Transcript Slide 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
© http://histgeo.ac-aix-marseille.fr
Maltese Islands
0 300 km
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
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)
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
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
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 buildings and monuments “
franka
” limestone 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
T. Mifsud & J. Cassar Institute for Masonry and Construction Research, University of Malta
Samples used
“Franka” Limestone Fresh quarry samples Naturally weathered samples Artificially weathered samples Non desalinated Soluble salts present Treated Not treated Desalinated Treated Not treated Non desalinated Soluble salts present Treated Not treated Desalinated Treated Not treated Non desalinated Soluble salts present Treated Not treated Desalinated Treated Not treated 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
T. Mifsud & J. Cassar Institute for Masonry and Construction Research, University of Malta
Results
Treated sample type
Quarry Desalinated Quarry Non Desalinated Naturally Weathered Desalinated Naturally Weathered Non Desalinated Artificially Weathered Desalinated Artificially Weathered Non Desalinated Quarry “Soll” (Croveri 2004) The Victory Monument, Birgu The Zammit Monument, Valletta
Oxalate peak intensity/ calcite peak intensity
14 %
Halite peak intensity/ calcite peak intensity
< 2 % 35 % 12 % 2 % 3 % 15 % > 100 % 9 % < 2 % 10 % > 100 % 9 % 10 % 17 % 2 % 3 % 3 %
T. Mifsud & J. Cassar Institute for Masonry and Construction Research, University of Malta
Conclusions All the treated samples formed whereas weddellite was never formed whewellite,
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 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/ T. Mifsud & J. Cassar Institute for Masonry and Construction Research, University of Malta