Boone, Matthieu

Cnudde, Veerle

De Kock, Tim

Dewanckele, Jan

Jacobs, Patric

Van Hoorebeke, Luc

English

Ghent University Academic Bibliography

Geophysical Research AbstractsVol. 12, EGU2010-8189, 2010EGU General Assembly 2010© Author(s) 2010Porosity change after gypsum crust formation on macro-porouslimestonesJan Dewanckele (1), Veerle Cnudde (1), Tim De Kock (1), Marijn Boone (1), Matthieu Boone (2), Luc VanHoorebeke (2), and Patric Jacobs (1)(1) Ghent University, Geology and Soil Science, Ghent, Belgium (jan.dewanckele@ugent.be, +32/(0)9 264 66 97), (2) GhentUniversity, Subatomic and Radiation Physics, UGCT, Ghent, BelgiumThe deterioration of stone is a complex process in which physical, biological and chemical mechanisms are in-volved. In this research, pore structure changes inside two types of porous limestone were analyzed before, duringand after strong acid tests with SO2. Sulphatation and crust formation phenomena on natural building stones ex-posed to a polluted environment, are largely described in literature. As far as rocks rich in calcium carbonate areconcerned, the main processes involved are the dissolution of the calcium carbonate and the formation of gypsum(CaSO4.2H2O) in presence of an acid atmosphere. The low mobility of this newly formed salt favours its accu-mulation in porous materials and at the surface of less porous media. The main actor in the process of gypsumcrystallization on limestone is the aggressive sulphur dioxide gas (SO2).In this study, the Savonnières and Euville limestone were subjected to tests with a strong acid. According to thestandard EN 13919:2002E, samples were put in acid environment for 21 days. At the bottom of the containera mixture of 500 ± 10 ml H2SO3 and 150 ± 10 ml de-mineralized H2O was added. No airborne particles oroxides of nitrogen (NOx) were added. Before exposure, after 6 days in the polluted environment and at the endof the test, the two samples were scanned with X-ray computed tomography (X-ray CT) at the Centre for X-ray Tomography at Ghent University, Belgium (UGCT; www.ugct.ugent.be). This visualization technique allows2D and 3D reconstructions on a micrometer scale of the internal structure of an object without damaging thematerial. It thus enables to scan the same sample in a sequential way. In order to obtain information about thesample’s interior of which the characteristics can be compared before, during and after the test, the same scanningparameters (exposure time, amount of frames, energy, etc.) were used. In addition, the same adjustments like beamhardening correction, normalizing, ring and spot filter, etc. were applied for the reconstruction. Total porosity, openand closed porosity and radial porosity were calculated for each sample by using the in-house developed softwareprogram Morpho+.The analysis of the various scans revealed that the Euville limestone developed a distinct gypsum crust, behindwhich a secondary porous layer of 100 µm thickness had developed. Inside the sample the porosity decreasedby infilling of the large pore spaces with gypsum. However, after 6 days exposure the total porosity of the sampleincreased from 5.70% to 8.45%. In this case, the formation of secondary porosity behind the newly formed exteriorgypsum layer prevailed upon the crystallizing of gypsum inside the pores located in the sample’s interior. Also, thefirstly formed gypsum crystals prevented the further interaction of the sulphuric acid with the stone material. After21 days, the total porosity of the sample still reached 8.45%. The results of the radial porosity measurements werealso the same after 6 and 21 days, indicating that the secondary porosity and the filling of pores inside the sampleswere stabilized.On the other hand, the gypsum crust on the Savonnières limestone was less visible. No secondary formed porouslayer was measured and the total porosity decreased from 12.10% to 10.94% after 6 days and further to 10.31%at the end of the test. The decrease of porosity was still measurable at a depth of 500 µm inside the sample. Thecombination of micro-CT, image analysis and induced weathering tests are a promising combination of tools andtechniques that allow for a better understanding of gypsum crust formation and pore structure change just behindthe crust and deeper inside the rock sample.

Porosity change after gypsum crust formation on macro-porous limestones

https://biblio.ugent.be/publication/1092475/file/1092501

Porosity change after gypsum crust formation on macro-porous limestones

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