Pore characteristic evolution and damage deterioration of granite subjected to the thermal and cooling treatments combined with the NMR method

Temperature disturbance could cause deterioration of rock mechanical parameters, and it is of great significance to establish the corresponding quantitative characterization method of rock damage to reveal the damage and instability mechanism of rock and guide engineering practice. In this study, fine-grained granite was selected as the object of research and has undergone different thermal treatments with different high-temperature and cooling methods. The formation and development of pores and microcracks were observed and analyzed by field emission-scanning electron microscopy (FE-SEM) and low-field NMR scanning experiments. Then, the rock damage coefficients based on macro and microdamage mechanisms were calculated, and the mechanism of microdamage was specifically analyzed based on the change of the characteristics of the small, medium, and large pores of the rock and the characteristics of fractal dimension. The results showed that the porosity of granite increased slowly first and then rapidly with the increase of heat treatment temperature, and the proportion of medium and large pores made the most significant contribution to the improvement of porosity. The damage value of granite increased continuously with the heat treatment temperature. The damage value of liquid nitrogen (LN 2 ) cooling was the highest, followed by water and natural cooling. After heat treatment and cooling treatment, the pore distribution of granite had good statistical fractal characteristics, and the fractal dimension decreased with the increase in heat treatment temperature, and the fractal dimension of the rock cooled by liquid nitrogen was the highest, followed by water cooling and natural cooling.