Ion transport in porous media studied by NMR

Moisture and salt transport in masonry can give rise to damages. Therefore a detailed knowledge of the moisture and salt transport is essential for understanding the durability of masonry. A special NMR apparatus has been made allowing quasi-simultaneous measurements of both moisture and Na profiles in porous building materials. Using this apparatus both the absorption of a 4 M NaCl solution in a calcium silicate brick and the drying of a 3 M NaCl capillary saturated fired-clay brick have been studied. It was found that during the absorption process the Na ions clearly stay behind, which this is caused by adsorption of these ions to the pore surface. For the drying it was found that at the beginning of the drying process the ions accumulate near the surface. As the drying rate decreases, diffusion becomes dominant and the ion profile levels off again.