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

• Published in: Minerals engineering Vol. 162; p. 106730
• Main Authors: Yao, Junhui, Tao, Ming, Zhao, Rui, Hashemi, Sam S., Wang, Yiqing
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.03.2021
• Subjects: Microwave heating; Red sandstone; Sample pre-treatment; Structural degradation; Temperature distribution; Temperature distribution; Microwave heating; Red sandstone; Sample pre-treatment; Structural degradation
• ISSN: 0892-6875; 1872-9444
• DOI: 10.1016/j.mineng.2020.106730

•A sample pretreatment method was used to represent real-world conditions.•Pore water has a significant effect on thermal response of rock subject to microwave.•Heating at higher power levels for short durations has a better energy utilization.•Development of intergranular fractures leads to rock damage and even fracture. 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 pre-treatment 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 (T2 greater than 100 ms) and mesopores (10 ms＜T2＜100 ms) increased after microwave treatment, and change in porosity led to forming transgranular fracture and increasing the number of intergranular fracture. Furthermore, intragranular fractures decreased with microwave propagation, which indicated that the rock damage was a result of the development of intragranular fractures.