Abstract: Rock engineering may be subjected to high temperature environment. Different cooling methods of high-temperature rock often lead to significant changes in the physical and mechanical properties of the rock, which will have an important impact on the stability and permeability of rock engineering. Magnetic resonance imaging (MRI), scanning electron microscope (SEM) and uniaxial compression test were used to study the porosity, pore size distribution, peak strength, peak strain, stress-strain relationship and microstructure changes of five temperatures for sandstone samples at 100, 300, 500, 600 and 800℃ under two cooling methods (natural cooling and water cooling). The test results show that: (1) When the rock samples used the natural cooling method, the strength of high-temperature sandstone does not decrease continuously with the increasing of temperature. However, rock samples using water cooling method show continuous decrease of sandstone strength, and the decreasing extent is far greater than that of natural cooling; (2) 500℃ can be considered as the critical value of the influence of different cooling methods on the porosity of sandstone. When the temperature is above 500℃, the water cooling method will cause the rock porosity increase rapidly, and the proportion of pores with large pore diameter (Ф>10 μm) is also higher than that of the natural cooling method. In this consideration, in the field of high-temperature sandstone engineering, the possible seepage hazards should be fully considered when water cooling is used (i.e., fire extinguishing with water after a tunnel is on fire); (3) The SEM test results shows that when the temperature is above 500℃, water cooling promotes the widening and expansion of cracks. When the temperature reaches to 800℃, the pore size of water-cooled sandstone becomes larger, and the fracture is largely developed and connects into a network. This will lead to a substantial increase in water permeability. At the same time, it is one of the reasons for the sharp decrease in rock strength that caused by water cooling at this temperature.

Key words: sandstone, heat treatment, cooling conditions, pore, MRI, SEM