Thawing and Softening of Frozen Sandstone by Microwave Irradiation

The strength and hardness of rock are much higher at freezing temperatures than at room temperature. This results in high excavation costs and low excavation efficiency in frozen rock layers. This study proposes a novel way to thaw porous and water-bearing rock by microwave irradiation. It is applicable to a wide range of strata and is not dependent on whether the rock contains wave-absorbing minerals. Quartz sandstone specimens free from absorbing minerals and of different saturation levels were used in an investigation of thawing and softening behaviors under microwave irradiation. The rock pore structures were observed before and after irradiation. The results show that (1) frozen quartz sandstone irradiated by microwaves undergoes three stages: (i) rapid melting of pore ice, (ii) intense vaporization of meltwater, and (iii) drying. (2) Microwave irradiation significantly reduces the strength of frozen quartz sandstone. (3) The mechanisms are vaporization expansion , which causes the propagation of intergranular cracks, and thermal expansion , which induces trans-granular cracking. (4) Softening of 40–100%-saturated frozen quartz sandstone is caused by both vapor and thermal expansion, while 0–40%-saturated sandstone is mainly affected by thermal expansion. This study provides theoretical and experimental support for microwave-assisted breakage of frozen porous and water-bearing rock. Highlights Microwave irradiation is a promising auxiliary method in excavation of frozen rock. Microwave irradiation heating melts frozen sandstone completely within only 40 s. Unfrozen water in frozen sandstone is the fundamental cause of the rapid melting. Softening effect on frozen sandstone is enhanced by increasing saturation degree.