Gypsum crystallization on natural building stones due to sulphatation processes is a well known and widely described phenomenon in deterioration studies. The gaseous SO2 of the polluted environment reacts with the carbonate of the stone, resulting into gypsum (CaSO4.2H2O) crystallization on the surface and inside the porous structure of the material. In order to obtain quantitative information on porosity and pore structure change before, during and after sulphatation, the process should be monitored on the same sample.With the aid of non-destructive high-resolution X-ray computed tomography we investigated various types of porous limestones before, during and after in vitro-induced gypsum crystallization processes. A 3D image with a resolution of 2.5 µm was obtained during each step of gypsum crust formation. Besides the visualization of the gypsum crystallization, also quantitative information on porosity and pore structure changes could be obtained. For each stone type, radial porosity, open pore structure, partial porosity and distribution of equivalent and maximum opening of the pores has been calculated with the software program Morpho+. The blistering of the gypsum crust on the calcareous sandstone has been quantified, and the secondary porosity provoked by the dissolution of calcite has been calculated in 4 dimensions. The results of this research contribute to a better understanding of the evolution of pore structure changes due to deterioration. This process results in zones of changed porosity with a subsurface weakening due to loss of coherence and crustal induration. A better understanding thus allows quantification of future material loss and durability change.
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