Deep Mines Run Hot: Why 40°C Makes Coal Waste Concrete Stronger, Then Weaker

Deep beneath the surface of coal mining regions, the rocks are warm. As mines push deeper to meet global demand for minerals and energy, they enter high-geothermal zones where the surrounding strata can hold temperatures far above the mild conditions of a surface laboratory. That warmth matters enormously for one of green mining’s most promising technologies: gangue cemented backfill, a concrete-like material made from coal waste that is pumped into emptied underground voids to support the overlying rock. A new study published in Case Studies in Construction Materials has now mapped, with unusual precision, how curing temperature shapes the strength, energy behavior, and failure patterns of this material, and the results reveal a sharp tipping point at 40 degrees Celsius.

The research team, led by Teng Li and Yongqiang Hou of Anhui University of Science and Technology, prepared cylindrical specimens from a mixture of crushed coal gangue, Class II fly ash, and ordinary Portland cement in a mass ratio of 5:2:1:2, yielding a slurry with a solid mass concentration of 80 percent. The gangue came from the Xieqiao Coal Mine operated by Huaihe Energy Group and was graded into coarse, medium, and fine aggregate fractions of 35, 35, and 30 percent respectively. After 24 hours of initial setting, the specimens were demoulded and cured for 28 days in a programmable chamber held at 95 percent relative humidity and one of four temperatures: 20, 30, 40, or 50 degrees Celsius. Three parallel specimens were tested per condition to average out experimental scatter.

Under uniaxial compression in a displacement-controlled electronic universal testing machine, every specimen told the same four-part story: an initial compaction stage in which pre-existing pores and microcracks close, an approximately linear elastic stage, a concave-down yield stage where irreversible deformation accumulates, and a post-peak failure stage marked by flaking and blocky spalling. But the numbers behind those stages shifted dramatically with temperature. Compaction stress climbed from 0.4219 megapascals at 20 degrees to 1.4195 megapascals at 40 degrees, an increase of roughly 236.5 percent, before falling back to 0.9605 megapascals at 50 degrees. Peak stress followed the same arc, rising from 1.6645 megapascals at 20 degrees to a maximum of 5.682 megapascals at 40 degrees, then dropping 32.4 percent to 3.841 megapascals at 50 degrees.

The elastic modulus, a measure of the material’s stiffness, traced the same non-monotonic curve with an even tighter fit to a quadratic function. It rose from 239.86 megapascals at 20 degrees to 609.99 megapascals at 40 degrees, a gain of 154.4 percent, then fell 31.2 percent at 50 degrees. Interestingly, stiffness and strength did not respond to temperature in perfect lockstep. The modulus gained less than strength during the warming phase but lost about the same proportion during the overheating phase, suggesting that stiffness reflects the initial rigidity of the material skeleton and responds sluggishly to pore filling, while strength depends more directly on the cementitious bonds formed by hydration products and is therefore more sensitive to accelerated hydration.

The mechanism behind the reversal at 50 degrees is a classic case of too much of a good thing. Below 40 degrees, elevated temperature accelerates cement hydration, generating more calcium silicate hydrate gel and needle-like ettringite crystals that fill pores, bind the gangue aggregate, and densify the structure. Above that threshold, hydration races ahead so quickly that products cluster unevenly, encapsulating aggregates in some regions while thermal stresses from the exothermic reaction spawn fresh microcracks. Scanning electron microscopy at 5000 times magnification made the contrast vivid: at 20 and 30 degrees the 28-day microstructure remained loose and porous, with intact spherical fly ash particles showing limited pozzolanic activity, while at 50 degrees the fly ash was tightly enveloped in a thick hydration layer that looked dense but lacked strong chemical bonding, producing what the authors call a dense-but-not-strong weak interfacial structure.

Energy analysis added a thermodynamic dimension to the picture. Applying the first law of thermodynamics, the team split the mechanical work done on each specimen into releasable elastic strain energy and dissipated energy consumed by crack growth and friction. At the peak stress point, total strain energy density peaked at 40 degrees with 36.2804 kilojoules per cubic meter, alongside 26.4641 kilojoules per cubic meter of elastic energy and 9.8163 kilojoules per cubic meter of dissipated energy. All three indices rose and then fell with temperature, confirming that the 40-degree material could both store and dissipate the most energy. Beyond that point, brittleness increased, crack propagation shifted from gradual to sudden coalescence, and the dissipated energy at the peak collapsed to 2.7745 kilojoules per cubic meter, nearly the same as the room-temperature value.

The failure modes themselves transformed with heat. At 20 degrees, specimens failed in tension along semi-penetrating axial cracks with limited secondary fracturing. At 30 and 40 degrees, the failure shifted to a tensile-shear mixed pattern with dense, interconnected crack networks that broke the material into bulking blocks. At 50 degrees the fragmentation was most severe, with pronounced lateral dilatancy and intense bulking. In practical terms, hotter-cured backfill does not merely weaken; it fails more suddenly and more violently, a distinction that matters for engineers assessing the load-bearing stability of pillars of waste backfill supporting deep mine roofs.

Perhaps the study’s most technically ambitious contribution is a new piecewise damage constitutive model. Building on the Drucker-Prager strength criterion and assuming a power-law statistical distribution of micro-element strength, the model treats the initial compaction stage as a defect-closing phase without damage propagation and captures the elastic and yield stages with a statistical damage variable corrected by a coefficient accounting for the residual load-bearing capacity of failed elements. When fitted to the experimental curves, the model tracked the pre-peak stress-strain behavior closely at all four temperatures. Its shape parameter told a striking story of its own: it stayed relatively stable from 20 to 40 degrees but jumped sharply at 50 degrees, indicating that thermal microcracks had prematurely eliminated the weakest elements and left the survivors with a homogenized strength distribution, the statistical fingerprint of thermally induced embrittlement.

For the mining industry, the implications are concrete. As backfill operations descend into hot deep seams, the ambient rock temperature becomes a design variable rather than a background detail. The study identifies 40 degrees as the optimal curing temperature for this gangue-fly ash-cement system, where a structural strengthening mechanism gives way to a thermal damage deterioration mechanism. Curing regimes and mix proportions can now be tuned to the geothermal conditions of a specific mine, and the temperature-sensitive model parameters offer a way to predict pre-peak mechanical behavior without waiting for field failures. The authors note that their model covers only the pre-peak regime, since post-peak behavior is dominated by frictional sliding along macroscopic fracture surfaces, and they plan to extend the framework with discrete element methods and acoustic emission monitoring.

There is also a broader environmental resonance. Gangue cemented backfill simultaneously disposes of coal waste and restores structural integrity to mined-out voids, making it a cornerstone of green mining initiatives. Understanding exactly how temperature governs its long-term stability means that the mountains of gangue stacked around coal mines can be converted into reliable underground support even in the hottest, deepest workings. What this study makes clear is that the relationship between heat and strength is not a straight line but a curve with a summit, and knowing where that summit lies could determine whether the backfill holding up a kilometer of rock endures or crumbles.

Subject of Research: Effect of curing temperature on the mechanical properties, energy evolution, and damage behavior of gangue cemented backfill for deep coal mine backfilling

Article Title: Mechanical properties, energy evolution, damage characteristics of gangue cemented backfill under different curing temperatures

Article References: Li, T., Hou, Y., Yu, X., Xionggang, Z., Du, H., Wang, Y., & Zhang, X. Y. (2026). Mechanical properties, energy evolution, damage characteristics of gangue cemented backfill under different curing temperatures. Case Studies in Construction Materials, 25, Article e06589. https://doi.org/10.1016/j.cscm.2026.e06589

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06589

Keywords: gangue cemented backfill, curing temperature, coal mining, thermal damage, uniaxial compression, energy dissipation, damage constitutive model, hydration, fly ash, deep mines, brittle failure, elastic modulus