Weathering processes have been studied in detail for many natural building stones. The most commonly used analytical techniques in these studies are thin-section petrography, 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 structural evolutions like porosity changes and growth of weathering crusts in function of time, non-destructive techniques become necessary. In this study, a Belgian historical calcareous sandstone, the Lede stone, was exposed to gaseous SO(2) under wet surface conditions according to the European Standard NBN EN 13919 (2003). Before, during and after the strong acid test, high resolution X-ray tomography has been performed to visualize gypsum crust formation to yield a better insight into the effects of gaseous SO(2) on the 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, partial porosity changes were calculated in different stadia of the process. Increasing porosity has been observed visually and quantitatively below the new superficial formed layer of gypsum crystals. In some cases micro-cracks and dissolution zones were detected on the grain boundaries of quartz. By using Morpho+, an in-house developed image analysis program, radial porosity, partial porosity, ratio of open and closed porosity and equivalent diameter of individual pore structures have been calculated. The results obtained in this study are promising for a better understanding of gypsum weathering mechanisms, porosity changes and patterns on natural building stones in four dimensions.

Black crust growth mechanisms on three French building stones are described using diagenetic models that reveal the close links between the crust–stone interfaces and the microfacies of the host limestone. Each limestone is representative of a specific sedimentary facies and displays mixed pore structure: crinoidal limestone (Euville limestone), oolitic limestone (Savonnières limestone) and bioclastic matrix-supported limestone (Courville limestone). The crinoidal limestone is mainly made of well-developed calcitic cement (spar syntaxial calcite) with low macrocroporosity (15–20 vol. %). The oolitic limestone is macroporous (30–40 vol. %), oolite nucleus being partially or completely dissolved. The third building stone studied is less porous (14 vol. %) but presents a significant microporosity.Weathering of the Euville limestone proceeds primarily through preferential exploitation of cleavages and microcracks and secondly by progressive recrystallization in the areas separated by previous gypsum fill-in (micro-box work). In the Savonnières limestone (oolitic limestone), gypsum recrystallization could occur without microcracks: elements are sometimes nearly totally weathered, while the palisadic calcitic cement surrounding the oolites was still preserved. In the matrix-supported limestone (Courville limestone), weathering could deeply affect the matrix while elements are not weathered. When a layer of microcrystalline calcite is observed on the surface of the limestone, however, the black crust growth seems to be limited to the external part of the stone.Porous characteristics of limestones directly depend on sedimentary and diagenetic phases developed. The pore network controls moisture movement and also determines the reactivity of the stone to gypsum recrystallization.

Quantitative investigation of the crystallisation of NaCl from evaporating droplets.-Infrared thermography revealed different crystal growth processes.-The thermosignal depended on variations in the emissivity and temperature.-Intermittent drops in the thermosignal revealed solution creeping processes.-Infrared thermography permitted, for the first time, the in-situ observation of the creeping phenomenon.

Lutetian limestones of the Paris Basin were used from antiquity to today as building materials (e.g. Paris and Rheims Notre-Dame cathedrals), but quarries gradually closed down and started to disappear from the landscape and hence from memory. These limestones show important vertical and lateral variations and shift of facies, and, within the same area, the building stones can have very different petrographical and petrophysical properties. The use of sometimes very different stones may cause problems for the conservation of cultural heritage monuments because of their great response variability to treatments. In our study area, the building stone's sedimentological and petrophysical properties were characterized, as well as their behaviour in construction, in particular with respect to their durability or state of conservation. The cartography of these various microfacies, from abandoned quarries and old buildings, would allow a better management o fmineral resource sin this region.

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