs furthermore suffer from intense shrinking and fracturing during drying. The very high hydric expansion superimposes the salt crystallization pressure and the effect of thermal dilatation.

Introduction

Zeolites have an important influence on the weathering behaviour of natural building stones. Korkuna et al. (2006) found very small pore sizes for zeolite-rich samples (< 2 nm) resulting in a high specific surface area. The specific surface area, along with the pore size and connectivity of the pores is a controlling factor for water transport and retention in porous rocks and has a great influence on values like capillary water uptake, water vapor diffusion and hygroscopic water sorption (Siegesmund and Dürrast 2011).

Hydric dilatation can cause a significant volume change in tuff rocks and is therefore an important weathering factor. Reasons for hydric expansion may be the presence of swellable clay minerals or a large percentage of micropores (e. g. Gonzales and Scherer 2004). The swelling of natural building stones with a high percentage of micropores may be explained by the process of disjoining pressure, but the process is still under discussion (e.g. Ruedrich et al. 2011; Wedekind et al. 2013; Pötzl et al. 2018a; Pötzl et al. 2018b).

Zeolites adsorb and desorb water molecules reversibly (Di Tchernev 1978). They are therefore able to store heat during desorption and give it back to the environment during adsorption. Between 0 °C and 90 °C zeolites counter the expected thermal 120expansion because of shrinkage due to reversible water loss (Colella et al. 2001). Zeolites accordingly have an influence on the thermal expansion behavior of tuff rocks.

In this study zeolite and clay-rich tuff rocks from the states of Oaxaca in Southern Mexico and Queretaro in Central Mexico were investigated and compared with zeolite-free samples.

Petrography

The investigated samples are mainly felsic pyroclastic rocks. Cantera Verde Oaxaca (CV_O), Cantera Amarilla Oaxaca (CA_O) and Cantera Rosa Oaxaca (CR_O) are tuff rocks from the city of Oaxaca de Juárez. Cantera Verde Etla (CV_E), Cantera Amarilla Etla (CA_E) and Cantera Rosa Etla (CR_E) are from the city of Etla (20 km to the northwest of Oaxaca de Juárez). Mitla Gris (MG) and Mitla Rosa (MR) are from the village Mitla 45 km southeast of Oaxaca. Querétaro Blanco (QB), Querétaro Melon (QM), Querétaro Amarilla (QA) and Querétaro Naranja (QN) are from the state of Querétaro in Central Mexico. Lithoscans and thin section images of all samples are presented in figures 1 and 2.

Cantera Verde Oaxaca (CV_O) is a homogenous, dark-green vitric, rhyolitic ash tuff with massive to dense texture and weak orientation of pumice clasts (Fig. 1a). The glassy matrix is strongly altered to zeolite and clay minerals and is partly devitrified. Cantera Verde Etla (CV_E) is a homogenous, light-pistaccio green vitrophyric ash tuff of rhyolitic composition. Noticable macrosopically are white clay lenses (Fig. 1b), which disintegrate when in contact with water. CV_E has a hypocrystalline-cryptocrystalline matrix with vitrophyric texture. The crystals are often strongly weathered (Fig. 2b). The matrix is rich in zeolites (clinoptilolite and mordenite). SiO₂ is present in the form of crystobalite. Pablo-Galán (1986) describes the formation of zeolites in the Etla tuff by alkaline diagenesis of rhyolitic glass. He distinguishes between two varieties of tuff, a clinoptilolite-rich variety and a mordenite-rich variety.

Cantera Amarilla Oaxaca (CA_O) is a homogenous, yellow to white vitric ash tuff of rhyolitic composition (Fig. 1c). The rock is very soft and contains a high amount of swellable clay minerals (Tab. 1). Sometimes foliation can be observed in the ash layers. Lithoclasts of up to 0.5 cm are present in the cryptocrystalline to glassy matrix (Fig. 2c).

Figure 1: Lithoscans of the investigated tuffs. a) CV_O, b) CV_E, c) CA_O, d) CA_E, e) CR_O, f) CR_E, g) MG, h) MR, i) QB, j) QM, k) QA, l) QN.

Cantera Amarilla Etla (CA_E) is a yellow to orange rhyolitic, vitric ash tuff with characteristic layering (Fig. 1d). The orange layers or rings are colored by iron oxides and make the orientation of the sample clearly visible. The grain size is small (up to 3 mm) and the glassy matrix is homogenous (Fig. 2d). The tuff contains zeolites and swellable smectite (Tab. 1).

Cantera Rosa Oaxaca (CR_O) is a homogenous, fine- to medium-grained rhyolitic ash tuff of red to pink color with a glassy matrix and small crystals < 1 cm (Fig. 1e). The rock easily loses sand-sized particles on the surface. The thin section reveals an eutaxitic texture (Fig. 2e) with high amounts of welded pumice and glass. The sample is strongly devitrified in parts and contains clay lenses where the glass is altered.

121Table 1: CEC, BET as well as clay mineral content and zeolites of the investigated tuff rocks.

Cantera Rosa Etla (CR_E) is a rhyolitic, vitric ash tuff and very similar to CR_O in terms of texture, grain size and color (Fig. 1f). The thin section shows the similarity of CR_O and CR_E (Fig. 2e–f). CR_E contains smaller grains with smaller pores and the color is slightly brighter than CR_O.

The Mitla Tuff Gris (MG) is a rhyolic to dacitic crystal tuff of gray to white color (Fig. 1g). It has very variable porosities depending on the origin within the quarry. MG contains pumice clasts and angular crystals in a glassy matrix (Fig. 2g).

Mitla Tuff Rosa (MR) is a dense, rhyolitic tuff with small pores and crystals of up to 3 mm size. It has a pink matrix, which is in some parts discolored to white (Fig. 1h). The thin section (Fig. 2h) shows an overall small proportion (10 %) of cryptocrystalline to microcrystalline matrix with seriate texture. The crystals are hypidiomorphic and poorly altered.

Querétaro Blanco (QB) is a white to gray rhyolitic lapilli tuff with a high amount of pumice and glass within the matrix. Lithic fragments of more than 1 cm size are macroscopically visible in the lithoscan (Fig. 1i). The thin section of QB shows a vitrophyric texture with very few crystals and lithoclasts embedded in a glassy matrix (Fig. 2i).

Figure 2: Thin sections of he investigated tuffs. a) CV_O, b) CV_E, c) CA_O, d) CA_E, e) CR_O, f) CR_E, g) MG, h) MR, i) QB, j) QM, k) QA, l) QN with parallel nicols except g and h with crossed nicols.

Querétaro Melon (QM) is a rhyolithic, vitric lapilli tuff of orange-melon color with lithic fragments, as well as small crystals and big clasts of pumice in a glassy matrix (Fig. 1j). QM shows an eutaxitic texture with firmly welded glassy matrix and few smaller crystals under the microscope (Fig. 2j).

Querétaro Amarillo (QA) is a dacitic, heterogenous tuff breccia with clasts of several centimeters in size. The matrix is yellowish-brown and contains clasts that are often whitish-gray. The clasts are partly pumice, mostly xenoliths and a small amount of crystals (Fig. 1k). The matrix of QA is 122strongly altered to clay minerals. The pumice are mostly unwelded and show little signs of elongation or compression (Fig. 2k).

Querétaro Naranja (QN) is a vitric lapilli tuff of trachydacitic composition with lithic fragments in the centimeter scale. Small fragments of orange pumice and strongly altered crystals in a devitrified matrix of orange color are visible in Figure 1l. The thin section shows round and angular xenocrysts and altered feldspar in the fine-grained matrix (Fig. 2l).

In sum, all samples have an acidic composition. Particularly glass-rich samples are CR_O, CR_E, QM, QA and QN. Feldspar could be detected in all samples, while quartz was not detected by XRD in CV_E, CR_E, QB and QM. CV_E and QB instead contain the high temperature modifications of SiO₂ (tridymite and cristobalite). The cation exchange capacity (CEC) and the specific surface area (BET), as well as the content of clay minerals and zeolites in all samples are presented in Table 1.

Samples with significant amounts of zeolites are CV_O, CV_E and CA_E. The specific surface area (BET) is high for these samples. The main zeolites are heulandite and clinoptilolite, minor amounts of mordenite were detected in CV_E. Clay minerals were detected in all samples. The content of swellable clay minerals indicated by a high CEC value is especially high in CA_O, MG and CV_O. Smectite and muscovite-illite make up the main content of the clay fraction. Chlorite was only detected in CV_E and kaolinite was found in MG and could be present in small amounts in CV_O, CV_E, CA_O and QA.

Influence of zeolites and swellable clay minerals

Scanning electron microscopy (SEM) images reveal the type and location of clay minerals in MG and CV_O (Fig. 3a–d). Swellable smectite, muscovite and kaolinite are located in both pore space (Fig. 3a) and in the matrix (Fig. 3b,d) as well as on grain boundaries (Fig. 3c). Smectite has the ability to cause volume increase by swelling, especially when it is located on grain contacts. The alteration of feldspar to clay minerals often leads to the formation of intracrystalline porosity and therefore increases the effective porosity (Fig. 3c).

The scanning electron microscopy (SEM) images (Fig. 3e–h) reveal very small mordenite needles and clinotilolite laths in CV_E, which cause a high specific surface area (Fig. 3e–h).

The zeolite-rich samples CV_O, CV_E and CA_E show the highest hygroscopic water sorption and hydric expansion of all samples (see Kück et al. 2020a in this issue). During thermal expansion, the zeolite-rich samples (CV_O, CV_E and CA_E) show a large difference between the first and the second heating cycle, with high residual strain (Fig. 4). After the first wet cycle the samples show a decrease in residual strain. CA_O and CA_E do not recover from contraction after the second drying cycle. The Mitla samples and QB are rather unaffected by both thermal and thermohyric dilatation (see Kück et al. 2020a).

Figure 3: a–d: Different appearances of swellable clay minerals. a): smectite appears as a ‘spiderweb’ in the pore space as relict of weathered pumice clast in MG, b): kaolinite booklet and smectite in densely packed matrix, c): dissolved feldspar with clay minerals on the grain boundary reveals intracrystalline porosity, d): smectite in the matrix of CVO; e–h): Zeolites in CVE cause a high specific surface area. e): flaky zeolite in the matrix, f): zeolites grow into pore space, g): clinoptilolite laths in the matrix, h): 100 nm thin mordenite needles in a pore.

123

Figure 4: Residual strain [mm/m] of CVE in the X- and Z-direction for two heating cycles from 20–90 °C. Noticeable are intense shrinking and non-reversible thermal dilatation.

Discussion and conclusions

The cation exchange capacity (CEC) value depends on both clay minerals and zeolites, however, the CEC method used is not suitable for CEC analysis of zeolites because the large colored Cu-trien cations cannot enter the cavities of zeolites (Meier and Kahr 1999). For the analysed samples a clear trend between CEC and hydric expansion could not be observed (Fig. 5a). On the other hand, a linear dependency of hydric expansion and hygroscopic sorption on the specific surface area is observable (Fig. 5b, Fig. 6b). The specific surface area increases when clay minerals and zeolites are present in a volcanic tuff, because they yield a high share of micropores (often in the nanometer-scale) within the rock fabric. The higher the specific surface area is, the more water can interact with the rock and the expansion is resultingly larger. Zeolites can adsorb and release substantial amounts of water and therefore passively lead to hydric expansion, whenever water is released during dehydration.

The results of this study show, that the presence of both zeolites and swellable clay minerals has a combined effect on the water-related weathering behavior of tuff building stones. The porespace properties (e. g. pore radii, specific surface area) and the water transport and storage properties 124of volcanic tuff rocks are significantly influenced, by either of them. Volcanic tuff rocks with a significant amount of swellable clay minerals as well as zeolites show extremely high hygroscopic water sorption and hydric swelling. Thermal effects like shrinkage and fracturing during drying are particularly high. This study showed that the identification of the clay mineral and zeolite content, as well as their location and shape within the rock fabric is an important measure for the prediction of the weathering behavior of tuff building stones. For the planning of effective conservational treatment of tuff building stones a clay mineral and zeolite analysis should be considered substantial.

Figure 5: Correlation between a: hydric expansion [mm/m] and CEC [meq/100 g], b: hydric expansion [mm/m] and BET [m²/g].

Figure 6: Correlation between a) hygroscopic water sorption at 95 % rh [wt.-%] and CEC [meq/100 g], b) hygroscopic water sorption at 95 % rh [wt.-%] and BET [m²/g].

Acknowledgements

This work was supported by the German Research Foundation (Si-438/52-1) and the German Federal Environmental Foundation (AZ20017/481). We would like to thank J. L. Noria Sánchez, A. E. Andrade Cuautle and M. Á. González for their support of the onsite work. For the laboratory support and helpful comments we thank K. Wemmer, J. Menningen, W. Wedekind and C. Gross.

References

Colella, C., Gennaro, M. d.’ and Aiello, R. (2001), Use of zeolitic tuff in the building industry, Reviews in Mineralogy and Geochemistry, Vol. 45 No. 1, pp. 551–587.

Di Tchernev (1978), Solar energy application of natural zeolites, Natural zeolites: Occurrence, properties, use, Vol. 474, p. 485.

Gonzalez, I. J. and Scherer, G. W. (2004), Effect of swelling inhibitors on the swelling and stress relaxation of clay bearing stones, Environmental Geology, Vol. 46 No. 3–4, pp. 364–377.

Korkuna, O., Leboda, R., Skubiszewska-Zie, J., Vrublevska, T., Gunko, V. M. and Ryczkowski, J. (2006), Structural and physicochemical properties of natural zeolites: clinoptilolite and mordenite, Microporous and Mesoporous Materials, Vol. 87 No. 3, pp. 243–254.

Kück, A., Pötzl, C., López-Doncel, R. A., Dohrmann, R. and Siegesmund, S. (2020a), Tuffs in pre-Columbian and colonial architecture of Oaxaca, Mexico, in Siegesmund, S. and Middendorf, B. (Eds.), Monument future: Decay and conservation of stone, Göttingen, Kassel, Mitteldeutscher Verlag, Halle.

Pablo-Galán, L. de (1986), Geochemical trends in the alteration of Miocene vitric tuffs to economic zeolite deposits, Oaxaca, Mexico, Applied geochemistry, Vol. 1 No. 2, pp. 273–285.

Pötzl, C., Dohrmann, R. and Siegesmund, S. (2018a), Clay swelling mechanism in tuff stones: an example of the Hilbersdorf Tuff from Chemnitz, Germany, Environmental earth sciences, Vol. 77 No. 5, p. 188.

Pötzl, C., Siegesmund, S., Dohrmann, R., Koning, J. M. and Wedekind, W. (2018b), Deterioration of volcanic tuff rocks from Armenia. Constraints on salt crystallization and hydric expansion, Environmental earth sciences, Vol. 77 No. 19, p. 660.

Ruedrich, J., Bartelsen, T., Dohrmann, R. and Siegesmund, S. (2011), Moisture expansion as a deterioration factor for sandstone used in buildings, Environmental earth sciences, Vol. 63 No. 7–8, pp. 1545–1564.

Siegesmund, S. and Dürrast, H. (2011), Physical and mechanical properties of rocks, in Siegesmund, S. and Snethlage, R. (Eds.), Stone in architecture, Springer, pp. 97–225.

Snethlage, R., Wendler, E. and Klemm, D. D. (1995), Tenside im Gesteinsschutz-bisherige Resultate mit einem neuen Konzept zur Erhaltung von Denkmälern aus Naturstein, Denkmalpflege und Naturwissenschaft-Natursteinkonservierung I. Verlag Ernst & Sohn, Berlin, pp. 127–146.

Wedekind, W., López-Doncel, R., Dohrmann, R., Kocher, M. and Siegesmund, S. (2013), Weathering of volcanic tuff rocks caused by moisture expansion, Environmental earth sciences, Vol. 69 No. 4, pp. 1203–1224.

125

PHYSICO-CHEMICAL CHARACTERIZATION OF THE CARTAGENA WALL AND QUARRY MATERIAL STONE USED FOR ITS RESTORATION

Manuel Saba¹, Juan Lizarazo-Marriaga², Nicole Hernández-Romero², Cristina Tedeschi³, Edgar Quiñones-Bolaños¹

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 University of Cartagena, Research Group Esconpat, Faculty of Engineering, Cartagena, Colombia

2 Universidad Nacional de Colombia, Department of Civil and Agricultural Engineering, Bogotá, Colombia

3 Department of Civil and Environmental Engineering, Politecnico di Milano, Italy

4 University of Cartagena, Research Group Environmental Modelling, Faculty of Engineering, Cartagena, Colombia

Abstract

The deterioration of historical structures is a topic of primary importance due to their historical, cultural and economic importance. Aggressive environments around structures can depend on anthropogenic factors such as NO₂, SO₂ gases among others, and environmental factors such as differential dilation, differential hydric dilation and crystallization. In the present study is proposed a physical-chemical characterization of the structure and quarry stone used for the replacement of deteriorated blocks of the defensive Wall of Cartagena, Colombia, UNESCO Cultural Heritage since 1984. X-ray and ionic chromatography identification were done in the structure stone. Furthermore, is proposed a Petrographical comparison between structure and quarry material as well as mechanical characterization of the quarry material.

The structure is composed mainly by soft limestone. Similar physico-chemical characteristics were found through X-Ray and Petrographical comparison, while salt crystallization is found playing a secondary role in the structure deterioration.

Introduction

According to the United Nations’ Educational, Scientific and Cultural Organization (UNESCO), there are around 869 sites of cultural interest around the world (UNESCO 2019). They were selected for their historical relevance and uniqueness. All of them struggle with surrounding aggressive climate conditions that accelerate their deterioration. Currently, 53 of those sites are rated as being in imminent danger due to anthropogenic and natural factors (UNESCO 2019).

For example, the Fortifications on the Caribbean Side of Panama (Portobelo-San Lorenzo), part of the defensive colonial system built by the Spanish Crown to protect transatlantic trade, constitute a valuable example of 17th and 18th-century military architecture, whose integrity has been compromised by environmental factors, uncontrolled urban extension, development and the lack of maintenance and management. Dozens of structures have been destroyed in the past in the name of Civilization, or because of high deterioration and/or earthquakes. An example of this is the Noto Cathedral, Italy, a historic masonry building that suddenly collapsed in 1996 (Binda et al. 1999) 126producing during centuries a redistribution of stresses from the core of lime mortar concrete to the external cladding of stiff masonry. This is likely one of the causes of long-time damage of some ancient masonry towers. With these motivations, coupled processes of moisture diffusion, carbon dioxide diffusion and carbonation reaction are analyzed numerically. Due to the absence of models and data for lime mortar, one of the simplest models proposed for Portland cement concrete is adapted for this purpose. The results reveal the time scales of the processes involved and their dependence on wall thickness (size. Factors responsible for over-stressing and damaging the monuments and thus inhibiting their conservation are high levels of air pollution in the atmosphere, daily and seasonal cycles of temperature and humidity, sea spray (or salt spray), rising damp, atmospheric conditions (rainwater, wind and sunlight exposure (Kameni & Orosa 2016)).

During the process of restoration of cultural heritage buildings, it is usual to replace highly deteriorated stones and mortars with new ones. Unfortunately, often the choice of replacement materials is done without sufficient preliminary investigation of the properties of the existing materials. In order to come to a selection of replacement stones compatible with the existent ones, several material properties need to be taken into account, such as petrographic properties, mechanical strength, moisture transport behaviour, colour, texture etc., (Baronio G., et al. 2003), (Binda L. et al. 2003).

In the present research the study case of the Fortresses of Cartagena is reported. They are UNESCO Cultural heritage since 1984, nevertheless, UNESCO inspectors identified and high level of deterioration and lack of long-term maintenance plans, (UNESCO, 1984). The structure stands in front of the Caribbean Sea, in a tropical area, therefore, salt crystallization process was studied on the structure surface to define its role in the structure deterioration.

Quarry samples were taken from an ancient quarry in Cartagena (Tierrabomba Island) in the geological Formation called La Popa. La Popa Formation rests on the Bayunca Formation (of the Pliocene) and it was formed during the Upper Pleistocene. It is conformed by coral reefs formed on an underwater platform in an area with little contribution of terrigenous sediments, clear waters and temperatures between 21 °C and 25 °C. High porous limestones (> 30 %) with bulk density < 1,500 kg m^–3 are common in the area. Similar physical-mechanical characteristics were identified in the Cartagena’s Fortification from previous analysis, (Saba et al. 2019).

Therefore, structure and quarry samples were physical-chemical compared to assess the reliability to use them as a replacement of the deteriorated structure stones.

Materials and Methods

5 structure samples were collected from the stone surface (5 mm depth). Additionally, thin sections were done on those samples following the ASTM C1721 – 15, (2015) standard using blue epoxydic resin. Thin sections were petrographical analysed with an Olympus CX 31 microscope with magnifications ranging from x5 to x100 for assessing the presence of bioclasts, type of cement, terrigenous, distribution and quantification of primary and secondary porosity. Each thin section has a dimension of 4.5 cm × 2.6 cm. Point counting technique was used in a mesh of 300 equidistant points.

On these samples, ion analyses were carried out using Ion Chromatograph (IC). Powdered samples were dried at 60 °C until constant weight. Saline solubilisation was achieved by shaking 1 g of each dried sample in 100 ml of ultra-pure water. The 10 ml of obtained supernatants were filtered through a 0.2 µm PTFE membrane. The separation of cations Na+, Mg+, K+ was achieved by using a stationary-phase featuring a CS12A 250*4 mm column with a 10*4 mm guard (Dionex). As for anions Cl–, SO₄–, NO₃–, the stationary phase featured a AS9-HC 250 *4 mm column with a 10*4 mm guard (Dionex), (Nasraoui M. et al. 2009)the standard analytical equipment as ion chromatography (IC).

80 cubes of 5.0 × 5.0 × 5.0 cm were selected in the original quarry of the structure for the physical-mechanical 127characterization. Specimens were Characterized following the Natural Stone Test Methods (UNI EN 1936:2007 Natural stone test methods – Determination of real density and apparent density, and of total and open porosity, 2007). Real Volume V_R (m³), Open Porosity P_o (%) and Apparent Density ρ_b (kg m^–3) were calculated, (1–3).

Where m_d (g) is the Dry mass, m_s (g) Saturated mass, m_h (g) water immersion mass, ρ_rh water density at 20 °C, 998 kg m^-3.

Stone Uniaxial Compressive Strength (SUCS) measurements were done on the quarry samples.

X-Ray analysis were done in the 5 quarry samples and in 2 structure samples.

Results and Discussion

Thin section analysis results showed in Table 1 highlights that structure and quarry sample are both classified as Packestone according to Dunham (1962). They have similar ranges of Bioclasts and Sparry cement, while Primary porosity is significantly higher in the quarry samples (see Figure 1). Increasing of stone porosity often is related to decreasing of mechanical properties, which is probably the reason why this specific quarry in Tierrabomba Island was abandoned for new ones in the same area at the middle of the XVII century (Álvarez-Carrascal 2018; Cabrera et al. 1995).

Figure 1: Thin Section structure and Quarry Stones, (Saba et al. 2019).

Table 1: Petrographycal analysis results.

Component	Structure stone (%)	Quarry Stone (%)
Bioclasts (B)	28.5±4.4	29.3±2.1
Terrígenous (T)	7.6±6.2	0.8±0.5
Autigens and Others (Au)	0.1±0.3	0.00±0.00
Primary Porosity (P)	34.7±5.9	45.3±2.1
Secondary Porosity (S)	0.0±0.0	0.0±0.0
Micrite Cement (M)	3.2±2.3	0.0±0.0
Sparry Cement (Sp)	27.2±6.4	21.47±1.6

From the quarry stone physical-mechanical characterization, can be highlighted that open porosity differs about 10 % from thin section analysis, which means that thin section measurements despite their low representativeness provide an acceptable approximation to porosity values compared with results coming from a large number of samples analysed whit gravimetrical measurements.

Structure and quarry stone are found as pure limestone from the X-ray analysis, with CaCO₃ higher than 98 %, and a presence of Quartz and Halite lower than 1 %, (Figure 2).

Figure 2: Structure samples X-Ray results.

Ion chromatography analysis on structure samples show a total range of salts between 0.4 % and 2.4 % of Mass on average 1.0 % in all 5 structure samples (Table 2). From the literature review it is difficult to assess if this is a high or low salt content because it should be compared with samples taken at the same depth.

Salts are usually found with higher concentration 128on the first millimetres of the material surface, while lower concentrations correspond to deeper sampling. In our case sampling depth has been 5 mm from the surface. Nevertheless, sampling depth in literature works usually it is not adequately reported (Ahmad & Haris Fadzilah 2010; C, Lopez-Arce et al. 2016). Hence, a comparison is risky and not appropriate (Figure 3).

Table 2: Stone Physical-mechanical Characteristics.

Characteristic	Quarry Stone
VR (mm3 × 105)	1.55±0.11
Ms (g × 102)	2.75±0.12
Mh (g × 102)	1.03±0.09
Md (g × 102)	1.55±0.12
P0 (%)	42.78±1.88
ρb (kg m–3)	1411±47
SUCS (MPa)	1.86±0.36

V_R is Real Volume P_o Open Porosity, ρ_b Apparent Density, m_d is the Dry mass, m_s Saturated mass, m_h water immersion mass, SUCS is Stone Uniaxial Compressive strength

Figure 3: Structure samples Ion Chromatography results elaborated with Runsalt^®.

Salt crystallization process is not directly observed in the structure surface of the area of study, nevertheless, a careful revision of the whole structure should be done to eventually detect this type of deterioration and compare salt concentrations with generic sections of the structure and define if ion chromatograph results are effectively reliable to correctly detect the deterioration process in this case of study.

Results were further analysed with Runsalt software (V. 1.9), (Bionda 2005). It is the graphical user interface to the ECOS thermodynamic model (Price 2000). It allows to define which salts are crystallizing from a mixture depending on temperature and RH conditions. The ion contents, as determined by IC, are input for the model. As an example, sample 5 ion and cation values were introduced as a input, as well as the average temperature in Cartagena, 27.7 °C according to DIMAR (2018).

Table 3: Ion Chromatography results (%), input data of Runsalt. N.= Number of Sample.

N	Cl–	SO4–	NO3–	Na+	Mg+	K+
1	0.044	0.171	0.048	0.074	0.025	0.042
2	0.196	0.15	0.005	0.25	0.036	0.003
3	0.861	0.587	0.016	0.814	0.11	0.077
4	0.481	0.162	0.035	0.431	0.037	0.064
5	0.115	0.134	0.008	0.207	0.04	0.007

Results are reported in Figure 3 and are similar for the whole structure samples. The mix of salts on the samples is identified and Relative Humidity of equilibrium (RH_eq) is reported for each salt in the solution. It is worth to mention that Cartagena stands in a tropical area, with a Relative Humidity range between 72 % to 90 % during the year, considering minimum and maximum values of daily and seasonal variations. From these preliminary studies, Thenardite and Bloedite could crystalize having a RH_eq of 75 %, although their role on the material deterioration should be further studied, for example pondering whether the number of crystallization cycles per year is relevant.

Regarding mechanical properties, SUCS varies between 1.5 MPa to 2.2 MPa which is typical of a soft pure limestone (Table 3).

Conclusions

In the present research structure sample were analysed and the problem of salt crystallization in the structure deterioration was approached.

It was found that salts on the structure surface rarely crystalize due to high environmental RH in the area of study. Therefore, it can be inferred that probably salt plays a minor role in the stone deterioration, while the main causes of deterioration should be further studied.

129Quarry and structure stone were physical-chemical compared through X-ray and petrographical analysis. Quarry stones were found compatible for replacement of deteriorated block, although further research is needed to include more relevant physical and aesthetic variables in the analysis of stone replacement such as colour, texture and moisture transport properties.

Acknowledgements

This Project has been funded by the Universidad de Cartagena, Colombia, through the resolution number 00473 of 2016 “Octava convocatoria a proyectos de investigación, para grupos de investigación visibles (categorizados o reconocidos) en la plataforma Scienti del departamento de Ciencia, Tecnología e Innovación-Colciencias y avalados por la Universidad de Cartagena”. Furthermore, this project has been funded with the support of the European Commission (Grant agreement no. 2014-0873/001-001). This publication only reflects the view of the author, and the Commission cannot be held responsible for any use which may be made of the information contained therein - Elarch Program (project reference number: 552129-em-1-2014-1-it-era mundus-ema21). Finally, Authors would like to acknowledge the development of this work to Universidad Nacional de Colombia by funding DIB 34835.

References

Álvarez-Carrascal, J. L. (2018). Retrospectiva de la construcción del cordón amurallado de Cartagena de Indias, su presente y su futuro. Universidad de Granada.

ASTM C1721 – 15 Standard Guide for Petrographic Examination of Dimension Stone, (2015).

Baronio G, Binda L, Tedeschi C, Tiraboschi C (2003) Characterisation of the materials used in the construction of the Noto Cathedral. Construction and Building Materials 17:557-571 doi: 10.1016/j.conbuildmat.2003.08.007.

Binda, L., Baronio, G., Gavarini, C., De Benedictis, R. & Tringali, S., (1999) Investigation on Materials and Structures for the Reconstruction of the Partially Collapsed Cathedral of Noto (Sicily). In 6th Int. Conf. Structural Studies, Repairs and Maintenance of Historical Buildings, STREMAH 99, Dresden, Germany, pp. 323–332.

Binda L, Baronio G, Tedeschi C, Tiraboschi C (2003c) Experimental research for the choice of adequate materials for the reconstruction of the Cathedral of Noto. Construction and Building Materials 17:629-639. doi:10.1016/S0950-0618(03)00059-X.

Bionda, D. (2005). A graphical user interface to the ECOS thermodynamic model for the prediction of the behaviour of salt mixtures under changing climate conditions.

Cabrera, A., Martelo, R., Martinez, A., & Martinez, R. (1995). La Ruta De Los Hornos.

UNI EN 1936:2007 Natural stone test methods – Determination of real density and apparent density, and of total and open porosity, 1 (2007).

DIMAR. (2018). Boletín Meteomarino del caribe Colombiano. No. 68 Agosto 2018. https://issuu.com/dimarcolombia/docs/boletin201808

Dunham, R. J. (1962). Classification of carbonate rocks according to depositional texture. In W. E. Ham (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir (pp. 108–121).

Ferretti, D., & Bažant, Z. P. (2006). Stability of ancient masonry towers: Moisture diffusion, carbonation and size effect. Cement and Concrete Research, 36(7), 1379–1388. https://doi.org/10.1016/j.cemconres.2006.03.013

Ghafar Ahmad, A., & Abdul Rahman, H. F. (2010). Treatment of Salt Attack and Rising Damp in Heritage Buildings in Penang, Malaysia. Journal of Construction in Developing Countries, 15(1), 93–113.

Kameni, M., & Orosa, J. A. (2016). Building construction materials effect in tropical wet and cold climates : A case study of office buildings in Cameroon. Case Studies in Thermal Engineering, 7, 55–65. https://doi.org/10.1016/j.csite.2016.01.007

Nasraoui, M., Nowik, W., & Lubelli, B. (2009). A comparative study of hygroscopic moisture content, electrical conductivity and ion chromatography 130for salt assessment in plasters of historical buildings. Construction and Building Materials, 23(5), 1731–1735. https://doi.org/10.1016/j.conbuildmat.2008.09.029

Price, C. (2000). Runsalt. An expert chemical model for determining the environmental conditions needed to prevent salt damage in porous materials, European Commission Research Report No 11, (Protection and Conservation of European Cultural Heritage). Archetype Publications.

Saba, M., Hernandez-Romero, N., Quiñones-Bolaños, E., & Lizarazo-Marriaga, J. (2019). Petrographic of Limestone Cultural Heritage as the Basis of a Methodology to Rock Replacement and Masonry Assessment: Cartagena de Indias Case of Study. Case Studies in Construction Materials. https://doi.org/https://doi.org/10.1016/j.cscm.2019.e00281

UNESCO. (1984). Port, Fortresses and Group of Monuments, Cartagena.

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Zoghlami, K., Lopez-Arce, P., & Zornoza-Indart, A. (2016). Differential Stone Decay of the Spanish Tower Façade in Bizerte, Tunisia. J. of Materials in Civil Engineering 1–14. https://doi.org/10.1061/(ASCE)MT.1943-5533.0001774.

131

COMPARATIVE ANALYSIS OF VOLCANIC TUFFS FROM EUROPE, ASIA AND NORTH-AMERICA

Ákos Török¹, Luigi Germinario¹, Rubén López-Doncel², Christopher Pötzl³, Siegfried Siegesmund³

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Budapest University of Technology and Economics, Department of Engineering Geology and Geotechnics, Megyetem rkp. 3, H-1111 Budapest, Hungary (torokakos@mail.bme.hu)

2 Geological Institute, Autonomous University of San Luis Potosí, Mexico

3 Geoscience Centre of the University of Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany

Abstract

In this study, volcanic tuffs from Europe (Germany and Hungary), Asia (Armenia) and North-America (Mexico) are compared. The physical properties (density, porosity and pore-size distribution, strength) and mineralogical composition of basic, intermediate and acid volcanic tuffs were measured in laboratory conditions aiming to evaluate these parameters in terms of stone durability. The colour of rhyolitic to andesitic tuffs, the amount of lithic clasts and the proportion of groundmass vary. The mineralogical composition, porosity and textural characteristics of the studied tuffs are very different. The comparative analysis suggested that clay content and distribution within the pores, the amount of glassy groundmass, porosity and pore-size distribution, as well as hygric swelling and thermal behaviour of the tuffs, are the main control factors of the durability.

Introduction

Volcanic tuffs are important lithotypes of our cultural heritage. The material characteristics and the worldwide occurrence allow a widespread use from sculptural elements to massive built structures such as castles and fortresses. Despite the extensive use of tuffs, these materials are prone to weathering and show various forms of decay due to their mechanical properties, textural variability and high porosity.

The current paper provides an overview of the petrophysical, mechanical and petrographic properties of tuffs from Europe, Asia, and North-America. It presents the geological setting and utilisation of these tuffs. It outlines the decay features that are linked to external factors and discusses the differences in durability linked to textural/mineralogical characteristics and stress conditions such as freeze-thaw and salt action. The study also provides a background for the material selections for restoration.

Materials

In the course of its geological history, immense amounts of volcaniclastic material were deposited in the territory of Armenia. Armenian tuffs show a wide variety of color, grain size, clast content and chemical composition. The two examples presented in this study, Hoktemberyan and Golden Armenia, are frequently used in the past and recent construction in many parts of the country (Figure 1a shows the utilization of Qasakh tuff).

In Mexico, volcanic tuffs were and are widely used in the construction of pre-Hispanic, colonial and 132modern monuments. The type and variety of these tuffs exceed half a hundred. In the present study, five tuffs of Central Mexico (San Luis Potosí, SLP), are presented as examples (Figure 1b). The volcanic tuffs of SLP belong to the Paleogene Silicic Large Igneous Province, the Sierra Madre Occidental, the largest ignimbritic province in the world.

Figure 1: Examples of the historical use of the studied tuffs and relevant geographic locations: a) Hovhannavank Monastery, Ohanavan (Armenia), 5th c.; b) Templo del Carmen, San Luis Potosí (Mexico), 17th–18th c.; c) Eger Castle, Eger (Hungary), 13th–18th c.; d) Porphyry house, Chemnitz (Germany), 1868.

Miocene volcanic activity produced a large amount of welded and unwelded pumiceous tuffs in Hungary. Besides the prevailing rhyolitic composition, dacitic and andesitic tuffs also formed in Eastern Hungary. Rhyolitic tuffs are particularly common in a 50 × 10 km area known as Bükkalja Volcanic Field in Northern Hungary.

Emblematic monuments such as the castle or the minaret of Eger were constructed from this material (Figure 1c).

Significant volcanic tuff deposits in Germany are mainly found in the eastern and western Central part of the country. Permian volcanism led to the deposition of the Hilbersdorf tuffs in eastern Central Germany, near the city of Chemnitz.

The Quaternary Weibern tuff is located in western Central Germany, in the municipality of Weibern.

Prominent examples of its application as building stone are, e. g. the Kaiser Wilhelm Memorial Church in Berlin, the Castle Church of Chemnitz or the porphyry house of master stonemason Findewirth in Chemnitz (Figure 1d).

Methods

The tuffs were described using polarized-light microscopy. Bulk and material density, porosity (EN 1936:2006), and water absorption at atmospheric pressure (EN 13755:2008) were measured. Pore-size distribution was analyzed by mercury intrusion porosimetry (MIP) (ASTM D4404-84). Ultrasonic 133p-wave velocity was recorded by direct transmission method (EN 14579:2004). Strength parameters such as uniaxial compressive strength (UCS), tensile strength (ASTM D3967-16) were measured, and Young’s modulus of elasticity was also recorded (ASTM D7012-14). Durability was assessed by resistance to freeze-thaw (EN 12371:2001 with modifications) and resistance to salt attack (EN 12370:1999).

Petrology and Mineralogy

The creamy rhyolitic tuffs from Northern Hungary are characterized by a porphyritic texture with plagioclase phenocrysts. The pumice content is high, and the groundmass is mostly glassy (Figure 2). Critical parameters associated with high textural variability are crystal-groundmass-pumice ratio; grain size; welding degree. These features may vary in outcrops a few kilometres away, and even within the same quarry, to a lesser extent (Germinario & Török 2019).

The German Hilbersdorf Regular tuff, appears in irregularly speckled and marbled in pale pink to dark purple and bright beige to greenish colours. A variety of elongated lapilli inclusions are embedded in the groundmass and millimetre to centimetre-large cavities are partly filled with loose, clayey material. It has a porphyritic texture with mono- and polycrystalline quartz, muscovite, hematite, calcite, feldspar relics and lithic clasts in the glassy matrix (Figure 2). The porphyritic Weibern tuff consists of a fine-grained yellowish-brownish matrix in which pumice lapilli of yellow colour and partly elongated clasts of different but mostly grey colour are embedded. The rock fragments are mostly sandstone and shale as well as basalt and volcanic glass fragments. The matrix mainly consists of analcime, muscovite/illite, quartz and calcite (Wedekind et al. 2013).

The Armenian Hoktemberyan tuffs are trachydacites, while the Golden Armenia varieties have rhyolitic composition. The most popular Hoktemberyan tuff is of characteristic brick-red color, but transitions to orange-brownish and blackish also exist, often given different local trade names. In its fine-grained brick-red groundmass (~75 %), small elongated pumice clasts of red and black colour are embedded. Grey to black rock fragments, as well as huge amounts of white, elongated feldspars and glass particles in the millimetre range give a slightly speckled appearance. Thin-section analyses show feldspar and amphibole phenocrysts as well as volcanic lithoclasts and hematite located in a cryptocrystalline matrix (Figure 2). The yellowish-golden groundmass of Golden Armenia tuff embeds beige, grey and slightly reddish clasts. In thin section, volcanic lithoclasts, quarz phenocrysts, feldspar relics and vitric fragments are oberserved in a glass-rich matrix. Considerable amounts of swellable clay minerals (corrensite) are verified in Pötzl et al. (2018b).

All the Mexican volcanic tuffs of SLP have rhyolitic composition. The percentage of crystals and matrix vary of 30 %–70 %. The rocks contain mainly 134quartz, with an average abundance of 45 %, alkali feldspar (mostly sanidine and orthoclase) with 35 %, and plagioclase (oligoclase to anorthite) not exceeding 30 %. The texture of these rhyolitic ignimbritic tuffs varies from porphyritic hypocrystalline to vitrophyric (Figure 2).

Figure 2: Thin-section photomicrographs in plane- and cross-polarized light from a selection of the studied tuffs (see detailed explanations in the text).

Physical properties

The Hungarian tuffs generally have high effective porosity, from 17 % to 30 % approximately, with a well interconnected pore network, accounting for the almost exclusive presence of open pores in the rock volume. Capillary pores (> 0.1 µm) represent distinctively the most abundant size. The low bulk density, 1.5 g/cm³ on average, is directly related to the high porosity, as well as to the abundance of low-density pumice clasts and glass shards. Considering the mechanical properties, the studied tuffs are weak to moderately strong rocks with compressive strength of 7 and 28 MPa. Saturated conditions produce an extreme deterioration of the mechanical properties, with the strength that may decrease even by 90 % in the weakest tuff varieties (Table 1).

The Hilbersdorf and Weibern tuffs of Germany have high effective porosities of 26 % and 37 %, respectively. While Hilbersdorf tuff contains substantial amounts of micropores (43 %), Weibern tuff is characterized by huge portions (> 86 %) of capillary pores. In comparison to the other tuffs of this study, the Hilbersdorf tuff shows considerably high bulk (1.9 g/cm³) and matrix (2.6 g/cm³) densities, as well as moderate to high p-wave velocity (2.6 km/s), tensile (4 MPa) and compressive strength (32 MPa). The Weibern tuff, on the other hand, is characterized by lower densities and p-wave velocity and tensile strength (1.5 MPa) (Table 1). Both tuffs suffer a strong decrease (up to 40 %) in their mechanical properties when tested under saturated conditions (Table 1).

The Armenian tuffs show a broad range of petrophysical properties. The tuffs of this study have a high effective porosity of 21 % to 36 %. However, the Hoktemberyan tuff is characterized by a much lower ratio of micropores (9 %) and bulk density (1.6 g/cm³). Mechanical properties display the Armenian tuffs as moderately strong with maximum tensile strength values of 5 MPa. While Golden Armenia suffers from a distinct strength decrease under saturated conditions, the Hoktemberyan tuff does not seem to be affected by water in its mechanical properties.

The porosity of the Mexican tuffs ranges from 18 % to 36 % and the density values vary from 2.3 g/cm³ to 2.6 g/cm³, respectively. The pore-size distribution of the studied tuffs are unimodal and bimodal, in addition the average pore radius fluctuates from 0.15 µm to 4.02 µm, dominating the capillary pores. SLP volcanic rocks are also very soft to moderately hard, depending on the degree of welding and showing average values of tensile strength in dry conditions of around 8 MPa, falling extremely to values down to 1.33 MPa in water-saturated conditions. Well-welded ignimbritic tuff samples of SLP can have uniaxial compressive strength of 90 MPa (Wedekind et al. 2013).

Water saturation reduces the strength (both uniaxial compressive and tensile strength) of the tuffs with one exception (Hoktemberyan) (Table 1). The strength of the saturated samples can be as low as one-fourth, but in general, the value is half or two-thirds of the dry one. The porosity of the tuffs is in between 18 to 37 vol%, while matrix densities are relatively uniform with 2.3 to 2.6 g/ cm³. The P-wave velocity of the studied tuffs is in between 2.3 and 3.9 km/s. However, these values do not necessarily indicate differences in porosity (Table 1).

Durability

The durability of the Hungarian rhyolitic least porous welded tuffs may reach 90 freeze-thaw cycles and 20 salt crystallization cycles with minor or no decay in structure and technical properties. However, the most porous softest varieties, which have found a larger application in the historical built heritage, can withstand only few weathering cycles, before disintegration, erosion, and cracking. The magnitude of the differences in porosity, and consequently in the absorption of water and salt solutions, controls primarily the diverse durability.

135Secondary factors are pore-size distribution especially in the range 0.1–10 µm – critical for ice and salt crystallization damage. Considering the textural features, the most significant discriminating factor is the proportions between crystals and the weaker pumice and groundmass.

Table 1: Physical properties of studied tuffs (data of Hungarian rhyolitic tuffs is from Germinario and Török 2019, Hungarian andesite tuff is from Török 2007, data of the German and Armenian tuffs are from Pötzl et al. 2018a and Pötzl et al. 2018b, data marked by * is from López-Doncel et al. 2016; and marked by + is from Wedekind et al. 2013).

The Hilbersdorf tuffs withstand 11 to 21 cycles before complete disintegration due to its high porosity, bimodal pore radii distribution and high amounts of micropores.. The hydric and thermal expansion of these tuffs is greatly influenced by their clay mineral content and ratio of micropores. Maximum hydric expansion can reach values of 7.5 mm/m (Pötzl et al. 2018a).

Armenian tuffs also show great variation in their durability and weathering behaviour, dependent on their petrophysical properties. The resistance against salt attack varies from 16 to way over 150 cycles. Even considerably soft tuffs, like the Hoktemberyan varieties, are not strongly attacked by the salt crystallization and withstand more than 150 cycles. The Golden Armenia, on the other hand, disintegrates after 34 cycles. The main difference between both tuffs is the clay mineral content as well as the much higher ratio of micropores in Golden Armenia.

Regarding the durability of the Mexican tuffs, the conducted salt bursting tests show that after 14 cycles the most porous sample was completely destroyed and two samples, both with a bimodal pore distribution, have resisted at the end of the test 47 and 48 cycles, respectively. All SLP samples 136were tested for hydric and thermal expansion and all of them did not show any expansion, and one sample showed even hydric contraction with values around 0.03 mm/m.

Conclusions

The tuffs are broadly used in the monuments of Europe, Asia and North-America. The physical properties of the studied mostly acid tuffs are very different. Despite the high porosity (from 17 to nearly 37 vol%), tuffs can have relatively high tensile strength in dry conditions. This is significantly reduced with water saturation. The loss in strength can be one-fourth of the original one, but for most of the studied tuffs, it is half or two-thirds of the initial dry value. Not only water saturation but also freeze-thaw cycles and salt crystallization lead to a loss in strength or even an ultimate disintegration. There are tuff lithologies that can survive more than 100 salt crystallization cycles or 90 freeze-thaw cycles, while others disintegrate after few cycles. These differences in durability are attributed not only to porosity but also to pore-size distribution and micro-fabric, and are influenced by formation processes such as welding. Despite these findings and variations of durability, the usage of tuffs is still common, but at several sites the tuff elements show signs of deterioration.

Acknowledgements

ÁT acknowledges the financial support of National Research, Development and Innovation Fund of Hungary (K 116532). CP was supported by the German Federal Environmental Foundation (AZ20017/481).

References

Germinario L., Török Á. (2019). Variability of technical properties and durability in volcanic tuffs from the same quarry region – examples from Northern Hungary. Engineering Geology, 262, 105319.

López-Doncel, R. A., Wedekind, W., Cardona-Velázquez, N., Gon-Zález-Sámano, P. S., Dohrmann, R., Siegesmund, S. Pötzl, Chr. (2016): Geological studies on volcanic tuffs used as natural building stones in the historical center of San Luis Potosí, Mexico. Proceedings of the 13th International Congress on Deterioration and Conservation of Stone, vol. I, 107–115 pp., Glasgow. ISBN: 978-1-903978-59-7

Pötzl, C., Dohrmann, R. and Siegesmund, S. (2018a). Clay swelling mechanism in tuff stones: an example of the Hilbersdorf Tuff from Chemnitz, Germany. Environmental Earth Sciences, 77 (5), p. 188.

Pötzl, C., Siegesmund, S., Dohrmann, R., Koning, J. M. and Wedekind, W. (2018b) Deterioration of volcanic tuff rocks from Armenia. Constraints on salt crystallization and hydric expansion. Environmental Earth Sciences, 77 (19), p. 660.

Stück H., Forgó L. Z., Rüdrich J., Siegesmund S., Török Á. (2008). The behaviour of consolidated volcanic tuffs: weathering mechanisms under simulated laboratory conditions. Environmental Geology, 56, 699–713.

Török Á. (2007). Geology for Engineers. (in Hungarian with English summary) Müegyetemi Kiadó, Budapest, Megyetemi Kiadó, Budapest, 384 p.

Wedekind, W., López-Doncel, R., Dohrmann, R., Kocher, M. and Siegesmund, S. (2013). Weathering of volcanic tuff rocks caused by moisture expansion. Environmental Earth Sciences, 69 (4), 1203–1224.

137

THE CATHEDRAL-RUINS OF ZVARTNOTS AND AVAN (ARMENIA) – A COMPARATIVE STUDY ON DECAY AND RESTORATION

Wanja Wedekind¹, Emma Harutyunyan², Siegfried Siegesmund¹

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Geoscience Centre of the University of Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany

2 National University of Architecture and Construction of Armenia, Teryan 105, 0009 Yerevan, Armenia

Abstract

The Zvartnots Cathedral was built from basalts and tuffs in the 7th century. Since the year 2000, it has been listed on the UNESCO World Heritage list. The Avan Cathedral is even older and was built in the 6th century, mainly from tuff. Both monuments are now ruins and have been restored several times with different materials. This applies primarily to the restoration mortar used.

The tuff material used for both cathedral ruins is comparable.While damage to Avan Cathedral is dominated by cracks and static problems, the Zvartnots ruin is primarily damaged by widespread salt contamination.

In this study, the restoration history of both cathedral ruins were worked out and the petro-physical properties of the building stones were investigated. The historical and modern mortars used in both structures were analysed and examined in the laboratory. Onsite object investigations were carried out, which includes damage mapping, surface hardness, electrical conductivity and the sampling of salts. The investigation clarify the high quality and weathering resistance of the historical mortars.

Introduction

The Zvartnots and Avan Cathedrals are two outstanding examples of the high point of Armenian medieval architecture (Fig. 1a). Today both buildings are only preserved as ruins.

Zvartnots Cathedral

The ruins of Zvartnots Cathedral are located near Echmiadzin, about 17 km west of Yerevan. The complex consists of the ruins of a round cathedral, which was built in the mid 7th century and dedicated to Saint Gregory. Beside the cathedral there are also the ruins of the palace of the Catholic Nerses III, who officiated from 641 to 662.

The church of St. Gregory in Zvartnotz was destroyed in the 10th century and is considered to be the oldest and largest tetrakonchos in the Caucasus Region. The structure has been imitated several times with similar forms. The four conches of the central building were surrounded by a circular corridor, the diameter of which was 37.7 meters, with a presumed height of the triple-tiered cylindrical structure of around 45 meters (Fig. 1a).

The Gregory Church is mentioned in several medieval sources and is one of the best-known architectural symbols of the Armenian Apostolic Church. The Armenian Apostolic Church is the oldest “self-sufficient” state church in the world.

138Archaelogical investigations and restoration

After the first excavation work in 1901, a systematic archaeological investigation began in 1904 and was lead by Toros Toramanian (1864–1934). He presented a three-stage reconstruction model, that initially raised doubts, but was accepted by most experts as probably the right one in 1906. This model is considered a pilot project of reconstructive restoration work in Armenia (Fig. 1a).

From 1958 to 1967 the remains of the wall were being secured. Further restoration measures were carried out in the 1980 s, with large areas of cement-containing mortar and suspensions being used in both cases (Fig. 2f). The most recent restorations were carried out under the direction of Grigor Nalbandyan using cementless materials.

Avan Cathedral

The Catholic Cathedral of the Holy Mother of God of Avan is located in the Avan Yerevan district on the outskirts of the capital of Armenia. The Avan Cathedral is the oldest surviving church within the city limits of Yerevan.

The church was built in the late 6th century, between 591 and 602. Construction began shortly after the Byzantine-Sasan War ended (572–591), which led to the expansion of direct Byzantine rule over all western regions of Armenia.

Figure 1: a) The cathedral ruin of Zvartnots and the reconstruction model by Toramanian with the floor plan of the buildung. b) The cathedral ruin of Avan, a hypothetical reconstruction and the floor plan of the church.

The architectural historian Toros Toramanian (1864–1934) made the assumption that the Avan Church once had five domes (published in Maroutyan 1976). A single larger dome in the middle and four smaller domes over each corner of the church, over the round corner chambers. Thus, Avan would be the first such example of a church with five domes. The church has a four-pass floor plan with an octagonal central bay over which there was probably originally a dome (Fig. 1b).

A low, arched door leads from the west wall into the church and is surrounded by an ornate covering with three-quarter columns, which are decorated with capitals and bezels. A vischap stone from pagan times was used as a lintel above the main portal. Another door on the north wall, which was probably built at a later date, after the church was built, leads to the Catholic Palace. The church is also of immense architectural importance as the prototype of the much better known masterpiece, the St Hripsime in Ejmiatzin, Armenia.

Restoration history

In 1940–1941, 1956–1966 and 1968 restoration work was carried out on the church. Of note here is the oldest restoration from 1940–41, which was carried out according to the then most modern and still current principles of restoration theory (Brandi 2006). Compatible dry slaked lime mortars were used, which also has to be emphasized in terms of materials.

Damage forms and deterioration

The damages observed at Avan Cathedral are dominated by cracks and crackles of single ashlar, back-weathering, scaling and static problems of the top of the single building walls and pillars (Fig. 2c). At the Zvartnots ruins, the damage is primarily caused by widespread salt contamination associated with back-weathering and rounding (Fig. 2d, 4).

Historical photographs from the beginning of the 20th century show, that at some parts of the original walls even the historical whitish plaster was 139preserved (Fig. 2e). Today all the plaster surfaces are destroyed. Many of the stones are damaged by typical weathering forms associated with the presence and crystallization of salt (Fig. 2d, 4).

Figure 2: a) The remains of medieval architectural structures in Sanahin, Armenia shows the huge amounts of mortar within the building structure. b) Cross section of a wall at the Avan Cathedral. c) The critical static situation at the top of the walls at Avan Cathdral. Restoration mortar from the 1941 campaign is exposed in the lower parts of inner masonry. d) Widespread salt contamination and weathering at the Zvartnots Cathedral. e) Huge remnants of the inner masonry and preserved plaster on some walls at the Zwartnots Cathedral shown in the museum of the site (early 20^th century). f) Cement mortar injections done by restoration in the 1960 s also documented in the museum.

Methods of investigations

Onsite investigations were done by comparable observations, damage mapping, electrical capacity and conductivity measurements on single building stones.

The electrical conductivity and capacity is basically dependent on the material and is also influenced by the moisture content of the material and the presence of ionic compounds. The used device (surveymaster/protimeter) measures moisture in buildings and other related structures both on (conductivity) and below (capacity) the surface.

Sampling of the drilling powder was also done. Field studies were done on a southwestern wall of the Zwartnotz ruins and the westen front facade of the Avan Cathedral.

This study concentrates on the historical mortars of the Zvartnots Cathedral (ZM), the Avan Cathedral (AM) and the restoration mortar (RM) from 1941 used for Avan (Fig. 4). From these three mortars an analysis of their aggregate/binder ratio and the grading curve of its aggregate were done in a mobile laboratory. Petrographical and fabric analyses of the building materials were performed on thin sections under a polarization microscope and cathodoluminescence (CL) microscopy. Hydrostatic weighing on the samples was carried out to acquire the particle and bulk density as well as the porosity (DIN 52102). Mercury intrusion porosimetry was used to acquire the pore radii distribution (Fig. 3d, h, l).

Results
Methods of construction

Double shell masonry structures are characteristic of many ruins of the rich cultural heritage in Armenia. The building stones that form the exterior and interior masonry are held together between the two masonry shells by lime mortar and quarry stone masonry. The percentage of both structural elements in rising walls is about 50 % (Fig. 2a, b and e).

The inner shell of the masonry has a very solid and durable mortar. This mortar seems to literally glue the two wall shells together and seems to be the reason why many church buildings, as ruins, have still been partly preserved (Fig. 2a).

The mortars

The mortars of both historical buildings are lime mortars with volcanic aggregates with a high amount of glass-rich, amorphous material. These are characterized by a predominantly dense and binder-rich, light and dense matrix surrounded by aggregates of different sizes and color (Fig. 3a–3h). In contrast, the restoration mortar of the Avan Cathedral shows single quartz grains, feldspar crystals and sub-rounded inclusions clearly identified as lime accumulations (Fig. 3i). The historical Avan mortar (AM) mostly shows reddish, large pumice-rich grains (Fig. 3e). They reach a size up to 5 mm. The Zvartnots mortar shows more basaltic and andesitic fragments of sand particle size (0.063–2 mm). The matrix of both historical 140mortars is extremely dense and shows a firm connection to the grains (Fig. 3b, c and f and g). The Zvartnots mortar shows larger accumulations (–1 mm) of crystallized lime, which show multicolored effects in polarized light (Fig. 3b). These accumulations show a distinct red color under cathodoluminescence (CL) and could be a dolomitic lime. The matrix of the historical mortar shows a slight dark reddish structure, that can be identified as calcite-silica compounds (3c). A similar slight dark red color of the matrix could be observed for the mortar of the Avan Cathedral, while only light reddish lime accumulations are visible in CL-light (3g). The lime inclusions within the restoration mortar (RM) shows a similar intense light reddish color but no crystallization (Fig. 3k). The binding cement of the mortar is made from fine crystals, showing a multicolored birefringence in polarized light (Fig. 3j). These crystals are identified as calcium hydroxide or portlandite, which only forms single grain contacts. The blue glowing areas are probably claystone fragments (Fig. 3k) or alteration products (Fig. 3c).

Figure 3: The three investigated mortars. a–d) The mortar of the Zvartnots Cathedral (ZM), a) hand specimen, b) thin section under polarized light and c) under cathodoluminescence. d) The pore size distribution. e–h) The mortar of the Avan Church (AM) and i–l) the restoration mortar of Avan (RM) in the same order.

The pore size distribution of the three mortars is different. In the Zvartnots mortar smaller capillary active pores dominate the pore structure attaining 73 %. The microporosity (0.001–0.1 µm) reaches 20 % for the ZM and 28 % for the AM. Both mortars are characterized by a bimodal pore size structure, while the finer pores dominate (Fig. 3d, 3 h). The restoration mortar of the Avan Cathedral attains an amount of micropores of 64 %. A bimodal pore size structure is also formed in this case, dominated by small pores as well (Fig. 3l).

The historical mortars are lime mortars and show different binder contents. In the case of the original Avan Cathedral mortar, the lime content is 60 %, and only 25 % for the Zvartnots Cathedral. However, it must be taken in mind that the percentages are by weight and not by parts by volume. The Zwartnots mortar has a comparatively heavy aggregate mainly from basalt, which is why it also has the highest density of all mortars (Tab. 1). Therefore, the binder content appears relatively low. The binder-aggregate ratio of the restoration mortar (RM) shows a lime content of 34 %. In terms 141of the grading curve, there are huge differences between the historical mortars and the restoration mortar of the 1941 work. The historical mortars show a falling aggregate content from 0.5 mm to 63 µm and only a small amount of fine sands (Fig. 4). In contrast, the restoration mortar (RM) is dominated by fine sands with a clear domination of the grain size > 5 mm.

Figure 4: The grading curve of the three mortars.

Volcanic building stones

The tuffs, which were used in the Yerevan region in historical times, belong to the varieties of the Hoktemberyan Brown (HB) and the Hoktemberyan Red (HR) or the Hoktemberyan Black (HBl). Basalt is partly found at the Zvartnots Cathedral and can be compared with the Armenia Black (AB) variety. A detailed description of all building stones is given in Pötzl et al. 2018.

The porosity of the building stones ranges between 14.1–35.6 %, the particle density between 2.46 and 2.93 g/cm³ (Tab. 1). The porosity of the different mortars differs between 31.3 and 47.6 %, with densities between 1.2 and 1.7 g/cm³. Their particle densities range between 2.2 and 2.6 g/cm³ (Tab. 1).

Table 1: Porosity and density of the investigated stone and mortar samples. * Pötzl et. al 2018

Sample	Porosity (%)	Density (g/cm3)	Particle density (g/cm3)
HB*	35.0	1.62	2.49
HR*/Bl	35.6	1.59	2.46
AB*	15.9	2.25	2.67
ZM	31.3	1.78	2.60
AM	43.4	1.29	2.28
RM	47.6	1.28	2.45

Onsite investigations

The results of the mapping of the south-west wall of the Zvartnots complex show, that back-weathering, concentrated at the lower part of the wall is the main damage form covering more than 13 square meters of ashlar and more than 70 % of the whole study object (Fig. 5c).

These damages can be clearly associated with the effloresences of salts. Using test stripes, high amounts of sulfate (> 1,600 mg/l), chloride (1,000 mg/l) and nitrate (500 mg/l) could be detected. Loss of material, rounding and scaling only took place at around 3.5 m² of the investigated wall. Biological growth is not observable (Fig. 5c).

In a comparison of both buildings, which were built with comparable rock materials, values between 0–15 RF and 0–200 % WME speak for a largely intact condition.

For the Zwartnots cathedral electrical conductivity and capacity shows critical values in all measurement points by reaching more than 15 (% WME) in case of the conductivity and more than 400 (RF) for the capacity.

At the Avan Cathedral crack formation is the predominant weathering form, following by back-weathering and scaling. In Figure 6, these damage forms are depicted at one half of the façade, where also no relevant salt weathering was found. The electrical conductivity and capacity show critical values only in the lower part of the building, which is also partly effected by rising dampness (Fig. 6).

Discussion

Due to the lime inclusions, the RM mortar can be classified as a dry slaked mortar, that is known due to its sandwich-like preparation (Wedekind 2014). The RM mortar was made in a traditional way, but its durability is limited and not comparable with a hot mixed mortar by using amorphous aggregates, like the two described historical mortars. However, 142the RM mortar from the 1941 restoration work was still preserved in huge amounts. The RM mortar does not show any harmful side effects unlike the modern, cement-containing mortars used for the main restoration works in Zvartnots.

Figure 5: Damage mapping of a wall at the Zvartnots Cathedral ruin.

The two different historical mortars can be classified as hot-prepared mortars. In this case, the burned lime was probably mixed and slaked together with the amorphous siliceous aggregate. This could be the reason why no visible lime accumulation is visible. It is known, that the solubility of amorphous silicate increases sharply at a high pH in combination with heat (Iler 1979). All conditions are present during the mortar preparation, heat, a high pH of 14 and a lot of amorphous silicate material used as aggregate.

The high temperature reached by the exothermic reaction of the burned lime slaked with water and the high pH (14) seems to soften and chemically desolve the fine amorphous silicious particles within the aggregate and to form calicite-silica compounds. These cement-like, but salt-free and highly porous mortars seem to be the main reason, why very large parts of the Armenian cultural heritage is still preserved. These mortars also show a perfect compatibility with the highly porous building stones (Tab. 1). The tuff material has a high water uptake capacity and shows a comparably high resistance agains salt weathering (Pötzl et. al 2018).

Conclusions and further investigations

The high values of sulfate measured at the Zvartnots Cathedral could probably be due to the use 143of cement mortar, implemeted during restoration works at the ruins in the 1950s–1980s (Fig. 2f). Rain water could also be one possible source, since immense infiltration took place at many walls of the ruins. However, aggressive magnesium sulfpher salts could be formed if a dolomitic lime was used for the historical mortar. This should be clarified by further investigations.

Figure 6: Damage mapping of the right half of the main façade at the Avan Cathedral.

A proper desalination method has to be tested and applied. The first experimental conservation works during the summer school 2019 using poultices and the sprinkling method (Wedekind, Ruedrich 2006) have shown promising results.

The observations and results show that the restoration mortar, which is comparable to the historical mortar could be a proper material for both historical objects.

To discover the secret of its eternity and to prepare and develop a suitable and sustainable mortar for restoration, should be the main goal of future research.

Acknowledgements

We would like to thank all the participants of the 1th Summer School “Stones in Armenian Architecture” for their great work. We also thank the administration of heritage preservation of the Zvartnots side for their friendly cooperation, especially Grigor Nalbandyan as well as the Armenian Apostolic Church. We are grateful to Alfons van den Kerkof for allowing us access to the cathodoluminescence microscope at the Geoscience Centre of the University of Göttingen and for his friendly help and cooperation.

This work was generously supported by the Volkswagen Foundation (AZ93919).

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145

NOTES OF A BOWING BEHAVIOR ON LIMESTONE

Luís Sousa^1,2, Johanna Menningen³, Siegfried Siegesmund³

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 University of Trás-os-Montes e Alto Douro, Quinta de Prados, 5001-801 Vila Real, Portugal

2 Centro de Geociências da Universidade de Coimbra, Coimbra, Portugal

3 Geoscience Centre of the Georg August University Göttingen, Germany

Abstract

Eleven Portuguese limestones were evaluated regarding their bowing behavior. Only one variety showed meaningful bowing values under thermohydric conditions. The slabs of this variety, named Valverde, show a constant rate of deformation and reach bowing values of about 8 mm/m. While under dry heating cooling cycles no deformation was observable, the addition of water lead to increasing deformation values. On the contrary, alternating dry and wet cycles enable the limestone samples to recover during the dry cycles. Only continuously wet conditions imprint permanent bowing.

Petrographic observations and dilatometry tests allow to identify several stylolites filled with iron oxides as a possible reason for this behavior. These irregular fractures open under thermohydric conditions. Beyond a certain opening limit, the asperities inhibit the minerals to retake the initial position and deformation will be permanent. Therefore, this stone should not be used under wet conditions combined with higher temperatures. Those conditions would cause a bowing behavior as reported in several cases.

Keywords: limestone; bowing; petrographic properties

Introduction

Bowing behavior has been described for different rock types. Nevertheless, marble is the stone mostly prone to such a permanent deformation. The specific textural characteristics of marble together with anisotropic calcite crystal properties causes thermal strain within grain to grain contacts and originates bowing (Siegesmund et al. 2000; Siegesmund et al. 2008). Although this phenomena was described for many rock types, as for example 146even weathered granites can display bowing behavior (Siegesmund et al. 2018), there are only scarce cases reported for limestones (Siegesmund 2008).

Figure 1: Macroscopic appearance of Valverde limestone (honed surface; size of the photo: 10 cm × 10 cm).

Table 1: General characteristics and classification (according Dunham (1962) and Folk (1962)) of the selected limestones. The sample VAV is the Valverde variety.

Sample	General characteristics	Classification
ALP	Grey limestone composed of a micritic groundmass (95 %) and 5 % of components	Pelagic Mudstone (after Dunham, 1962) and Micrit (after Folk, 1962)
ATAZ	Light cream limestone with 50 % groundmass and 50 % components	Peloidal wackestone (after Dunham, 1962) and Pelmicrit (after Folk, 1962)
ATCR	Cream coloured limestone with 60 % groundmass and 40 % components	Peloidal wackestone and packstone (after Dunham, 1962) and Pelmicrit (after Folk, 1962)
CODFV	Light grey limestone composed of 60 % groundmass and 40 % components	Ooid-peloid grainstone (after Dunham, 1962) and Oopelsparit (after Folk, 1962
LIOZ	Fully recrystallized limestone composed of calcite microcrystals (sparit and microsparit)	Dolosparit (after Folk, 1962)
MCCT	Light cream limestones composed of 50 % groundmass and 50 % components	Bioclastic grainstone (after Dunham, 1962) and Biopelsparudit (after Folk, 1962)
SBM	Light cream limestone composed of 40 % groundmass and 60 % components	Peloidal grainstone (after Dunham, 1962) and Pelsparit (after Folk, 1962)
SBR	Light cream limestone composed of 40 % groundmass and 60 % components	Ooid grainstone (after Dunham, 1962) and Oosparudit (after Folk, 1962)
VAV	Light to medium grey limestone composed of 70 % groundmass and 30 % components.	Bioclastic packstone/grainstone (after Dunham, 1962) and Biosparit/Biodismicrit (after Folk, 1962).
VPAZ	Medium grey limestone composed of a micritic groundmass (> 90 %) and less than 10 % of components	Pelagic Mudstone (after Dunham, 1962) and Micrit (after Folk, 1962)
VPCR	Light grey limestone composed of 60 % groundmass and 40 % components	Bioclastic floatstone (after Dunham, 1962) and Biomicrudit (after Folk, 1962).

For a Portuguese limestone (usually named Valverde) several stoneworkers described bowing phenomena. In order to evaluate these observations systematically, eleven limestone varities were selected and analysed: Alpinina (ALP), Ataíja Azul (ATAZ), Ataíja Creme (ATCR), Semi-rijo Codaçal (cut pararellel to the sedimentary lamination, CODFV), Lioz (LIOZ), Moca 4M (cut perpendicular to the sedimentary lamination, MCCT), Semi-rijo Branco Mais(i) (SBM), Semi-rijo Branco Real (SBR), Vidraço Azul Valverde (VAV); Vidraço Portela Azul (VPAZ); Vidraço Portela Creme (VPCR).

The limestone samples used in this study where collected in the Maciço Calcário Estremho (MCE), a Jurassic limestone massif located in the Portuguese Lusitanian Basin, with the exception of one sample (LIOZ) collected in a Cretaceous Unit near Lisbon (Silva, 2017). MCE is one of the world’s leading regions producers of limestones for ornamental purposes. The quarries develop along the hillside, with several benches, can reach depths around 50 m and are grouped in several exploitation sites. Several lithostratigraphic units are exploited and originate different ornamental varieties, from cream laminated calcarenites to grey calciclastic micritic limestones (Carvalho and Lisboa, 2018). The samples selected from MCE show this high variability.

Figure 2: Microstylolites (right in the photo) and non-disturbed matrix (left in the photo) in the sample VAV (crossed polars).

147Besides a petrographic characterization, the hydric expansion caused by moisture exposure was evaluated as well as the bowing potential and thermal expansion measurements were performed.

Material

The detailed petrographic characterization of the Valverde sample highlights some interesting features; the limestone shows a partially recrystallized groundmass, consisting of around 50 % micritic and 20 % of dismicritic components and peloidal microsparit.

Microscopically it is possible to recognize that there are zones (patches) composed of micritic matrix and an other one composed of sparitic cement, giving the rock an inhomogeneous effect similar to bioturbation. All components are smaller than 2 mm composed of bioclasts, pelloids, pseudo-pelloids, benthic foraminifers, echinoderms and several spicules of crinoids. The fabric of the sample varies locally from mud- to grain- supported.

An interesting aspect of this sample is the presence of disturbed areas where parallel microstylolites appear, affecting the groundmass. The microstylolites consist of an opaque material (black) probably accumulations (laminaes) of organic material and clays.

According to Flügel (2004) this limestone belongs to the RMT 14, Bioclastic packstone and wackestone with skeletal grains, various amounts of intraclasts and some ooids (shoal-near). These sediments are common for the open marine, inner ramp zone.

Methods

The hydric expansion was determined for all limestones on cylindrical samples (Ø 15 mm × 100 mm). The length change of those cylinders was measured under water-saturated conditions. The samples were completely immersed in demineralized water for 24 hours, meanwhile the expansion of the samples was measured with a displacement transducer. The resolution of the transducer is 0.1 µm and the accuracy is about 0.5 µm.

Figure 3: Evolution of the moisture expansion during a 24 hours test.

Figure 4: Bowing behavior of the selected limestones under wet and dry cycles.

Figure 5: Bowing effect after heating cycles.

To determine the bowing potential and its directional dependence, slabs of 40 × 10 × 3 cm were exposed to heating cycles (20–80–20 °C per day) simulating in an accelerated procedure the conditions at the building. Additionally, the specimens were 148exposed to thermohydric conditions, as the slabs were moistened on one side and cyclically heated on the reverse slab surface. The slabs were heated for 6 hours before they cool down to ambient temperature and the water was refilled 2 hours before the next heating cycle started. The bowing was measured by means of a measuring bridge with an accuracy of l µm/35 cm. A total of 91 cycles were performed (for details see Koch and Siegesmund 2002).

Figure 6: Progressive increase of residual strain for 20–90–20 °C cycles of the VAV samples. Investigated as a function of the number of heating cycles under dry (3 cycles, red background) and wet (4 cycles, blue background) conditions.

Thermal expansion measurements took place in the temperature range of 20–90 °C using a pushrod dilatometer (for details see Koch and Siegesmund 2004). The specimens had a length of 50 mm and a diameter of 15 mm. The residual strain (RS) was determined within seven heating–cooling cycles: 3 dry cycles 20–90–20 °C and 4 wet cycles 20–90–20 °C, in the Z and XY directions. RS is determined by the ratio between the sample length change after the heating–cooling cycles and the initial length (Shushakova et al. 2013).

Results and Discussion

The values of hydric dilatation are lower than 0.09 mm/m, with excepetion of the Valverde (VAV) samples which reach 0.22–0.26 mm/m.

Usually limestones show low hydric dilatation values, because these rocks have low clay content (Siegesmund and Dürrast, 2014). The dilatation of the VAV sample could be explained by some possible clay content within the dark material observed in microstylolites. Further investigations must be performed to clarify this hypothesis.

The bowing tests show the absence of permanent changes in almost all of the selected limestone. In fact, only the Valverde (VAV) sample is affected by this experiment. In the first 24 wet cycles the bending increases continuosly and reaches the value of 7 mm/m. Followed by dry conditions, the sample shows some recovery down to 6 mm/m within the first cycles. This value remains stable during the next 41 dry cycles. When the wet cycles start again a new increase of the bowing is evident with a maximum expansion around 9 mm/m.

149High values of bowing are frequently observed in marbles (e. g. Siegesmund et al. 2008) but unknown in limestones. Furthermore, as the mineralogy of all selected limestones is similar the bowing of the VAV sample turns strange. The only different factor are the microstylolites, which therefore seem to be the reason for the bowing potential. Since no bending occurs when dry cycles are performed, the water is an important factor. The interaction of heat and water seems to open and offset the microstylitic cracks, avoiding the recontraction and resulting in an irreversible deformation.

The thermal expansion measurements for the VAV sample show some residual strain about 0.25 mm/m after 4 wet cycles, while the performed dry cycles do not exhibit permanent deformation. Additionally, the sample has been measured in two directions, Z and XY, perpendicular and parallel to the microstylolites considering any effect caused by directional dependence. As depicted in Fig. 4, the expansion of both directions is comparable. Consequently no directional dependence of the expansion can be observed.

Residual strain is only observed under wet conditions as well as the bowing behavior is only evident under wet conditions. Despite the values of residual strain are lower than the range of values found in marbles exhibiting bowing (Mennigen et al. 2018), they confirm the bowing behavior of this sample. The areas with parallel microstylolites affecting the groundmass are aleatory distributed in the limestone and each one possibly contributes the total bending of the sample.

These preliminary results show the influence of the water in the bowing behavior under thermal cycles. However the nature of the microstylolites needs to be completely analysed, especially if clays are present. Therefore, more studies are necessary to understand completely the anomalous bowing behavior of the Valverde limestone.

Conclusions

The boeing potential of eleven varieties of Portuguese limestones were studied. Only one variety (Valverde) show evident bowing, reaching values around 9 mm/m after 91 cycles. This variety is also the only one showing hydric dilatation with values around 0.26 mm/m after 24 hours. Residual strain happens only in wet conditions, with values around 0.25 mm/m.

Petrographic observations of this variety highlight the presence of disturbed areas with parallel microstylolites. The microstylolites are of an opaque material, probably accumulations of organic material and clays. Both water and heat may cause the opening and offsetting of the microstylolites causing irreversible deformation.

Further studies are necessary to fully explain this bowing behavior. Meanwhile, this stone should not be used as a cladding material in a humid and warm environment, where bowing behavior can occur more easily.