Damage caused by freeze-thaw treatment with liquid nitrogen on pore and fracture structures in a water-bearing coal mass

The purpose of the study was to explore the microscopic and mesoscopic damage mechanisms of water-bearing coal mass in the freeze-thaw process with liquid nitrogen. For this purpose, by using a Zeiss microscope, a nonmetal ultrasonic velocity detector and a nuclear magnetic resonance (NMR) spectrometer, the changes in the structures of surface fractures and pores before and after coal masses with different moisture contents when subjected to freeze-thaw treatment with liquid nitrogen (FTTLN) were tested. The result showed that under the freeze-thaw effect of liquid nitrogen, the structures of surface fractures and internal pores of coal mass were both changed. Upon increasing the moisture content of the coal, the width of surface fractures increased; the surface fractures showed significant fractal characteristics: The higher the moisture content of the coal, the greater the rate of change of fractal dimension after FTTLN; the ultrasonic wave velocity in the coal mass gradually decreased after FTTLN, and the greater the moisture content, the more significant the decrease in ultrasonic wave velocity; under the freeze-thaw effect of liquid nitrogen, the pore structure in the coal mass was transformed from micropores and small pores to mesopores and macropores and certain new micropores and small pores were generated. Upon increasing the moisture content of the coal mass, the peak area of micropores and small pores and their proportion decreased at first and then increased (reaching the minimum at 9.12% of moisture content); the peak area, peak area proportion, and porosity of mesopores and macropores always increased. The damage factor attained based on the width of fractures, longitudinal wave velocity, fractal dimension of surface fractures, and porosity was favorably correlated with the moisture content: With increasing moisture content, the coal mass became more severely damaged.