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 MgSO 4 ·H 2 O, 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 MgSO 4 + H 2 O derived from a careful evaluation of the available thermodynamic data, all experiments were carried out under appropriate conditions in a controlled environment. Hydration of MgSO 4 ·H 2 O and formation of MgSO 4 ·6H 2 O (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 hexahydrite from a highly supersaturated solution. The hydration of kieserite in confined spaces generates substantial stresses, resulting in considerable deformations of the glass filters that were used as porous substrate. The deformation measurements confirm that the deliquescence−recrystallization pathway is more efficient than the direct solid state reaction in generating stress. Finally, theoretical considerations confirm that the stress generated during the hydration is sufficient to damage nearly every building material.