THE TREATMENT OF WEATHERED GLOBIGERINA LIMESTONE: THE SURFACE CONVERSION OF CALCIUM CARBONATE TO CALCIUM OXALATE T. Mifsud & J. Cassar Institute for Masonry and Construction Research, University of Malta, Malta HWC 2006 – MADRID

T. Mifsud & J. Cassar Maltese Islands Institute for Masonry and Construction Research, University of Malta T. Mifsud & J. Cassar

Globigerina Limestone – geology and use Forms part of the geological sequence composed of, from top to bottom - Upper Coralline Limestone - Greensand - Blue Clay - Globigerina Limestone - Lower Coralline Limestone Globigerina Limestone is found in three layers (upper, middle and lower) The lower Globigerina Limestone is used in construction due to its homogeneity It is the main building stone of the Maltese Islands both in the past as well as today Institute for Masonry and Construction Research, University of Malta T. Mifsud & J. Cassar

Globigerina Limestone – composition Fine-grained limestone, full of Globigerinae and visible fossils (scallop shells and burrowing sea urchins) Primarily composed of calcium carbonate; calcite crystals cemented together by non-crystalline calcium carbonate Clay minerals (up to 12% depending on stone type) Quartz (up to 8 %) Feldspars, apatite and glauconite Porosity = 32 to 41% Micro-pore structure = majority of pores ≤ 4 µm Institute for Masonry and Construction Research, University of Malta T. Mifsud & J. Cassar (Cassar 1999 & 2002, Cassar & Vannucci 2001)

Globigerina Limestone – “franka” and “soll” Occurs as two types “Franka” type: good quality limestone, weathers well “Soll” type: poor quality limestone, weathers badly “Franka” and “soll” differ in their mineralogical composition and physical properties “Soll” limestone is richer in the non-carbonate fraction “Soll” limestone has a lower overall porosity “Soll” limestone has a higher proportion of small pores Institute for Masonry and Construction Research, University of Malta T. Mifsud & J. Cassar

Globigerina Limestone – deterioration The historical buildings and monuments were built without damp proof courses Typical local construction includes a double skin of masonry with soil infill The local marine environment is a source of soluble salts Physical degradation thus results due to salt damage of the highly porous Globigerina Limestone Chemical degradation also results from acidic conditions resulting from polluted environments Deterioration manifestations include powdering, flaking, alveolar decay, back weathering and erosion Institute for Masonry and Construction Research, University of Malta T. Mifsud & J. Cassar

Methodology of the study It is believed that many of the surviving historical buildings and monuments are composed of the “franka” limestone type due to their reduced deterioration Due to the context of the local “franka” limestone buildings and monuments ammonium oxalate treatment seems promising Studies with ammonium oxalate treatment on Globigerina Limestone have so far included fresh quarry “franka” and “soll” and weathered “soll” types The investigation of an induced calcium oxalate surface of weathered “franka” limestone was the next step that has led to this research Institute for Masonry and Construction Research, University of Malta T. Mifsud & J. Cassar

Samples used “Franka” Limestone Fresh quarry samples Naturally weathered samples Artificially weathered samples Non-desalinated Soluble salts present Desalinated Treated Not treated Institute for Masonry and Construction Research, University of Malta T. Mifsud & J. Cassar

After treatment the samples, both treated and untreated, were tested Treatment and testing A 5% ammonium oxalate poultice was applied for 5 hours at 28°C and 75% RH by means of a cellulose pulp After treatment the samples, both treated and untreated, were tested This first phase concerns the verification of the conversion from carbonate to oxalate using X-Ray Diffraction Also included in the testing were 2 exposed Globigerina Limestone monuments and “soll” limestone samples, all treated with an ammonium oxalate poultice in 2003 by others Due to the small amounts of sample available for testing from the monuments, Synchrotron analysis was opted for Institute for Masonry and Construction Research, University of Malta T. Mifsud & J. Cassar

Results T. Mifsud & J. Cassar Treated sample type Oxalate peak intensity/ calcite peak intensity Halite peak Quarry Desalinated 14 % < 2 % Non Desalinated 35 % 2 % Naturally Weathered 12 % 3 % 15 % > 100 % Artificially 9 % 10 % Quarry “Soll” (Croveri 2004) The Victory Monument, Birgu The Zammit Monument, Valletta 17 % Institute for Masonry and Construction Research, University of Malta T. Mifsud & J. Cassar

The non-desalinated samples formed larger amounts of whewellite Conclusions All the treated samples formed whewellite, whereas weddellite was never formed The non-desalinated samples formed larger amounts of whewellite It is hypothesised that this is due to the larger surface area available for reaction with the ammonium oxalate poultice, present in the non-desalinated samples The presence of sodium chloride does not inhibit the successful formation of whewellite Institute for Masonry and Construction Research, University of Malta T. Mifsud & J. Cassar

Acknowledgements The authors would like to thank: Dr. Emmanuel Pantos from the Daresbury Synchrotron Radiation Source (SRS), UK Architect Chris Falzon, Chief Executive Officer of VISET (Malta) plc. Agius Stone Works Ltd. Dr. Ray Bondin, Executive Coordinator of the Valletta Rehabilitation Project Dr. Paola Croveri Architect Tano Zammit Architecture Project (AP), Malta The Institute for Masonry and Construction Research of the University of Malta http://home.um.edu.mt/masonry-construction/ Institute for Masonry and Construction Research, University of Malta T. Mifsud & J. Cassar