Influence of microwave treatment on mechanical behaviour of compact basalts under different confining pressures

The realisation of microwave-induced fracturing of hard rocks has potential significance for microwave-assisted mechanical rock fracturing and stress release in deep rock masses. In this context, compact basalts were treated by microwave heating in a multi-mode cavity at a frequency of 2450 MHz, and then, we investigated the mechanical behaviour of basalt samples after microwave treatment under uniaxial compression and conventional triaxial compression (CTC) tests. After microwave exposure, cracks appeared on the surface and inside of the rock sample, and the temperature of the sample's surface was unevenly distributed. The results show that the conventional triaxial compressive strength (CTCS) of basalt samples decreased linearly with microwave exposure time, and the higher the confining pressure, the smaller the reduction in the strength of basalt samples after microwave treatment. Under uniaxial compression, microwave exposure greatly affected the axial deformation, suggesting that deformation resistance of the samples gradually decreases with increasing microwave exposure time. Under triaxial compression, some microcracks induced by microwave exposure closed due to the effect of confining pressure, resulting in the confining pressure inhibiting any rightward shift of the axial deformation curve. Furthermore, under uniaxial compression, the elastic modulus and Poisson's ratio of basalts also decreased in a quasi-linear manner with elapsed microwave exposure time. Under triaxial compression, microwave exposure has slight influence on elastic modulus and Poisson's ratio. After microwave treatment, the changes in rock strength and deformation mainly result from changes in between the mineral structures. Confining pressure results in the closure of microcracks produced by microwave exposure, so that effects of microwave treatment on strength and deformation decrease, thus reducing the influence on elastic constants. The cohesion decreases with increasing microwave exposure time and shows an approximately linear decrease over time. In the basalt samples, new microcracks in various directions generated by microwave exposure can increase the discreteness of test results, while the discreteness of test results caused by microcracks gradually reduces with increasing confining pressure.