Variations of Physical and Mechanical Properties of Heated Granite After Rapid Cooling with Liquid Nitrogen

To determine the effect of liquid nitrogen (LN2) cooling on the damage of heated rock, we conducted a series of physical and mechanical tests on Shandong granite samples. These granites were first slowly heated to the target temperatures (25600 degrees C) and held for 10h, followed by rapid cooling with a coolant. Three coolants were used and compared in our experiment: air, water and LN2. Physical properties and mechanical properties were tested after thermal treatments. Microstructural changes were also observed using scanning electron microscope and optical microscope. According to experimental results, permeability of the heated granites increases significantly after LN2-cooling, while density, P-wave velocity, strength and elastic modulus reduce. As heating temperature rises, changes in these properties become more pronounced. Compared to air-cooling and water-cooling, LN2-cooling induces greater changes in the physical and mechanical properties at any target temperature. This indicates that LN2-cooling can damage the heated rocks more remarkably than the other two cooling treatments. According to microscopic analysis, inter-granular cracking is the primary failure mode during thermal treatment, and most of the inter-granular cracks distribute at the boundaries of quartz. Our results in this paper are of great value for understanding the characteristics of thermal damage induced by rapid cooling.