Measurement of the Pressure induced by salt crystallization in confinement

Salt crystallization is a major cause of weathering of artworks, monuments and rocks. Damage will occur if crystals continue to grow in confinement, i.e. within the pore space of these materials generating mechanical stresses. We report on a novel method that allows to directly measure, at the microscale, the resulting pressure while visualizing the spontaneous nucleation and growth of alkali halide salts. The experiments reveal the crucial role of the wetting films between the growing crystal and the confining walls for the development of the pressure. The results suggest that the pressure originates from a charge repulsion between the similarly charged wall and the crystal separated by a ~1.5 nm salt solution film. Consequently, if the walls are made hydrophobic, no film and no crystallization pressure are detected. The magnitude of the pressure is system-specific and explains how a growing crystal exerts stresses at the scale of individual grains in porous materials.