The researchers used weathered coal gangue that had been stored in the open air for two years at the Jilin Coal Energy Reserve Base in China. The particles were light gray, rough, loosely structured and unevenly graded, with most concentrated in the 9.5–19.0 mm range. Untreated material had a crushing index of 23.96%, above the 20% limit for Class II crushed stone specified in GB/T 14685-2022, and was therefore considered unsuitable for direct use as a pavement-concrete aggregate. Its other measured properties were a 28.61% Los Angeles wear rate, 24.86% firmness index, 84.32 kgf/mm² Vickers hardness and 7.32% aggregate impact value.

The treatment combined a physical cement coating with chemical hardening. Cement paste was prepared at a water-to-cement ratio of 0.75, and 1.5% magnesium aluminum silicate was added to stabilize the paste and suppress bleeding. Weathered gangue was mixed into the paste for two minutes, soaked for 30 minutes, removed and air-dried for two days. A water-based lithium-based curing agent, diluted at a curing-agent-to-water ratio of 1:2, was then sprayed evenly over the particle surfaces. After another 30 minutes of room-temperature drying, the material was cured for 28 days. The agent’s lithium silicate can react with calcium hydroxide in the cement coating to form calcium silicate hydrate, while cement reactions can produce calcium silicate hydrate and calcium aluminate hydrate on and beneath the aggregate surfaces.

After consolidation and 28 days of curing, the gangue’s crushing index declined from 23.96% to 16.05%, placing it below the cited Class II threshold. Vickers hardness rose from 84.32 to 141.43 kgf/mm², Los Angeles wear loss fell from 28.61% to 19.88%, and the aggregate impact value declined from 7.32% to 5.66%. These laboratory measurements showed greater hardness and resistance to wear and impact-induced fragmentation. Density increased from 2.4751 to 2.619 g/cm³, while water absorption rose slightly from 4.32% to 4.82%, meaning that the treatment strengthened the particles without removing their porous character.

To examine how particle size affected concrete behavior, the team prepared one natural-stone control and four mixtures in which consolidated weathered gangue progressively replaced conventional aggregate over wider size intervals. The Fuller target gradation contained 0–2.36, 2.36–4.75, 4.75–9.50, 9.50–19.0 and 19.0–31.5 mm fractions. The gangue mixtures cumulatively replaced conventional aggregate through 2.36–4.75, 2.36–9.50, 2.36–19.0 and 2.36–31.5 mm, representing 11.46%, 27.55%, 50.29% and 72.63% of the total aggregate. The 0–2.36 mm fraction was not replaced, and the study did not separately test different replacement ratios within an individual size fraction.

Each mixture and specimen type included four replicates, with reported values and force–displacement data calculated as arithmetic means. According to GB/T 50081-2019, 100 mm cubes were used for compressive and shear testing, 150 mm cubes for splitting tensile testing and 100 by 100 by 400 mm prisms for flexural testing. The pavement mix contained 315 kg/m³ of cement, 120 kg/m³ of water, 1% water-reducing admixture, 632 kg/m³ of fine aggregate and 1,283 kg/m³ of coarse aggregate. Specimens were tested after 28 days, using a loading rate of 0.5 MPa/s for compression and shear and 0.05 MPa/s for splitting tension and flexure.

Under the study’s secondary-road criteria, the maximum admissible replacement extended from 2.36 to 19.0 mm. At that setting, the concrete achieved 27.81 MPa compressive strength, 2.13 MPa splitting tensile strength, 4.67 MPa flexural strength, 2.93 MPa cohesion and an internal friction angle of 66.20°. Replacing only the 2.36–4.75 mm fraction yielded flexural and splitting tensile strengths of 5.24 and 3.13 MPa, compared with 5.22 and 3.08 MPa in the control. Because the splitting-strength difference was only about 1.6% and the error bars overlapped, the researchers interpreted it as essentially unchanged. The 2.36–9.50 mm mixture also retained 5.18 MPa flexural strength, above the study’s heavy- and higher-traffic criterion of more than 5.0 MPa.

Mechanical performance generally weakened as larger gangue fractions entered the concrete. Splitting tensile strength was 2.78 MPa for the 2.36–9.50 mm replacement, 2.13 MPa for 2.36–19.0 mm and 1.52 MPa for replacement through 31.5 mm. The latter two values represented reductions of 30.8% and 50.6% relative to the unreplaced control. Flexural strength fell to 3.44 MPa in the fully replaced mixture, 34.1% below the control. The researchers associated the reductions with the lower strength and higher porosity of coarse gangue, which promoted aggregate fracture and stress concentration. They also observed that gangue absorbed water during curing, potentially lowering the local water-to-cement ratio and strengthening the aggregate–mortar bond.

Failure morphology changed alongside the strength measurements. Natural-aggregate concrete developed an X-shaped fracture that began near the corners and propagated through interfacial transition zones, or ITZs, toward the center. In gangue mixtures, cracks increasingly traveled through the gangue as well as the ITZ and mortar, and the overall response became more plastic. Under full replacement, the outermost large gangue particles failed first, after which damage progressed inward. Splitting-tensile failure displacement decreased from 10.43 mm in the control to 4.19 mm in the fully replaced group, indicating reduced deformation capacity. Cohesion declined from 4.23 MPa in the control to 1.75 MPa at full replacement, while the internal friction angle increased to 71.43°; the authors noted that this occurred alongside deterioration in other mechanical properties.

A two-dimensional finite-element model represented the 100 mm compression specimens with 40,000 CPS4R elements and four material phases: gravel, mortar, consolidated gangue and a weakened ITZ. Simulated compressive strengths exceeded the corresponding laboratory means by 5.61–10.68%. For example, the model estimated 44.62 MPa for the control, compared with a measured mean of 41.32 MPa, and 23.68 MPa at full replacement, compared with 21.41 MPa. The study attributes the difference partly to the idealized model, which does not reproduce specimen unevenness and machine error and cannot fully capture thickness-direction behavior, lateral diffusion or all cracking effects. Despite these limitations, the simulations reproduced the observed shift in failure location and crack path.

At the maximum replacement range of 2.36–19.0 mm, the model indicated that natural aggregate in the 19.0–31.5 mm fraction carried 58.31% of the stress contribution. The corresponding shares were 22.88% for 9.50–19.0 mm, 13.43% for 4.75–9.50 mm and 5.38% for 2.36–4.75 mm. Mean stress was 22.54 MPa in the largest natural-stone fraction, compared with 9.30, 9.77 and 9.68 MPa in the progressively smaller fractions. The analysis supports a gradual replacement strategy under the tested conditions: consolidated weathered gangue can occupy smaller aggregate fractions while the largest conventional stones provide much of the modeled load-bearing capacity. Durability under traffic, repeated loading, environmental exposure and field-scale construction was not evaluated, leaving those questions beyond the evidence presented by this laboratory study.

Subject of Research: Mechanical performance of mining-area pavement concrete containing consolidated weathered coal gangue

Article Title: Mechanical behaviors of weathered coal gangue filled concrete pavement based on surface solidification

Article References: Li, Y., Guo, W., Miao, X., Tao, X., Wang, Q., Zhang, J., Chen, W., & Wei, Z. (2026). Mechanical behaviors of weathered coal gangue filled concrete pavement based on surface solidification. Frontiers in Materials, 13, Article 1954367. https://doi.org/10.3389/fmats.2026.1954367

Image Credits: AI Generated

DOI: 10.3389/fmats.2026.1954367

Keywords: weathered coal gangue, concrete pavement, surface consolidation, lithium silicate, recycled aggregate, compressive strength, flexural strength, failure modes, finite-element simulation, mining infrastructure, Mechanical, behaviors

The research team, led by Hangli Yang and Haoxiang Luan with colleagues including Fei Geng, Yupeng Xu, Hongke Zheng, and Jianguang Xie, built a three-dimensional discrete element model, or DEM, of macroporous recycled concrete and constrained it with an unusually rich set of laboratory measurements. Unlike continuum models that treat concrete as a smeared material, DEM represents the structure as thousands of individual particles connected by bonds that can stretch, shear, and snap. That makes it possible to watch the internal load-bearing network, known as force chains, reorganize as frost damage accumulates. Because no existing simulation platform can directly solve heat transfer, ice–water phase change, and moisture migration together, the team used an experimentally calibrated equivalent-damage approach, updating contact stiffness and bond strengths cycle by cycle rather than applying a single blanket reduction.

The experimental foundation came from the group’s earlier tests on three concrete mixes, labeled M1–20%, M2–25%, and M5–30%, where the numbers denote paste type and designed porosity. The recycled coarse aggregate, crushed from demolished concrete with particles between 9.5 and 37.5 millimeters, consisted of a natural stone core wrapped in a layer of old cement paste covering 23.1 percent of its surface. The researchers measured an impressive array of properties at different numbers of freeze–thaw cycles: compressive, splitting tensile, and shear strengths of the new paste; nanoindentation elastic moduli of the paste, the old paste, and their interfaces with natural aggregate; the rate at which old paste detached from recycled aggregate; and the mass change, strength, and elastic modulus of full specimens.

One of the most striking findings emerged immediately. The specimens did not simply decay monotonically. Instead, their mass and elastic modulus both rose during the first 40 cycles before declining. The explanation lies in a chemical race: water entering the connected pores promoted continued hydration of residual unhydrated cement, and the additional hydration products partially filled small pores, stiffening the paste and its interfaces faster than early frost microdamage could soften them. Only after about 40 cycles did accumulated microcracking, interfacial debonding, and surface spalling win out, sending stiffness into a steep decline. Freeze–thaw deterioration, the study shows, is a staged process of competing mechanisms rather than a steady slide.

To capture the geometric side of damage, the team converted measured mass loss into a count of lost surface paste particles. Because spalling occurred mainly in new-paste bridge regions rather than in aggregate cores, the net solid mass loss could be mapped onto removal of surface paste elements from the model. The algorithm allowed only particles on the current external surface, identified by a bond coordination number below six, to be deleted, so erosion progressed gradually inward without creating nonphysical internal voids. By 160 cycles, the models had shed 817 particles for the densest mix and up to 1,775 for the most porous one, each removal representing a real loss of load-bearing contact.

Validation against compression tests was convincing. Mean errors in peak stress across all groups and cycle counts ranged from 5.2 to 6.0 percent, while peak-strain errors stayed between 12.0 and 15.5 percent. The simulated elastic moduli, though noisier with errors of 10.8 to 18.1 percent, reproduced the characteristic rise-and-fall trend, and a new porosity-controlled prediction equation for post-freeze–thaw elastic modulus achieved a mean calculation error of just 7.6 percent. After 160 cycles, the peak-stress losses reached 38.3 percent for M2–25%, 35.8 percent for M5–30%, and 32.6 percent for M1–20%, matching the laboratory ordering and confirming that freeze–thaw resistance is governed jointly by porosity and the quality of the paste and its interfaces.

The deepest insight came from tracking force chains, the quasi-linear chains of highly stressed contacts that carry most of the load through granular skeletons. In undamaged and early-cycle specimens, load flowed through a few dominant strong chains concentrated in the specimen center and along inclined diagonal paths, producing the classic X-shaped shear failure. The coefficient of variation of force-chain intensity peaked at 1.847 at 40 cycles, when stress concentration was greatest. As cycling continued, however, the degradation of paste-related interfaces and the loss of surface particles progressively severed these dominant chains, forcing load to redistribute from a few high-capacity paths to many dispersed low-capacity ones. The coefficient of variation fell to 1.609 by 160 cycles.

That decline might superficially look like homogenization, as if the material were becoming more uniform and therefore tougher. The researchers show it is anything but. They call the phenomenon damage-induced pseudo-homogenization: the apparent evening-out of the force-chain distribution arises purely because the strongest load paths have failed, at the very moment peak stress and elastic modulus continue to fall. Simultaneously, the proportion of tensile bond failures grew, rising by 4.3 to 6.3 percent across the groups after 160 cycles, and the macroscopic failure mode shifted from X-shaped shear to longitudinal splitting as cracks found weakened interfaces and coalesced parallel to the loading direction. The toughness index dropped by up to 33.1 percent in the most porous mix, meaning the extra deformation capacity reflected damage, not ductility.

The bond-failure statistics also pinpointed the material’s Achilles’ heel. Across all freeze–thaw states, the failure rate ordering never changed: the interfaces between new and old paste, and between new paste and natural aggregate, failed most often, followed by paste-to-paste bonds, with old-paste-related bonds failing least. After 160 cycles, the fastest-growing failure rates belonged precisely to those new-paste interfaces, identifying them as the weak zones through which frost damage first develops and then transfers into the load-bearing skeleton. Notably, the tensile-failure proportion began climbing after 120 cycles for the densest mix but after only 40 cycles for the most porous one, showing how higher porosity concentrates stress and accelerates the onset of terminal damage.

Beyond its scientific contribution, the work carries practical weight for cold-region infrastructure. Permeable pavements, municipal and airport drainage bases, and sponge-city surfaces built with recycled aggregate can now be evaluated with a model that distinguishes which phases and interfaces are degrading, rather than relying on bulk indicators like mass loss or dynamic modulus that blur the picture. The authors suggest their framework can guide material selection, pore-structure design, durability assessment, and maintenance planning. The study’s validation remains an internal consistency check within a single material system, so independent confirmation on other mixes and laboratories will be the next step. But the central message stands: when ice attacks porous recycled concrete, the most dangerous change is not the damage you can weigh, but the silent rewiring of the force chains that hold the pavement together.

Subject of Research: Mesoscopic discrete element modeling of freeze–thaw deterioration mechanisms in macroporous recycled concrete

Article Title: Mesoscopic DEM study of freeze–thaw deterioration in macroporous recycled concrete: Interfacial damage, particle spalling, and force-chain reconstruction

Article References: Yang, H., Luan, H., Geng, F., Xu, Y., Zheng, H., & Xie, J. (2026). Mesoscopic DEM study of freeze–thaw deterioration in macroporous recycled concrete: Interfacial damage, particle spalling, and force-chain reconstruction. Case Studies in Construction Materials, 25, Article e06561. https://doi.org/10.1016/j.cscm.2026.e06561

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06561

Keywords: macroporous recycled concrete, freeze–thaw cycles, discrete element method, force chains, interfacial transition zone, recycled aggregate, surface spalling, pseudo-homogenization, permeable pavement, sponge city, elastic modulus, bond failure