The use of high resolution X-ray CT and image analysis to quantify open porosity changes after induced weathering tests on porous geomaterials

Weathering processes of geomaterials have been studied in detail on many natural building stones. The most commonly used analysis techniques in these studies are thin-section petrogra-phy, SEM, XRD and XRF. Most of these techniques are valuable for chemical and mineralogical analysis of the weathering patterns. However, to obtain crucial quantitative information on struc-tural evolutions, such as porosity changes and growth of weathering crusts in function of time, non-destructive techniques become necessary. In this study, a Belgian historical calcareous sand-stone, the Lede stone, was exposed to gaseous SO2 under wet surface conditions according to the European Standard NBN EN 13919:2002. Before, during and after sulphatation processes, high resolution X-ray CT was performed to visualize gypsum crust formation to obtain a better insight into the effects of gaseous SO2 on pore modification in 3D. The tomographic scans were taken at the Centre for X-ray Tomography at Ghent University (UGCT). With the aid of image analysis software, partial porosity changes were calculated at different stages of the process. In this study, several X-ray CT scans of the same sample were taken with a laboratory based X-ray source. The great advantage of this approach is the ability to measure changes in porosity, evolutions of micro-cracks and disintegration in the same rock sample over a specific time period. Additionally, the relationship between days of weathering and pore network evolution was investigated with a resolution of 2.5 µm. Increasing porosity was observed visually and quantita-tively below the newly formed superficial layer of gypsum crystals. In some cases micro-cracks and dissolution zones were detected on the grain boundaries of quartz. With the aid of Morpho+, an image analysis program developed in-house, radial porosity, partial porosity, ratio of open and closed porosity and volume of individual pore structures were calculated. This non-destructive imaging approach turned out to be very valuable for qualitative (visualiza-tion of dissolution processes, study of micro-crack initiation, pore infilling, etc) and quantitative (calculation of partial porosity, estimation of dimensional changes, computation of interconnec-tivity, etc.) characterization of the pore modification processes in a natural building stone in full 3D. By combining the results with traditional techniques, including polarization microscopy and SEM, a better interpretation of the weathering processes has been obtained. The approach outlined relies on a combination of high resolution X-ray CT, 3D image analysis and the use of traditional techniques that can be employed on a variety of rocks and weathering processes. Because water flow and thus weathering depends acutely on the pore structure inside materials, changing structures as described and quantified in this study are of the highest im-portance for better understanding of the weathering processes, which involve moisture or water. However, it should be noted that this study is primarily concerned with the time-lapsed quantifi-cation of changing pore structures in three dimensions. Pore modifications due to weathering are complex processes of which high resolution X-ray CT can provide a deeper insight. However, some care must be taken, because different processes are responsible for the alteration of the stone. In this study only SO2 is taken into account while in realistic situations biological activity, temperature changes, rain, sheltering etc. can also play an important role