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| Titel |
Porosity change after gypsum crust formation on macro-porous limestones |
| VerfasserIn |
Jan Dewanckele, Veerle Cnudde, Tim De Kock, Marijn Boone, Matthieu Boone, Luc Van Hoorebeke, Patric Jacobs |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038791
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| Zusammenfassung |
| The deterioration of stone is a complex process in which physical, biological and chemical
mechanisms are involved. In this research, pore structure changes inside two types of porous
limestone were analyzed before, during and after strong acid tests with SO2. Sulphatation
and crust formation phenomena on natural building stones exposed to a polluted
environment, are largely described in literature. As far as rocks rich in calcium
carbonate are concerned, 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 accumulation in
porous materials and at the surface of less porous media. The main actor in the
process of gypsum crystallization 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 the standard EN 13919:2002E, samples were put in acid environment for
21 days. At the bottom of the container a mixture of 500 ± 10 ml H2SO3 and 150 ± 10 ml
de-mineralized H2O was added. No airborne particles or oxides of nitrogen (NOx) were
added. Before exposure, after 6 days in the polluted environment and at the end of
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 allows 2D and 3D reconstructions
on a micrometer scale of the internal structure of an object without damaging the
material. It thus enables to scan the same sample in a sequential way. In order to obtain
information about the sample’s interior of which the characteristics can be compared
before, during and after the test, the same scanning parameters (exposure time,
amount of frames, energy, etc.) were used. In addition, the same adjustments like
beam hardening correction, normalizing, ring and spot filter, etc. were applied for
the reconstruction. Total porosity, open and closed porosity and radial porosity
were calculated for each sample by using the in-house developed software program
Morpho+.
The analysis of the various scans revealed that the Euville limestone developed a distinct
gypsum crust, behind which a secondary porous layer of 100 μm thickness had
developed. Inside the sample the porosity decreased by infilling of the large pore
spaces with gypsum. However, after 6 days exposure the total porosity of the sample
increased from 5.70% to 8.45%. In this case, the formation of secondary porosity
behind the newly formed exterior gypsum layer prevailed upon the crystallizing of
gypsum inside the pores located in the sample’s interior. Also, the firstly formed
gypsum crystals prevented the further interaction of the sulphuric acid with the stone
material. After 21 days, the total porosity of the sample still reached 8.45%. The
results of the radial porosity measurements were also the same after 6 and 21 days,
indicating that the secondary porosity and the filling of pores inside the samples were
stabilized.
On the other hand, the gypsum crust on the Savonnières limestone was less visible. No
secondary formed porous layer 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. The combination of micro-CT,
image analysis and induced weathering tests are a promising combination of tools
and techniques that allow for a better understanding of gypsum crust formation
and pore structure change just behind the crust and deeper inside the rock sample. |
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