Response of the mechanical properties of impurity-bearing gypsum rocks with a porphyritic structure to long-term water immersion and softening mechanisms

The effect of long-term water immersion on the mechanical properties of porphyritic gypsum rock containing coarse-grained gypsum minerals and a variety of fine-grained minerals was explored using uniaxial and triaxial compression tests. XRD, SEM, and EDS analyses were utilized to discuss the water-softening mechanism in relation to the rock fabric. The results show that the elastic modulus decreases while Poisson’s ratio increases for rocks without lateral confinement after long-term immersion, which exerts a weak effect on the internal friction angle of rocks but greatly weakens the cohesion. The softening effect of water on rocks attenuates nonlinearly with increasing immersion time, and the compressive strength changes as a logarithmic function of immersion time. As the confining pressure increases, the sensitivity of the compressive strength to immersion time tends to weaken. The increase in the confining pressure causes the transition from brittleness to ductility of rocks, and long-term water immersion leads to a decrease in the transitional confining pressure. The softening of rocks is attributed to structural damage caused by the dissolution of gypsum minerals and the disintegration of material zones composed of fine-grained minerals under immersion conditions. Water infiltration is facilitated by intergranular pores within the fine-grained mineral zone as microcracks are distributed along the gypsum cleavages and contact boundaries between coarse- and fine-grained mineral zones. During prolonged water immersion, the water activity shifts from a predominance of infiltration to that of dissolution and disintegration, resulting in a transient increase followed by a gradual decrease in the mass of gypsum rock.