Damage in porous media: role of the kinetics of salt (re)crystallization

We present multiscale experiments assessing the impact of the recrystallization dynamics of sodium sulfate and sodium chloride on damage observed in sandstone during humidity cycling. The deliquescence of both salts leads to the formation of homogeneous salt solutions which, upon drying, can reach high concentrations before (re)crystallization. For sodium chloride that has only a single crystalline phase, precipitation manifests itself as the fast growth of a small number of nuclei which grow to larger sizes in time. For sodium sulfate, with different crystalline phases, we observe that kinetically the precipitation of the thenardite (the anhydrous phase with the higher solubility) appears to be favored. The comparison of these results with those obtained when rewetting of crystals is done with liquid leads to the conclusion that the size of crystals that precipitate upon recrystallization is an important ingredient for explaining the damage observed in stones, besides the crystallization pressure that is usually invoked.