THE TREATMENT OF WEATHERED GLOBIGERINA LIMESTONE: THE SURFACE CONVERSION OF CALCIUM CARBONATE TO CALCIUM OXALATE

1. 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

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

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

4. T. Mifsud & J. Cassar Institute for Masonry and Construction Research, University of Malta 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 (Cassar 1999 & 2002, Cassar & Vannucci 2001)

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

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

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

8. T. Mifsud & J. Cassar Institute for Masonry and Construction Research, University of Malta Samples used

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

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

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

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