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

3. 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

4. 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)

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

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