Pre-Soaked Ceramic Particles Turn Low-Carbon Cement Into Lightweight, Chloride-Proof Concrete

Concrete is the most widely used building material on Earth, but its weight is a silent tax on every structure it forms. Heavy concrete demands beefier foundations, adds seismic mass in earthquake zones, and complicates the retrofitting of aging bridges and towers. A new study published in Case Studies in Construction Materials reports a way to slash that weight without sacrificing strength or durability, by pairing two sustainability workhorses of modern construction: limestone calcined clay cement, known as LC3, and pre-wetted shale ceramsite, a porous, lightweight aggregate that doubles as an internal water reservoir. The result is a mortar that is lighter than conventional mixes, stronger per kilogram than ordinary C50 concrete, and dramatically more resistant to chloride ion penetration, the chief enemy of reinforced structures in marine environments.

The research team, led by Qiaoyun Wu, Zhifeng Xu, Xuyong Chen, and Shukai Cheng, set out to resolve a long-standing trade-off. Lightweight aggregate concrete reduces structural dead load, improves thermal insulation, and lowers seismic forces, which is why it appeals to builders of high-rises, long-span bridges, and coastal infrastructure. But lightweight aggregates are inherently weaker and more porous than natural sand and stone, so piling them into a mix eventually erodes mechanical performance. Earlier work on ultra-high-performance concrete showed that strength first rises and then falls as lightweight aggregate content increases, peaking at moderate replacement levels. The question was whether the superior chemistry of LC3 could compensate for higher doses of a weak, porous aggregate, and whether pre-soaking that aggregate could tip the balance further.

LC3 itself is one of the most promising low-carbon binders in cement science. It blends ordinary Portland cement with calcined clay and limestone powder, typically replacing 30 to 50 percent of the clinker. That substitution cuts carbon dioxide emissions by roughly 30 percent, from about 834 kilograms of CO2 per tonne of cement down to about 556 kilograms, while delivering 28-day strengths comparable to ordinary Portland cement. The magic lies in a synergistic reaction: aluminate species released by the calcined clay react with carbonate ions from the limestone to form carboaluminate hydrates, known as hemicarbonate and monocarbonate phases, alongside additional calcium-aluminosilicate gel. These phases fill pores, consume portlandite, and refine the microscopic architecture of the hardened paste.

Shale ceramsite, the second ingredient, is an expanded, sintered clay aggregate with an apparent density of 1720 kilograms per cubic meter and a water absorption capacity of 5.3 percent. Its porous interior can soak up water and release it later, a property the researchers exploited through internal curing. In the experiments, the ceramsite was either used dry or fully pre-wetted to its saturated surface-dry condition. The pre-wetting water was carefully subtracted from the mixing water so that every mixture shared the same effective water-to-binder ratio of 0.4, ensuring that any performance differences could be attributed to the internal reservoir effect rather than to extra free water. The sand-to-binder ratio was varied from 1.1 to 1.5, and the binder was held constant at 420 parts cement, 120 parts metakaolin, and 60 parts limestone powder per cubic meter.

The mechanical results were striking. All mixtures weighed in below 1700 kilograms per cubic meter when oven-dried, comfortably satisfying the lightweight mortar criterion of 2000 kilograms per cubic meter under EN 206-1. As expected, increasing the aggregate content reduced both density and strength. But the pre-wetted series consistently outperformed its dry-aggregate twin at 7 and 28 days, with the best formulation showing a 10.8 percent gain in 28-day compressive strength at the lowest aggregate ratio. Flexural strength told a subtler story: it fell by 25 to 35 percent as ceramsite content rose, a steeper decline than the 17 to 18 percent seen in compression. The porous particles act as preferential crack initiation sites under tension, while under compression the matrix can redistribute load around the softer inclusions. Pre-wetting strengthens the interfacial transition zone through enhanced local hydration but cannot fully erase the aggregate’s intrinsic weakness.

Perhaps the most commercially compelling number is specific strength, the ratio of compressive strength to density. Across a 36 percent increase in ceramsite dosage, the specific strength of both series remained remarkably stable, and the optimal pre-wetted mixes exceeded 50 megapascals with specific strengths above 0.03 megapascals per unit density. Conventional C50 concrete manages only 0.017 to 0.021, meaning the new material delivers roughly 1.7 to 2.0 times the strength per unit weight. That efficiency translates directly into slimmer columns, lighter foundations, and smaller seismic inertia forces, savings that compound across an entire structure.

Durability results were even more dramatic. In capillary water absorption tests, the pre-wetted mixes absorbed 64 to 67 percent less water over 150 hours than their dry-aggregate counterparts at identical aggregate ratios. Electric flux measurements, a standard proxy for chloride penetrability, told the same story: the pre-wetted mortars showed 59 to 80 percent lower flux, and the best mix registered below 100 coulombs, placing it in the very low permeability class, a level normally associated with high-performance concrete. For marine and coastal construction, where chloride-induced rebar corrosion is the dominant failure mode, that classification carries real practical weight.

The microstructural evidence explains why. X-ray diffraction revealed stronger hemicarbonate and monocarbonate peaks in the pre-wetted samples, confirming that the sustained internal water supply accelerates the signature LC3 reaction between metakaolin-derived aluminate and limestone carbonate. Thermogravimetric analysis quantified the shift: the best pre-wetted mix contained more carboaluminate hydrates and more calcium-aluminosilicate gel while consuming portlandite, converting a relatively inert crystalline byproduct into pore-filling binding phases. Non-evaporable water content, a proxy for hydration degree, was consistently higher in the pre-wetted series, and scanning electron microscopy showed fewer unhydrated clinker grains, fewer portlandite crystals, and a denser, crack-free matrix.

Mercury intrusion porosimetry completed the picture. The pre-wetted mortar with the lowest aggregate ratio achieved the highest gel pore fraction, 47.2 percent, and the lowest macroporosity, just 3.1 percent, with the pore size distribution shifting decisively toward fine capillary and gel pores. Because capillary water uptake is governed by large capillary pores while chloride migration depends on the connectivity of the whole network, this refinement suppresses both transport mechanisms simultaneously. The authors are careful to note that electric flux reflects pore solution chemistry as well as pore structure, and they recommend confirmation through long-term chloride migration and diffusion testing, along with studies of carbonation, freeze-thaw, and sulfate resistance.

The sustainability ledger is nuanced but favorable. Sintering ceramsite consumes about 0.534 gigajoules per tonne, nearly eight times the energy of natural sand, because the process requires kilns running at 1100 to 1250 degrees Celsius. Yet the aggregate can be made from industrial wastes such as fly ash and coal gangue, its lightness shrinks foundation and reinforcement demands, its insulating blocks cut building heating and cooling loads, and its low bulk density reduces transport emissions per cubic meter. Weighed over a full life cycle, the authors argue, these operational and structural savings outweigh the production penalty. Combined with LC3’s 30 percent carbon advantage at the binder stage, the system points toward a generation of lightweight, high-strength, chloride-resistant concrete suited to the decarbonized, resilient infrastructure the coming decades demand.

Subject of Research: Internal curing and phase assembly in limestone calcined clay cement mortar with pre-wetted shale ceramsite

Article Title: Synergistic internal curing and phase assembly in limestone calcined clay cement mortar incorporating pre-wetted shale ceramsite: Mechanical properties, chloride resistance, and pore structure refinement

Article References: Wu, Q., Xu, Z., Chen, X., & Cheng, S. (2026). Synergistic internal curing and phase assembly in limestone calcined clay cement mortar incorporating pre-wetted shale ceramsite: Mechanical properties, chloride resistance, and pore structure refinement. Case Studies in Construction Materials, 25, Article e06607. https://doi.org/10.1016/j.cscm.2026.e06607

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06607

Keywords: limestone calcined clay cement, LC3, shale ceramsite, lightweight aggregate concrete, internal curing, chloride resistance, pore structure, carboaluminate hydrates, sustainable construction, compressive strength, capillary absorption, low-carbon cement