Effect of microwave treatment on thermal properties and structural degradation of red sandstone in rock excavation

Weakening rocks using microwave irradiation has been a major topic in research for the past 50 years. However, the thermal effect of microwaves on rocks is mostly studied with a multi-mode metal cavity, in which rocks absorb microwaves reflected from all directions, and this method cannot represent how the microwave pretreatment affects the rock mass in the rock excavation process. To simulate microwave treatment before an excavation, we wrapped a sample of red sandstone in a copper foil to avoid microwave overheating and the thermal properties and structural deterioration of the sample were investigated. Collected temperature data showed that pore water not only elevated the heating rate significantly but also it related to the temperature distribution within the rock sample. With the continuous removal of moisture during the heating process, the temperature distribution changed from exponential to linear equation. Besides, increasing the microwave power was beneficial to improve the heating rate and energy concentration on the rock fragmentation. Combining the results from nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), it was found that the porosity of macropores (T-2 greater than 100 ms) and mesopores (10 ms<100 ms) increased after microwave treatment, and change in porosity led to forming transgranular fracture and increasing the number of inter granular fracture. Furthermore, intragranular fractures decreased with microwave propagation, which indicated that the rock damage was a result of the development of intragranular fractures.

Automated summary

Research on weakening rocks using microwave irradiation has been ongoing for the past 50 years. The study found that increasing microwave power improved heating rate and energy concentration on rock fragmentation, leading to increased porosity and forming fractures within the rock. Furthermore, microwave treatment reduced intragranular fractures, indicating that rock damage was a result of the development of intragranular fractures.