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

The research team, led by Kunchen He, Haichun Hao and Bengao Yang, together with colleagues from institutions including Sichuan University’s State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of Deep Underground Engineering, started from an awkward truth about existing coring technology. Pressure-holding coring tools date back to the 1960s and were refined through the Deep Sea Drilling Project, the Ocean Drilling Program and later the European HYACE and HYACINTH systems, which added the Pressure Core Analysis and Transfer System. More recently, a so-called five-preservation concept has been proposed to maintain pressure, temperature, mass, moisture and light simultaneously. But comprehensive fidelity comes at a price: costs balloon and success rates fall. Because prior work had already shown that in weakly water-bearing formations the dominant deep factors are temperature and pressure, and that these two factors interact competitively rather than additively, the researchers set out to determine exactly when each one rules the behavior of deep coal.

The experimental material came from the No. 8 cross-heading in the southern part of a colliery in the Qinshui coalfield, sampled at a vertical depth of 568.2 meters within the No. 3 coal seam of the Lower Permian Shanxi Formation. The seam sits in a weak aquifer horizon, so the natural moisture content of the fresh samples was only 2.67 to 3.18 percent, allowing the team to justify excluding pore water pressure and focusing on the coupling of confining pressure and temperature. The coal is a bright, hard black anthracite with an original gas pressure of 2.5 to 2.75 megapascals and gas contents between 20.62 and 27.49 cubic meters per ton. Proximate and elemental analyses confirmed the material’s high maturity: fixed-carbon contents near 81 percent, very low to low total sulfur of 0.41 to 0.45 percent, and average maximum vitrinite reflectance between 3.15 and 3.17 percent.

Because deep anthracite is notoriously brittle and riddled with endogenetic fissures, only 27 standard cylindrical specimens of 50 millimeters in diameter and 100 millimeters in height could be prepared from the block samples, which were sealed in bubble wrap, taped, and packed in foam-lined wooden boxes on site. Physical properties varied considerably, with densities between 1.38 and 1.5 grams per cubic centimeter and P-wave velocities spanning 1.19 to 2.43 kilometers per second, a fluctuation of up to 51.2 percent. To prevent this heterogeneity from contaminating the results, the team applied a local outlier factor algorithm using density and wave velocity as characteristic variables. The filter retained 19 statistically similar specimens, cutting the mean square deviation of density by 58.4 percent and that of wave velocity by 60.6 percent, a crucial step for a study that hinges on detecting subtle temperature-driven changes.

The core of the work was a two-factor, four-level orthogonal scheme spanning 16 experimental groups, with temperatures of 20, 50, 80 and 95 degrees Celsius crossed with confining pressures of 1, 22, 44 and 55 megapascals, chosen using formation gradients of 30 degrees Celsius per kilometer and 22 megapascals per kilometer to represent depths down to 2500 meters. Testing used the RTRX-140-65 GCTS rock mechanics system, capable of 1000 kilonewtons of axial load, 70 megapascals of confining pressure and heating to 140 degrees Celsius, with a Micro-II acoustic emission system tracking crack growth in real time through six sensors. A loading path of temperature first, confining pressure second, was adopted to protect the apparatus, with a gentle heating rate of 0.5 degrees Celsius per minute, followed by a two-hour temperature-pressure retention experiment and then real-time strain-controlled triaxial loading at 0.05 percent per minute.

The results revealed a stark antagonism. At a confining pressure of just 1 megapascal, temperature ran riot: peak strength fell by 14.73 to 69.65 percent and elastic modulus by 5.5 to 32.6 percent as temperature rose, with the decline accelerating, so that the strength loss jumped from 15.95 percent between 50 and 80 degrees Celsius to 57.65 percent between 80 and 95 degrees Celsius. Stress-strain curves at 80 degrees Celsius and above developed periodic fluctuations, the first acoustic emission event arrived progressively earlier, from 5.41 to 1.4 minutes, and the fraction of shear cracks climbed 2.8 times to 51.62 percent, signaling a shift from brittle toward ductile failure. But once the confining pressure reached 22 megapascals, the thermal fingerprint vanished almost entirely. Strength and modulus values stayed within the normal fluctuation ranges across all temperatures, acoustic emission characteristics became nearly indistinguishable, and shear crack proportions settled between 26.38 and 32.82 percent regardless of temperature.

The mechanism behind this tug-of-war turned out to be largely a battle between internal pore pressure and external confinement. During low-pressure, high-temperature retention tests, the researchers observed mass loss of 0.55 percent, a collapse of moisture content by 91.79 percent, a 22.44 percent drop in volatile components, and thermal shrink film inflated by escaping colorless gas. Drawing on the classic firedamp drainage observation that anthracite desorbs roughly 0.8 percent of its methane per degree Celsius, and applying the ideal gas law, the team estimated that pore pressure could rise more than six-fold as temperature climbs from 20 to 50 degrees Celsius. Above roughly 60 degrees Celsius, desorbing methane and water vapor generate pressures strong enough to carve gas-erosion crack networks along natural weak directions, explaining the earlier crack initiation, larger fissures and erratic strain paths. At 22 megapascals or more, high confining pressure suppresses molecular activity, inhibits methane desorption and moisture evaporation, raises effective stress, and effectively strangles the gas-erosion process before it begins.

To quantify exactly where control flips from one field to the other, the team devised high-temperature confrontation tests in which samples were heated first and then subjected to gradually increasing confining pressure while deformation was tracked. By analyzing strain rate responses, they defined the controlling confining pressure, the pressure that fully cancels the thermal damage of a given temperature, and the controlling temperature, the temperature that overwhelms a given pressure. The numbers were striking: at 95 degrees Celsius, confining pressures up to 2 megapascals were powerless, with the absolute volumetric strain rate rising 4.5-fold, but at 15 megapascals the sample stabilized with deformation below instrument accuracy. The fitted relationships, a quadratic curve for controlling confining pressure and a linear one for controlling temperature, each achieved a correlation coefficient of 0.99.

Plotting these two curves together produced the study’s headline deliverable: a two-dimensional optimization atlas that divides the deep sampling environment into three preservation zones. Above the quadratic boundary, pressure governs and temperature damage is negligible, so a coring tool needs to maintain only the pressure field. Below the linear boundary, temperature dominates and only the temperature field must be preserved. Between the curves, both fields matter and dual preservation is required. For engineers designing the next generation of fidelity coring tools, the atlas promises a direct path to lower costs, higher success rates and scientifically valid cores, because it replaces blanket five-field preservation with a tailored strategy grounded in the genuine in-situ conditions of each target depth. As China’s mines push past 900 meters into first-level high-temperature zones, that kind of targeted efficiency may determine whether the deep coal frontier can be opened both safely and economically.

Subject of Research: An experimental and theoretical study of the antagonistic temperature-pressure mechanisms controlling the physical and mechanical properties of deep coal and the resulting selection strategy for fidelity coring.

Article Title: Selection strategy for preserving physical fields in deep coal fidelity coring

Article References: He, K., Hao, H., Yang, B., Xie, J., Xu, L., Duan, H., & Gao, M. (2026). Selection strategy for preserving physical fields in deep coal fidelity coring. Results in Engineering, 32, Article 113000. https://doi.org/10.1016/j.rineng.2026.113000

Image Credits: AI Generated

DOI: Not provided

Keywords: deep coal, fidelity coring, temperature-pressure coupling, thermal damage, confining pressure, acoustic emission, methane desorption, pore pressure, triaxial testing, anthracite, pressure preservation, coring technology