The research, led by Jiazhen Sun and colleagues, tackles a class of materials known as low-cement concrete, or LCC. Instead of relying on ordinary Portland cement as the dominant binder, these mixes replace most of it with a carefully orchestrated blend of supplementary cementitious materials: ground granulated blast-furnace slag, fly ash, limestone powder, and iron tailing powder. The team designed four strength grades, from LC20 to LC50, in which cement content fell as low as 71.8 kilograms per cubic meter, compared with 359 kilograms in the reference mix. By weight, that means some of the mixes contained barely a fifth of the cement of conventional concrete, yet all met the compressive strength requirements of the corresponding Chinese standard grades at 28 days.

The chemistry behind this achievement is a story of staggered reactions. Slag, rich in calcium and already glassy in structure, activates early in the alkaline pore solution and begins forming calcium-aluminate-silicate-hydrate gel, the sticky glue of hardened concrete. Fly ash and iron tailing powder, which demand higher alkalinity before they react, hold back and then contribute steadily at middle and later ages. Limestone powder, largely inert, plays a physical role as a nucleation surface and microfiller while also reacting with aluminate phases to form carboaluminate compounds that add solid volume. The result is a binder system in which different components fire at different times, like relays in a long race, sustaining hydration for months.

A central worry with such extreme cement reduction is whether the pore solution stays alkaline enough to keep these secondary reactions running. The team measured pore-solution pH directly and found that even the leanest mix maintained a pH above 12.2 through 56 days of curing, with a minimum of 12.25. That is sufficient to depolymerize the glassy phases of slag and fly ash and keep the reaction engine turning. In fact, the pH of the low-cement pastes declined continuously with age, a sign that hydroxide ions were being actively consumed by ongoing secondary hydration rather than lost to a dying system.

The spectroscopic and thermal evidence told a consistent story. X-ray diffraction detected no portlandite, the crystalline calcium hydroxide that cement hydration normally produces in abundance, in the leanest mixes, and only weak traces in the higher grades. In the pure cement paste, portlandite content climbed from about 42 to 55 percent of the crystalline phases over two months. In the low-cement pastes, the trend reversed: calcium hydroxide was consumed almost as fast as it was made, dropping to between 0.71 and 1.62 percent by 56 days. Infrared spectroscopy showed the silicate band shifting toward higher wavenumbers, a fingerprint of increasingly polymerized, aluminum-bearing C-A-S-H gel. Thermogravimetric analysis, carefully corrected for the overlapping decomposition of iron tailing minerals and for limestone-derived carbonate, confirmed the same depletion.

The most eye-catching findings concerned the interfacial transition zone, the thin shell of paste surrounding each aggregate particle. In classical concrete science, this zone is the material’s Achilles heel. Because aggregate surfaces act like walls during casting, cement particles cannot pack densely against them, leaving a porous band, often tens of micrometers wide, enriched in weak, plate-like calcium hydroxide crystals. Cracks preferentially initiate and propagate here, and durability problems follow. Using backscattered electron imaging and a strip-based porosity profiling method, dividing the region next to the aggregate into fifteen strips ten micrometers apart, the researchers quantified this zone with unusual precision.

What they found upends the textbook picture. At three days, the interface of the low-cement concrete was indeed more porous than that of ordinary concrete, because the slow-starting supplementary materials had not yet produced enough hydration product to fill the voids left by the wall effect. But by 28 days the situation had inverted completely. The interfacial zone of the LC30 mix narrowed to roughly 60 micrometers, ten micrometers thinner than a pure cement paste control and thirty micrometers thinner than an ordinary C30 concrete. Its average porosity fell to about 4.7 percent, lower than the 6.2 percent of the surrounding matrix. The weak link had become a strengthened region, denser than the bulk paste it surrounds.

The mechanism is a triple play of packing, chemistry, and time. Fine, multisized supplementary particles pack more efficiently against the aggregate, softening the wall effect from the start. As secondary hydration proceeds, the moderate residual calcium hydroxide near the interface, rather than accumulating into weakening crystals, serves as a calcium reservoir and alkaline buffer that feeds the formation of additional C-A-S-H gel exactly where it is needed. Unreacted ultrafine particles lingering near the aggregate continue to hydrate under the low water-to-binder conditions, providing a sustained densification drive. Porosity profiles even revealed a subtle non-monotonic pattern in ordinary concrete, with porosity peaking a short distance from the aggregate surface, a signature of the wall effect that the low-cement system largely erased.

The macroscopic payoff showed up in long-term strength. Between 28 and 180 days, the low-cement mixes gained between 16.6 and 42.6 percent in compressive strength, with the leanest LC20 mix climbing from 32.6 to 46.5 megapascals. The ordinary reference concrete gained only 8.2 percent over the same period. Statistical analysis by two-way ANOVA with Holm-adjusted post hoc contrasts confirmed that every low-cement mix’s later-age gain was significantly greater than the reference, with p values ranging from below 0.001 to 0.029. The same calcium hydroxide that ordinary concrete leaves idle becomes, in this system, the fuel for months of continued gel formation, filling capillary pores in both the matrix and the interface.

The climate arithmetic is equally compelling. A cradle-to-gate assessment of embodied carbon showed the low-cement mixes cutting emissions by 44.7 to 68.8 percent relative to the conventional reference, with the leanest mix emitting just 101 kilograms of carbon dioxide equivalent per cubic meter against 324 for the control. For an industry under intense pressure to decarbonize, the message of this study is that cutting cement does not have to mean cutting performance. With the right blend of reactive and inert powders, staged hydration kinetics, and attention to particle packing, the concrete of the future may be both dramatically greener and, in its most vulnerable microstructural region, genuinely tougher than what it replaces.

Subject of Research: Microstructural evolution and interfacial transition zone densification in low-cement concrete with high volumes of supplementary cementitious materials

Article Title: Microstructural characteristics of low-cement concrete：From matrix to interfacial transition zone

Article References: Sun, J., Li, X., Chang, Z., Yang, C., Zhang, Q., Tang, Z., Han, Y., Miao, C., Wang, R., & Hu, Z. (2026). Microstructural characteristics of low-cement concrete：From matrix to interfacial transition zone. Case Studies in Construction Materials, 25, Article e06604. https://doi.org/10.1016/j.cscm.2026.e06604

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06604

Keywords: low-cement concrete, interfacial transition zone, supplementary cementitious materials, ground granulated blast-furnace slag, fly ash, limestone powder, iron tailing powder, C-A-S-H gel, calcium hydroxide, compressive strength, embodied carbon, sustainable construction

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