Investigating the evolution of pore space and magnetization rate of rocks under the effect of coal fire burning

The burned rocks distributed widely in the coal fire regions exert significant influence on the mining of coal and other underground resources. In order to investigate the evolution of the physical properties of rocks under the effect of coal fire burning, widely distributed sandstones and mudstones were heated at different constant temperatures of 25, 200, 400, 600, 800, and 1000 ℃ for 2 h. In addition, the anomalous magnetization and thermotropic transformation of the minerals were studied using a magnetization rate meter. Low-temperature nitrogen adsorption was performed to observe the changes occurring in the pore size distribution with the burning effect. The results revealed that below 600 °C, the color of the rock samples intensified, and their brightness increased, the proportion of cracks in the rocks increased. These changes were attributed to the dehydration of the water-bearing minerals in the rocks and the thermal stress that occurred. Above 600 °C, the magnetic properties were greatly enhanced, the primary cracks in the rocks expanded and penetrated to form a macroscopic fracture network. These changes were attributed to the melting of the rock cement material accompanied by the appearance of strong magnetic minerals.