Experimental evidence of crystallization pressure inside porous media.

Crystallization pressure of salt in porous materials is one of the mechanisms that may induce serious damage, for example, weathering of buildings and monuments of cultural heritage. Since this pressure also causes the solubility of the salt inside a porous material to differ from the bulk solubility, it can be assessed experimentally by measuring the solubility inside the pores. We show that this is possible by NMR, and study Na(2)CO(3) and Na(2)SO(4) in a series of model porous materials. Using the solubility data the crystal-liquid surface energies are estimated as gamma = 0.09 N/m for Na(2)CO(3) . 10H(2)O and gamma = 0.06 N/m for Na(2)SO(4) . 10H(2)O. For pore sizes below about 30 nm, the resulting pressure exceeds the tensile strength of typical building materials (3 MPa). No pressure is induced by the metastable Na(2)SO(4) . 7H(2)O, which suggests for this crystal a value of gamma close to zero.

Sodium NMR relaxation in porous materials.

The NMR relaxation of hydrogen nuclei of a fluid in a porous material is generally interpreted in terms of the Brownstein-Tarr model, in which the relaxation rate of the signal is inversely proportional to the pore size. We have investigated whether this model can be applied to the relaxation of Na nuclei in a NaCl solution in a porous material. The results indicate that the ion distribution over the pores can be obtained from an analysis of the Na NMR signal decay, if the pore sizes are roughly below 1 microm. This information is very useful for studies of combined moisture and ion transport in porous building materials.

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.