CAN DRYING AND RE-WETTING OF MAGNESIUM SULPHATE SALTS LEAD TO DAMAGE OF STONE?

Magnesium sulphate salts have been linked to the rapid decay of stone in the field and in laboratory experiments, but the mechanism of damage is still poorly understood. Our dilatation experiments show significant expansion of magnesium sulphate salts occurs under drying and re-wetting conditions that may eventually lead to damage. To clarify the damage mechanism we investigated drying-induced crystallization and re-wetting of magnesium sulphate salts by using ESEM/STEM. We applied combined TGA/XRD/DSC to identify the salt phases. The results show that magnesium sulphate has 3 stable phases a Earth surface conditions but 8 metastable and 1 amorphous phase that form during the hydration and dehydration of stable salts. We conclude that hydration of lower hydrated single crystals is firstly a through-dissolution process that is only visible at a small scale (≈µm) by using STEM, followed by a swelling of the crystal. This demonstrates that the crystallization pressure is the main responsible for the stress induced by salt hydration. In addition, drying-induced crystallization is kinetically hindered at a high concentration, which we explain through the low nucleation rate in a highly viscous magnesium sulphate solution. Crystallization of epsomite takes place at lower solution concentrations and can lead to stress under drying-conditions.