Physico-mechanical and microstructure properties of cemented coal Gangue-Fly ash backfill: Effects of curing temperature

High environment temperatures in deep coal mines may affect the strength and stability of cemented Coal Gangue-Fly Ash Backfill (CGFB). This study aimed to analyze the consequences of temperature on the physical and mechanical properties and microstructure of CGFB. Uniaxial compression tests were carried out on CGFB specimens at different curing temperatures (i.e., 20℃, 35℃, and 50℃) and curing ages of 3, 7, 14, and 28 days. The moisture content of CGFB specimens was tested by the drying method. The microstructure and mineral composition of CGFB specimens were measured by X-Ray Diffraction (XRD), Thermogravimetric-Differential Scanning Calorimetry (TG-DSC), Mercury Intrusion Porosimetry (MIP), and Scanning Electron Microscopy (SEM). The results show that temperature has a significant effect on the strength and microstructure of CGFB. Higher curing temperatures can speed up the hydration reaction rate and generate more hydration products, making the pozzolanic effect of fly ash occur earlier. Early compressive strength of CGFB specimens increases, but later compressive strength decreases due to the decrease of the bonding of the internal structure due to the thermal damage. The change of curing temperature will lead to the change of failure mode of CGFB specimens. The failure mode of CGFB specimens changes from tensile failure at 20℃ to tensile-shear mixed failure at a higher temperature (35℃, 50℃). Under the influence of temperature, among the two physical property parameters (i.e., porosity and moisture content of CGFB), the porosity can better reflect the mechanical behavior of the backfill. This study provides a reference value for guiding filling in high ground temperature coal mine.