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

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

CALCIUM OXALATECALCIUM OXALATE

T. Mifsud & J. Cassar

Institute for Masonry and Construction Research, University of Malta, Malta

HWC 2006 – MADRID

© http://histgeo.ac-aix-marseil le.fr300 km0

Maltese Islands

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



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)



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



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



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



Samples used

“Franka” Limestone

Fresh quarry samples Naturally weathered samples Artificially weathered samples

Non-desalinatedSoluble salts present

Desalinated Non-desalinatedSoluble salts present

Desalinated Non-desalinatedSoluble salts present

Desalinated

Treated Not treated

Treated Not treated

Treated Not treated

Treated Not treated

Treated Not treated

Treated Not treated





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

Results

Treated sample type Oxalate peak intensity/ calcite peak intensity

Halite peak intensity/ calcite peak intensity

Quarry Desalinated

14 % < 2 %

Quarry Non Desalinated

35 % 2 %

Naturally Weathered Desalinated

12 % 3 %

Naturally Weathered Non Desalinated

15 % > 100 %

Artificially Weathered Desalinated

9 % < 2 %

Artificially Weathered Non Desalinated

10 % > 100 %

Quarry “Soll” (Croveri 2004) 9 % 2 %

The Victory Monument, Birgu

10 % 3 %

The Zammit Monument, Valletta

17 % 3 %



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



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/

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