Effect of the kinetics of crystallization on salt weathering of stones

Salt weathering is a major cause of deterioration of rocks and building materials. The precise mechanisms of why certain salts (e.g., sodium sulfate) cause more damage than others (e.g., sodium chloride) under the same environmental conditions still remain elusive. Here we show that for sodium sulfate, the existence of hydrated and anhydrous crystals and specifically their kinetics of crystallization are responsible for the damage. Using MRI and optical microscopy we show that during wetting and drying cycles large damage occurs in salt contaminated sandstone due to a very rapid growth of the hydrated phase of sulfate in the form of clusters that nucleate on anhydrous micro crystals. The rapid growth of the clusters generates stresses in excess of the tensile strength of the stone. Sodium chloride only forms anhydrous crystals that consequently do not cause damage in the experiments.