Mechanical improvement of modified coal-based solid waste composite material after exposure to elevated temperatures to control overburden failure

A systematic investigation was conducted on the effects of diatomite substitution rate and elevated temperature on key compressive performance indicators, including compressive strength, elastic modulus, peak strain and failure mode. The temperature of 400 °C is turning point for uniaxial compression strengths and residual strengths. The average uniaxial compression strength of specimens after exposure to 400 °C increases from 33.9 MPa to 47.2 MPa, increasing by 39.2 %. Then it decreases to 17.2 MPa when the temperature rises to 800 °C. At 400°C, the silica from diatomite reacts vigorously with the calcium hydroxide present in concrete, producing a more abundant and stable C-S-H. However, when the temperature exceeds 600°C, C-S-H begins to decompose, which is a strong destructive reaction and directly leads to a sharp decline in strength. A theoretical model is proposed to predict the compressive strengths of modified backfill material after exposure to elevated temperatures. Additionally, a positive correlation was observed between the steel fiber content and the uniaxial compressive strength. As the proportion of diatomite increases, the uniaxial compression strengths decrease in 1.0 % and 2.0 % steel fiber, but increase in 1.5 % steel fiber. The strengths of specimens after 800 °C heat treating are hardly affected by the diatomite. Furthermore, SEM and XRD were used to reveal compressive strength enhancing mechanism of specimens after exposed to high temperatures.