Micro-scale mechanisms controlling the deformation and failure of gypsum

Gypsum is a rock material with uncommon low strength and, in general, low porosity. Huge attention has been paid in the scientific community to its mechanical properties due to the role played in civil infrastructures (e.g. weakness zones in tunnel excavation), in orogenetic wedges (e.g. important detachment horizons in correspondence of gypsum bodies) and in oil and gas accumulations (e.g. large-scale gypsum-dominated structures working as trap). The large use of this material in construction industries requires a specific attention to the stability assessment of quarries, mainly in underground environment. The deformation and failure of gypsum rocks is controlled by the cooperation of intra-crystalline mechanisms (e.g. presence of preferential weakness surfaces according to the crystallographic orientation) and inter-crystalline mechanisms (e.g. interaction among different crystals). However, the nature of these mechanisms is not yet completely understood. In the present research, triaxial strength tests were performed on gypsum core samples. Following a multiscale approach, samples were analysed macroscopically and with optical and electron microscopes at the end of the tests. The particular crystalline structure of gypsum, with water molecules layered in the crystalline reticule, was recognized as an important parameter in controlling the meso-scale mechanical behaviour of the rock and the gradual transition from brittle to plastic strain regime.