Effects of high temperature on the linear thermal expansion coefficient of Nanan granite

The evaluation of physico-mechanical characteristics of rocks after thermal treatment is a key issue in underground rock engineering projects such as exploitation of geothermal resources and geological disposal of nuclear waste. In this research, the lengths of cylindrical Nanan granite specimens were obtained before, during and after thermal treatment (up to 1000 °C) to investigate their linear thermal expansion coefficients, and the variation mechanisms were revealed by optical microscopic observations. According to the experimental results collected from the extensive corresponding literature, the relationships between the linear thermal expansion coefficients of various granites were also elaborated. The experimental results demonstrated that the linear thermal expansion coefficients of the granite in this study both under and after thermal treatment increase with temperature. Meanwhile, the linear thermal expansion coefficients increase rapidly above 500 °C, which is because of the quartz phase transition from α–phase to β–phase. The increase of linear thermal expansion coefficients of granite under and after thermal treatment closely relates to the thermal expansion of mineral crystals and the development and coalescence of intergranular and transgranular microcracks. The experimental results are expected to provide a reference to analytical calculations of thermophysical processes in granite.