Hydration of MgSO4·H2O and Generation of Stress in Porous Materials

Crystals growing in confined spaces can generate stress and are a major cause of damage in porous materials. The present paper is an experimental study of the isothermal hydration of MgSO4·H2O, kieserite, in porous glass filters with different in situ techniques, i.e., X-ray diffraction under controlled conditions of temperature and humidity, scanning electron microscopy after cryofixation of samples, and electronic speckle pattern interferometry. Based on the phase diagram of MgSO4 + H2O derived from a careful evaluation of the available thermodynamic data, all experiments were carried out under appropriate conditions in a controlled environment. Hydration of MgSO4·H2O and formation of MgSO4·6H2O (hexahydrite) is a true solid state reaction below the deliquescence humidity of kieserite. This reaction, however, is kinetically hindered. Above the deliquescence humidity, the reaction proceeds via a two stage reaction pathway involving the dissolution of kieserite and the subsequent crystallization of hexahy...