Microstructural Changes of Anhydrite–Gypsum Samples During Water Immersion

Sulphatic evaporites represent a critical challenge for underground engineering due to their high solubility, swelling potential, and sensitivity to changing hydraulic and thermal conditions. In this study, we investigate the temperature-dependent dissolution behavior and microstructural evolution of Triassic sulphate rocks consisting of anhydrite and minor portions of gypsum from the Western Alps. Twelve cylindrical samples were immersed in CaSO4-saturated water solutions at 15 °C, 40 °C, and 60 °C for six months. Periodic mass and volume measurements were combined with Scanner Electron Microscope (SEM) imaging to quantify dissolution and document mineralogical transformations. All samples experienced progressive mass loss, whereas volumetric changes remained below measurement resolution. Dissolution pathways varied strongly with temperature. At 15 °C, dissolution occurred mainly along anhydrite grain boundaries, producing rounded crystal edges, while less effect was observed in the gypsum veins, leaving the intergranular layers preserved. In contrast, at 40–60 °C, gypsum was preferentially dissolved, generating porosity around comparatively unaltered anhydrite grains. These results qualitatively reproduce the temperature-controlled solubility inversion between gypsum and anhydrite predicted by thermodynamic models. No secondary gypsum precipitation or swelling features were observed. The experimental evidence highlights the role of temperature and hydraulic regime in controlling the stability of sulphate rocks and provides insights relevant to tunnel excavation, underground storage facilities, and geomechanical modeling in evaporitic settings.