Experimental Investigation on the Thermal, Mechanical and Cracking Behaviours of Three Igneous Rocks Under Microwave Treatment

Microwave heating is a promising technique in assisting the breakage of hard and abrasive rocks. It features rapid heating and selective heating, enabling generation of large enough thermal stresses to induce damage in rocks. This paper investigated the effect of microwave treatment on the heating rate, spatial temperature distribution, ultrasonic wave velocity, elastic modulus and load bearing capacity of three igneous rocks (i.e., gabbro, monzonite and granite). The crack pattern and crack density were also studied using petrographic thin-section observations. Experimental results indicate that the single-mode microwave system can effectively and efficiently weaken the three rocks by generating cracks or melting/shattering the specimens. The heating rates and thermal gradients increase as either exposure time or power level increases. When heated at 2 kW for 120 s, the P-wave velocities of gabbro and monzonite can be reduced to 44% and 51%, respectively, S-wave velocities reduced to 34% and 62%, Young's modulus reduced to 13% and 27%, load bearing capacity reduced to 34% and 43%. In contrast, granite specimens were more prone to violent failures at high power levels. Petrographic observations imply that the dominant crack types in gabbro, monzonite and granite are intergranular & transgranular cracks, intergranular cracks, and transgranular cracks, respectively.