Impact of the Kinetics of Salt Crystallization on Stone Damage During Rewetting/Drying and Humidity Cycling

Abstract

In this study, we show that the key to understand why the same salt can cause damage in some conditions and not in others is the kinetics of crystallization. We present experiments assessing the impact of the recrystallization dynamics of sodium sulfate on damage observed in sandstone after repeated cycles of rewetting/drying and humidification/drying. Macroscopic and microscopic scale experiments using magnetic resonance imaging and phase contrast microscopy demonstrate that sodium sulfate that has both hydrated and anhydrous phases can lead to severe damage in sandstone during rewetting/drying cycles, but not during humidity cycling. During rewetting (a rapid process) in regions (pores) that are highly concentrated in salt, anhydrous microcrystals dissolve only partially, giving rise to a heterogeneous salt solution that is supersaturated with respect to the hydrated phase. The remaining anhydrous crystals then act as seeds for the formation of large amounts of hydrated crystals, creating grape-like structures that expand rapidly. These clusters can generate stresses larger than the tensile strength of the stone, leading to damage. On the other hand, with humidification (a slow process) and after complete deliquescence of salt crystals, the homogeneous sodium sulfate solution can reach high concentrations during evaporation without any nucleation, favoring the formation of isolated anhydrous crystals (thenardite). The crystallization of the anhydrous salt generates only very small stresses compared to the hydrated clusters and therefore causes hardly any damage to the stone.