Investigation of high-temperature effects on the strengthening and degradation of mechanical property in sandstone

The underground coal gasification (UCG) mining technology constitutes an innovative form of clean energy technology pertaining to coal resources. The sandstone specimens from the roof and floor of coal seam were collected and heat treated at 25 °C–800 °C in laboratory. According to the test results of physical parameters, 400 °C is the temperature turning point for the physical parameters such as mass, longitudinal P-wave velocity, porosity, and water absorption of sandstone specimens to change from "slow" to "drastic". The uniaxial compression test showed that the uniaxial compressive strength (UCS) of sandstone specimens decreases by 3.22% when the temperature rises from 25 °C to 200 °C, and by 5.70% when the temperature rises from 600 °C to 800 °C, both of which show temperature degradation effect. However, in the range of 200 °C–600 °C, the average UCS of sandstone specimens increases from 122.80 MPa to 162.54 MPa, showing obvious strength strengthening effect. In order to fully elucidate the mechanism of mechanical properties strengthening and degradation of tunnel surrounding rock under high temperature, X-ray diffraction (XRD), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were combined in the experiment. We concluded that these two intensity effects can be explained by physical and chemical reactions, including mineral composition thermal expansion, thermally induced microcrack and healing effects, phase transformation of the rock-forming mineral quartz, dehydration and thermal decomposition of mica, as well as dehydration and dehydroxylation of illite. The research results can provide theoretical basis for UCG roof and floor disaster prevention evaluation and tunnel support system design.