Every year, coal mining leaves behind mountains of coal gangue, a rocky byproduct of mining, washing, and separation that piles up across vast tracts of land, sheds dust, risks spontaneous combustion, and leaches acidic drainage into surrounding soil and water. At the same time, the construction industry consumes enormous quantities of natural sand and gravel, driving demand for alternative aggregate resources. A new study published in Case Studies in Construction Materials offers a strikingly practical answer to both problems: crush coal gangue into concrete aggregate, then coat each particle with cheap, locally available minerals to fix the one weakness that has always held this recycled material back. The research, led by Yaoyu Wang and colleagues, demonstrates that the right surface treatment can transform coal gangue concrete from a material that crumbles under freezing conditions into one that survives dozens of punishing cycles with most of its strength intact.
The core challenge lies in something invisible to the naked eye: the interfacial transition zone, or ITZ, the thin band of cement paste that bonds directly to the surface of each aggregate particle. In ordinary concrete, the ITZ is often the weakest region of the whole material, but coal gangue makes the problem dramatically worse. The rock is riddled with pores, bedding-related defects, pre-existing microcracks, and weak mineral regions, and its exposed surface is dominated by aluminosilicate minerals that react only weakly under normal curing conditions. Although the angular, rough texture of crushed gangue does provide good mechanical interlocking with the surrounding paste, that advantage cannot compensate for the discontinuous, mechanically feeble boundary that forms where aggregate meets binder. When water finds its way into these surface-connected defects, differences in pore structure and stiffness between the aggregate and the paste concentrate stress near the interface and invite cracks to begin exactly where the material can least afford them.
The research team attacked this weakness with three humble modifiers, all readily available in coal-mining regions: sodium silicate, silica fume, and limestone powder. Each works through a different route. Sodium silicate can penetrate and seal open pores and microdefects, while its soluble silicate species participate in reactions near the aggregate surface that improve interfacial continuity. Silica fume, an ultrafine powder of nearly pure amorphous silica, physically fills surface irregularities and interfacial voids and can later react pozzolanically to refine the surrounding cementitious products. Limestone powder contributes mainly through particle packing, heterogeneous nucleation of hydration products, and carbonate-related interactions with aluminate-bearing phases. The researchers applied each modifier at dosages of 2, 3, and 4 percent by aggregate mass, dry-mixing the powder onto washed, surface-dried gangue particles before the concrete was batched.
The concrete itself was deliberately unconventional. Instead of ordinary Portland cement, the team used a coal-based solid-waste binder containing 50 percent benchmark cement, 25 percent calcined coal gangue powder, 10 percent limestone powder, 10 percent fly ash, and 5 percent flue gas desulfurization gypsum, a formulation that turns more mining and power-plant waste into a useful building material. The calcined gangue powder was produced by heating ground raw gangue first to 500 degrees Celsius and then to 800 degrees, activating its aluminosilicate content. All mixtures shared a fixed water-to-binder ratio of 0.40 and an aggregate-to-binder ratio of 1.50, so that the only variable was the type and dosage of the surface coating. Compressive strength screening at 3, 7, and 28 days identified the best dosage for each modifier: 4 percent sodium silicate, 3 percent silica fume, and 4 percent limestone powder.
The most revealing part of the study came from nanoindentation, a technique that presses a microscopic diamond tip into a polished specimen surface to measure local stiffness point by point. The researchers ran indentation paths of 25 points, spaced 10 micrometers apart, across the aggregate, the ITZ, and the paste on either side. In every mixture, the modulus held steady at roughly 50 to 55 gigapascals inside the aggregate, plunged within the interfacial zone, bottomed out near 110 micrometers from the boundary, and then recovered toward the paste. But the depth of that plunge depended strongly on the treatment. The unmodified concrete reached a minimum of about 18 gigapascals; limestone powder raised it to 22, silica fume to 26, and sodium silicate to 33 gigapascals, the latter producing the smoothest mechanical transition across the entire interface.
Scanning electron microscopy gave those numbers a visible structure. The unmodified interface showed coarse residual particles, loose agglomerates, open pores, and discontinuous patches. Sodium silicate treatment, by contrast, produced abundant flocculent, fibrous, and fine granular products distributed around the aggregate surface, yielding a comparatively continuous and compact boundary. Silica fume yielded a more uniform fine-grained morphology, though some spherical residual particles remained visible. Limestone powder produced a heterogeneous picture, with compact product-rich regions coexisting alongside loose, porous zones and needle-like, plate-like reaction products morphologically consistent with carbonate-bearing AFm-type phases such as monocarboaluminate and hemicarboaluminate, though the authors caution that SEM alone cannot confirm phase identity.
Then came the durability trials, framed in an unusual but physically precise vocabulary. Rather than the conventional labels of wet-dry and freeze-thaw cycling, the researchers speak of liquid-vapor and liquid-solid phase cycling, emphasizing what the pore water itself is doing. In the liquid-vapor test, specimens spent 8 hours immersed in water, 8 hours drying naturally, and 8 hours in a 75-degree-Celsius oven, repeating the absorption-evaporation cycle up to 30 times. In the liquid-solid test, a rapid freeze-thaw apparatus swung the specimen core temperature between minus 35 and plus 20 degrees Celsius every 5 hours, forcing pore water to freeze, redistribute, and melt again and again. The results diverged sharply. After 30 liquid-vapor cycles, silica fume-modified concrete lost only 9.3 percent of its compressive strength, dropping from 27.9 to 25.3 megapascals, while the unmodified control lost 18.8 percent. Freezing proved far more brutal: after 15 liquid-solid cycles, the unmodified and limestone-modified specimens had lost roughly 70 and 67 percent of their strength, and by 30 cycles both had collapsed beyond the point of valid testing.
The survivors told the most important story. Sodium silicate-modified concrete, which had not been the best performer in the wet-dry regime, proved the champion of freeze-thaw resistance, retaining the highest residual strength, 9.7 megapascals after 30 cycles, and enough structural integrity to keep testing. Post-cycling nanoindentation after 15 cycles confirmed that the damage was concentrated precisely where the team expected: the aggregate region barely changed, the paste weakened moderately, and the ITZ deteriorated most severely. Silica fume’s interface lost only 3.8 percent of its minimum modulus under liquid-vapor cycling, while sodium silicate’s interface retained both the highest absolute modulus and the smallest relative loss, 24.8 percent, under liquid-solid cycling. The lesson is that different coatings build different kinds of interfacial quality, and the best choice depends on the environment the concrete will face.
The mechanistic explanation ties the whole picture together. Surface modification does not eliminate water-induced deterioration; it delays its initiation, localization, and propagation within the ITZ. Silica fume’s fine particles plug the small voids where moisture redistribution would otherwise nucleate microcracks, making it ideal for hot, drying climates with repeated wetting. Sodium silicate, supplying soluble silicate species into an alkaline local environment, appears to foster a more continuous, product-rich interfacial fabric that can distribute the internal pressures generated by ice formation and delay debonding and crack coalescence, making it the better choice for cold regions. Limestone powder, with its more heterogeneous product distribution, offered the least protection against either regime. The authors note that water-migration pathways were inferred from combined macroscopic, microscopic, and nanoindentation evidence rather than directly measured, and that local alkalinity changes and individual reaction products were not quantified.
What makes this work resonate beyond the laboratory is its double dividend: it converts an environmental liability into infrastructure material while simultaneously making that material tougher than anyone expected. Because the binder itself is built from calcined coal gangue, fly ash, and desulfurization gypsum, an entire concrete could one day be assembled almost entirely from industrial byproducts, with nothing more exotic than sodium silicate, silica fume, or ground limestone standing between a weak interface and a durable one. For coal regions searching for a future after mining, and for a construction sector under pressure to cut its carbon and resource footprint, the humble act of coating a waste rock particle before it enters a mixer may prove to be one of the most consequential steps in sustainable materials engineering.
Subject of Research: Surface modification of coal gangue aggregate to strengthen the interfacial transition zone in coal-based solid-waste concrete
Article Title: Surface modified coal gangue aggregate in coal-based solid-waste concrete: ITZ strengthening and resistance to water phase-transition cycling
Article References: Wang, Y., Yang, Y., Yu, C., Li, R., Chang, J., & Zhang, T. (2026). Surface modified coal gangue aggregate in coal-based solid-waste concrete: ITZ strengthening and resistance to water phase-transition cycling. Case Studies in Construction Materials, 25, Article e06572. https://doi.org/10.1016/j.cscm.2026.e06572
Image Credits: AI Generated
DOI: 10.1016/j.cscm.2026.e06572
Keywords: coal gangue, concrete, interfacial transition zone, surface modification, sodium silicate, silica fume, limestone powder, freeze-thaw resistance, solid waste recycling, nanoindentation, sustainable construction, coal-based binder
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Neil Sanderson. (October 2, 2026). Turning Coal Waste Into Tougher Concrete: Coating the Weak Link at the Interface. Scienmag. https://scienmag.com/turning-coal-waste-into-tougher-concrete-coating-the-weak-link-at-the-interface/
Neil Sanderson. “Turning Coal Waste Into Tougher Concrete: Coating the Weak Link at the Interface.” Scienmag, 2 October 2026, https://scienmag.com/turning-coal-waste-into-tougher-concrete-coating-the-weak-link-at-the-interface/. Accessed 2 October 2026.
Neil Sanderson. “Turning Coal Waste Into Tougher Concrete: Coating the Weak Link at the Interface.” Scienmag. October 2, 2026. https://scienmag.com/turning-coal-waste-into-tougher-concrete-coating-the-weak-link-at-the-interface/
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Tags: coal ganguecoal gangue recyclingcoal waste concrete enhancementcoal waste utilization in constructioncoal-based binderconcretedurability of coal gangue concreteenvironmentally friendly concretefreeze-thaw resistancefreeze-thaw resistance in recycled concreteinnovative construction material technologiesinterfacial transition zoneinterfacial transition zone improvementlimestone powdermineral coating for aggregate strengthnanoindentationreducing coal mining waste environmental impactsilica fumesodium silicatesolid waste recyclingsurface coating for recycled aggregatessurface modificationsustainable constructionsustainable construction materials
