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.
The mechanical and deformative properties of gypsum rock have fascinated the scientific world for several decades (e.g. Craker and Schiller 1962; de Meer and Spiers 1997; Hoxha et al. 2006). The peculiarity of its mechanical properties (i.e. the low strength despite a general low porosity, the high plasticity even with a low confining pressure) implies technical problems in several application fields, in primis tunnelling excavation and stability of underground quarries. Gypsum is, indeed, involved in a wide range of industrial applications (e.g. it is a main component of stucco, wallboards, cements, paints and soil conditioner/fertilizer, it is used in food industry as a flocculant and in several clinical applications as regenerative material for bone defects, dental plaster, or as a delivery vehicle for pharmaceutical agents – Van Driessche et al. 2019). Quarry sites, both open pit and underground, are therefore largely exploited all over the world. The stability assessment of quarry environments, both during the active exploitation or in abandoned conditions, requires an accurate investigation of mechanical response of gypsum rock (e.g. Auvray et al. 2004; Caselle et al. 2019a; Caselle et al. 2020a; Castellanza et al. 2008; Castellanza et al. 2010).