Abstract: To investigate the water-induced ageing effect on gypsum rock, different water environment conditions were simulated in the laboratory, including different air humidity conditions and saturated groundwater conditions which may exist in the gypsum mines. Mechanical parameters of gypsum rock specimens were obtained under the pre-set conditions for different times. Meanwhile, the variation of pore structure in gypsum rock specimens was measured by nuclear magnetic resonance equipment. Then, the water-induced ageing mechanism of the gypsum rock was revealed. The results showed that the effect of water on the ageing of gypsum rock was obvious. Firstly, the ageing extent of gypsum rock increased with time, whereas mechanical parameters including uniaxial compressive strength, Brazilian tensile strength and elastic modulus decreased with time in a negative exponential relation. However, the Poisson’s ratio of gypsum rock had no clear relation with time. Secondly, the ageing extent and ageing rate of gypsum rock were closely related to the state of water. The higher the relative humidity was, the more pronounced the ageing extent was and the faster the ageing rate was. In particular, the ageing extent of gypsum rock was the most significant and its ageing rate was also the fastest when fully submerged in water. Generally, the water-induced ageing mechanism of gypsum rock was caused by the coupling of physical and chemical effects of water, and the chemical effect was the primary cause for the ageing effect. When the gypsum rock was exposed to water, the continuous dissolution and recrystallisation of the gypsum rock changed the mineral composition of the gypsum rock. As a result, the rock structure became loose, the porosity increased, and mechanical properties gradually weakened. The research results provide references for designing the gypsum mining and the evaluation of the long-term stability of gypsum goaf.

Key words: gypsum rock, ageing effect, nuclear magnetic resonance, dissolution, recrystallization, relative humidity