Abstract: Under nature and saturated conditions, a series of uniaxial and triaxial compression tests was conducted on gypsum rock mined from Jinghua mine in Jingmen. Mechanical properties of water-softening gypsum rock and the corresponding softening mechanism were studied by the scanning electron microscope (SEM). The results show that the strength and deformational properties of gypsum rock are tremendously weakened by water. With different concentrations of SO42- and Ca2+ in the solution, the softening coefficient of rock strength ranges from 0.6 to 0.72 and the softening coefficient of elastic modulus is in the range of 0.66-0.75. Besides, the cohesion c decreases significantly whereas the internal friction angle ? remains essentially unchanged. Since the gypsum rock has good ductility, the critical confining pressure of the elastic deformation to the ideal plastic deformation is found between 2.5 MPa and 5 MPa in nature sate, while the critical confining pressure is lower than 2.5 MPa for saturated gypsum rock. The softening mechanism of gypsum rock is closely related to the slight solubility of the calcium sulphate dehydrate, which is the major ingredient of rock. In distilled water, the rock structure is weakened by the dissolution process of gypsum mineral. In saturated CaSO4 solution, the dynamic equilibrium state reaches when the dissolution rate is equal to the re-crystallization rate. The continuous dissolution and re-crystallization processes tend to gradually loosen the original tight structure, resulting in softening mechanical properties. In conclusion, this study provides deep insights into the transformation process of the mechanical properties of the gypsum rock in accordance with different solutions and confining pressures, which is of great significance for the stability analysis of goaf area in gypsum mines.

Key words: gypsum rock, water-saturated condition, softening mechanism, mechanical properties, dissolution, recrystallization