Pore size distribution from hydrogen and sodium NMR using the transverse relaxation

Salts in porous building materials can crystallize during drying, which may occur at the surface, causing defacing, or just under the surface, where it may cause structural damages, e.g., delamination, surface chipping or disintegration. Therefore a detailed knowledge of the moisture and salt transport is essential for understanding the durability of building materials. Up to now only a few studies have been reported on unsaturated moisture and ion transport in porous media, because suitable measurements techniques are lacking. About the issues which still have to be studied experimentally and theoretically are: chemical and physical interaction of ions with a material, supersaturation of ions in a pore system and the growth of crystals in pores [1]. For these studies it is important to know the ion distribution within the pore system. Up to now usually only the transverse relaxation of hydrogen is used to determine the pore water distribution [2]. In this study the transverse relaxation of sodium as measured at 4.7 T was used to determine the distribution of the sodium ions in a saline solution in various porous materials. First the transverse relaxation of hydrogen and sodium were compared for Nucleosil samples of different pores sizes. These measurements show clearly that also the transverse relaxation of the sodium can be linked to the pore size. The upper limit is set by the bulk relaxation of sodium. These measurements were also done for materials having a bimodal pore size distribution like mortar and calcium silicate brick. Also in these cases the sodium measurements give a clear bimodal distribution.