Can drying and re-wetting of magnesium sulfate salts lead to damage of stone?

Magnesium sulfate salts have been linked to the decay of stone in the field and in laboratory experiments, but the mechanism of damage is still poorly understood. Thermomechanical analysis shows that expansion of stone contaminated with magnesium sulfate salts occurs during drying, followed by relaxation of the stress during dehydration of the precipitated salts. We applied thermogravimetric analysis and X-ray diffractometry to identify the salt phases that precipitate during drying of bulk solutions. The results show the formation of 11 different crystal phases. A novel experiment in which a plate of salt-laden stone is bonded to a glass plate is used to demonstrate the existence of crystallization pressure: warping of the composite reveals significant deformation of the stone during rewetting of lower hydrates of magnesium sulfate. Environmental scanning electronic microscope (ESEM)/STEM experiments show that hydration of single crystals of the lower hydrates of magnesium sulfate is a through-solution crystallization process that is only visible at a small scale (*lm). It is followed by growth of the crystal prior to deliquescence. This demonstrates that crystallization pressure is the main cause of the stress induced by salt hydration. In addition, we found that drying-induced crystallization is kinetically hindered at high concentration, which we attribute to the low nucleation rate in a highly viscous magnesium sulfate solution.