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

A team led by Chaojie Wang and Chengchao Guo set out to answer a deceptively simple question: how strong is the interface between polymer-stabilized soil and concrete, and how quickly does that strength fade when the interface is submerged? The researchers used a two-component permeable polymer grout, mixing a polyether polyol-based component with an isocyanate-rich component in equal masses. The liquid penetrates soil pores under controlled grouting pressure, then cures into a solid that binds soil grains together and glues them to concrete surfaces. This chemistry, based on urethane and urea linkages, gives the material its high bonding strength and low viscosity, but those same chemical bonds are vulnerable to hydrolysis, the slow reaction in which water molecules break chemical links apart.

The experimental program was meticulous. Soil excavated from a construction site in Zhengzhou, Henan Province, was air-dried, sieved, and re-wetted to controlled moisture contents of 6 and 11 percent. Concrete cubes of three strength grades, C30, C40, and C50, were cast, cured for 28 days, and then precision-cut with a CNC machine into half-cubes whose cut faces formed the test interfaces. The researchers machined grooves into some of these faces to create four levels of surface roughness, quantified using the joint roughness coefficient scale developed by Barton and Choubey, ranging from perfectly smooth to a JRC of 19.5. Soil was compacted in layers onto the concrete surfaces inside a custom mold, and a purpose-built constant-pressure grouting rig, monitored by a digital pressure sensor sampling at 1 hertz, injected the polymer until penetration was complete.

Once cured, the specimens faced their ordeal. After seven days of natural hardening, they were submerged in water for 7, 21, or 56 days, then sheared along the polymer-stabilized-soil-to-concrete interface in a servo-controlled direct shear machine under normal stresses of 200, 400, and 600 kilopascals. The resulting shear stress-displacement curves told a consistent story across every combination of concrete grade, soil density, grouting pressure, and roughness. Each curve rose elastically, peaked, softened, and settled to a residual plateau, the classic signature of a quasi-brittle interface that fails suddenly rather than stretching plastically.

The numbers reveal a striking three-stage decay pattern. The most dramatic loss of strength came early: within the first seven days of immersion, peak shear strength dropped by just over 20 percent on average, and the decline was steepest of all in that opening week. Between 7 and 21 days the erosion of strength continued but slowed, and between 21 and 56 days the curves flattened into a plateau. After 56 days underwater, the peak shear strength had fallen by roughly 30 percent overall, with a worst case of 35.15 percent. Crucially, the interface never collapsed entirely. The residual bonding capacity stabilized at approximately 60 to 65 percent of the original dry strength, suggesting that the polymer possesses an intrinsic water resistance that prevents complete bond loss.

Normal stress turned out to amplify the damage. Under identical immersion times, specimens sheared at higher normal stress lost a larger fraction of their strength than those tested at lower stress, because water lubricates the sliding surfaces more effectively when they are pressed together harder. The same pattern appeared in the residual strength, which is governed almost entirely by sliding friction once the bond has failed. After 56 days of immersion, residual shear strength had decayed by about 35 percent, exceeding the peak-strength decay, with a maximum of 38.44 percent. Fitting the data to the Mohr-Coulomb framework showed why: the interfacial friction angle, a measure of how much the rough surfaces resist sliding, proved more water-sensitive than cohesion, the chemical-glue component of strength. After 56 days, the friction angle at peak strength had decayed by up to 36.57 percent, and at residual strength by as much as 40.67 percent, while cohesion losses peaked near 26 percent.

Microscopy explained the mechanism. Scanning electron microscope images of the stabilized soil showed that water dissolved protruding polymer films, loosened surface soil particles, and smoothed the once-rough interface, eliminating the mechanical interlocking that had helped resist shear. ImageJ analysis of the micrographs revealed that apparent porosity climbed from about 25 percent in unimmersed samples to just under 27 percent after 56 days, with most of the growth occurring early. Mercury intrusion porosimetry confirmed the trend: the porosity of grouted soil rose from 21.49 percent before immersion to 26.49 percent after 56 days, while average pore size grew by 43 percent. Yet even the degraded soil remained far denser than the ungrouted silt, whose porosity of nearly 50 percent dwarfed anything measured after treatment. The water damage concentrated in pores between 7 and 200 micrometers, carving the fine through-cracks and voids that directly weaken the bond.

The failure surfaces themselves shifted character with soaking. In dry conditions, shear failure typically tore through the interior of the stabilized substrate, leaving soil smeared across the concrete, a mode the authors call type A. With longer immersion, less soil adhered to the concrete after failure, and the fracture plane migrated to the polymer-soil interface itself, the cleaner type B mode. Quantitatively, the proportion of soil clinging to the concrete surface fell steadily with immersion time and stabilized, while higher normal stress pushed failures back toward the substrate. The team also established a practical quality-control clue: specimens with total porosity above roughly 23 to 24 percent were far more likely to fail at the interface, and porosity correlated linearly with strength decay regardless of normal stress.

To turn these observations into a design tool, the researchers derived a three-stage damage constitutive model that captures the full bond-slip curve: a smooth rising branch, an S-shaped softening segment after the peak, and a frictional residual plateau described by the Mohr-Coulomb law. Fitted to the experimental data with the Levenberg-Marquardt algorithm, the model achieved coefficients of determination between 0.9275 and 0.9944 across all immersion times and stress levels, and it correctly reproduced the post-immersion shift toward greater ductility, in which the interface deforms further before reaching its diminished peak.

The authors are candid about the limits of their work. The five test combinations came from a preceding orthogonal design rather than a full factorial matrix, vertical displacement during shearing was not measured, and the hypothesis that strength stabilizes because accessible hydrolysable bonds are consumed early, leaving hydrophobic-protected links intact, rests on microscopy and literature rather than direct spectroscopy. Still, the practical message is clear. For trenchless tunnel repairs in water-rich ground, the finding that roughly 60 to 65 percent of interfacial strength survives 56 days of immersion offers a conservative starting point for design, while the porosity threshold gives inspectors a measurable warning sign. Water attacks the glue quickly, but it does not dissolve it completely, and knowing exactly where the decay stops may be the most valuable number this study delivers.

Subject of Research: Shear strength degradation and damage modeling of the polymer-stabilized soil to concrete interface under water immersion

Article Title: Research on the shear strength and damage constitutive model of interface between polymer stabilized soil and concrete under water immersion

Article References: Wang, C., Ding, L., Diao, Y., Zhang, X., Guo, C., Wang, F., & Du, X. (2026). Research on the shear strength and damage constitutive model of interface between polymer stabilized soil and concrete under water immersion. Case Studies in Construction Materials, 25, Article e06528. https://doi.org/10.1016/j.cscm.2026.e06528

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06528

Keywords: permeable polymer grouting, tunnel reinforcement, soil-concrete interface, direct shear test, water immersion, shear strength decay, Mohr-Coulomb parameters, damage constitutive model, mercury intrusion porosimetry, scanning electron microscopy, hydrolysis, geotechnical engineering