Mechanical Properties and Energy Evolution Law of Coal-Based Solid Waste Backfill at Different Temperatures and Ages

Abstract

The increasing use of deep coal mine backfilling operations under high geothermal conditions has drawn attention, The growing application of deep coal mine backfilling under elevated geothermal conditions has attracted increasing attention; however, the role of curing temperature in governing the mechanical response and energy evolution of coal-based solid waste backfill has not yet been fully clarified. In this study, coal gangue, fly ash, and desulfurization gypsum were selected as representative coal-based solid waste constituents to systematically examine the mechanical behavior and energy evolution of cemented backfill subjected to different curing temperatures (20, 27.5, 35, 42.5, and 50 °C) and curing durations (3, 7, 14, and 28 d). Material characteristics were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and particle size distribution analysis. Displacement-controlled uniaxial compression tests were conducted to obtain stress–strain responses, while elastic strain energy and dissipated energy were quantified and analyzed on the basis of the first law of thermodynamics. The findings highlight that curing temperature has a marked age-dependent effect on the mechanical behavior of the backfill. At early ages (3, 7, and 14 days), increasing the curing temperature enhances the elastic modulus, peak stress, and peak strain. At 28 days, both strength and deformation capacity reach their maximum values at 42.5 °C, whereas increasing the temperature to 50 °C causes post-peak softening and a reduction in load-bearing capacity. Energy analysis reveals that, at 28 days, both elastic strain energy and total energy at the peak stress first rise and then decline as curing temperature increases, reaching maximum values of 35.92 kJ/m³ and 56.45 kJ/m³, respectively, at 42.5 °C. The energy evolution process can be categorized into four distinct stages: pore compaction, linear elastic deformation, yielding damage, and post-peak failure. Higher curing temperatures significantly increase the proportion of energy dissipated in the compaction stage (from 37.5% to 59.3%) while reducing the energy dissipated in the elastic stage, which accelerates the damage process under high-temperature curing.