Characteristics and mechanism of rock 3D volume fracturing in microwave field

Efficient fracturing of hard rock is an essential prerequisite for exploring deep resources and developing deep underground space. The development of efficient rock-breaking technology has become a general trend. Microwave is considered as a highly efficient auxiliary rock-breaking technology with excellent development potential because of its high heating efficiency and freedom from secondary pollution. The interpretation of differential fracture behavior of rock samples under the microwave field is an essential basis for the practice of microwave-assisted rock-breaking technology. In this study, the cross-scale fracture behavior mechanisms of sandstone, granite, and gabbro under the microwave field based on macro-scale, meso-scale, and micro-scale were investigated. The results showed that at the macroscopic expansion fracture level of the rock, the sandstone exhibited a low-temperature collapse failure, and the maximum temperature was measured at 194 ℃, while gabbro and granite exhibited a high-temperature melting failure and the measured maximum temperatures could reach 367 ℃ and 492 ℃, respectively. The volume of the rock changed significantly when it broke away from its matrix to produce cracks or macroscopic fracture surfaces. The rock volume expansion rate was the largest when it was melted at high temperature, and the volume expansion rate of granite and gabbro reached 2.1% and 1.8% when they were broken. At the level of rock mesoscopic fracture surface characteristics, the value of the fractal dimensions of fracture surface caused by high-temperature melting in microwave field was 2.0% higher than that caused by low-temperature collapse failure. The fractal dimension values of the fracture surface of granite, gabbro and sandstone were 2.109 2, 2.070 4, and 2.066 0, respectively. In terms of the differential mechanism of rock microdamage, the fracture characteristics of the rock were closely related to the main components, rock-forming minerals content and the moisture content. On the one hand, when the quartz content in the rock was high, the thermal stress increased rapidly due to the significant difference in dielectric properties between quartz and other minerals. On the other hand, water vaporized rapidly under the microwave irradiation, resulting in high pore pressure in the rock, and the sample was prone to low-temperature collapse failure, where was no significant damage structure on the microfracture. However, when the difference in the dielectric properties of main mineral components in the rock was slight and the moisture content was minimal, the temperature inside the sample increased slowly. The temperature gradient of minerals was lower, and the whole rock showed a high-temperature melting failure in the later stage. A rock crystal chemistry and molecular structure analysis found that the content of the Si—O bond in the sample changed significantly, and the XRD diffraction peaks and corresponding diffraction angles of the quartz and silicate minerals shifted. The crystals were mainly affected by micro-compressive stress, showing a large number of microcracks or holes inside the quartz and plagioclase minerals of the granite and at the junction of two minerals. The damage structure in gabbro was mainly concentrated inside the pyroxene minerals. Therefore, the geological and lithologic conditions suitable for microwave-assisted rock breaking on this basis can be inferred to improve the rock-breaking efficiency of deep hard rock.