Effect of microwave treatment on thermal and ultrasonic properties of gabbro

The mining and tunnelling industries are looking for solutions to increase the rates of advance of mechanical excavators in hard and abrasive rocks. Microwave assisted rock breakage has substantial advantages in the volumetric heating, selective heating, instantaneous control and energy savings, which is regarded as a promising technology. A 2 kW single-mode industrial microwave system was used to heat the Austral Black gabbro specimens at the frequency of 2.45 GHz. The temperature and temperature distribution on the specimen surface after microwave treatment were immediately measured by an infrared thermal camera and a thermal gun. The P-wave velocity was measured using ultrasonic transducers to quantify the local and overall thermal-induced damage of specimens. We studied the influence of power levels (0.5-2 kW) and exposure times (30-120 s) on the temperature and P-wave velocity. The microwave induced macrocracks and microcracks were analysed using petrographic thin-section observations and the fluorescent resin technique. Testing results indicate that microwave can efficiently weaken rocks by either generating macro/micro cracks or melting specimens. Both the maximum and average temperatures increase linearly with the power level and exposure time. Non-uniformly spatial thermal gradients increase as the power level and exposure time increase. The overall P-wave velocity and the P-wave velocity in the middle of specimens are reduced by up to 55% and 80%, respectively, implying a significant reduction of strength. Experimental results also show that under the same energy, heating at higher power levels for shorter durations has a better weakening effect.