The research, published in the journal Construction and Building Materials, examined whether red mud, a highly alkaline residue generated in enormous quantities by aluminum refining, could be combined with dredged sediment to replace a substantial portion of conventional Portland cement in concrete formulations. The team, led by Faber Chaparro and Mohamed Osman, both MSc and PhD graduates of 2026 respectively, along with their colleague Mohammad Almakhadmeh and supervised by Ahmed Soliman, associate professor in Concordia’s Department of Building, Civil and Environmental Engineering, estimated that the substitution could reduce greenhouse-gas emissions by anywhere from 25 to 50 per cent, depending on how the mixture is formulated. That range is significant because it suggests the approach is not a marginal improvement but a potentially transformative one for an industry under intense pressure to shrink its environmental impact.

The logic behind the study rests on a class of materials known as limestone calcined clay cements, often abbreviated as LC3, which have emerged in recent years as one of the most promising routes to lower-carbon concrete. Rather than relying almost entirely on clinker, LC3 blends combine clinker with calcined clay and limestone powder, allowing a large fraction of the traditional binder to be replaced while retaining most of the mechanical performance. The catch is that the clay must be of reasonable quality, rich in the reactive minerals that participate in the pozzolanic reactions responsible for strength development. Low-grade clays and dredged sediments, which are abundant and cheap, often fall short of that standard because they contain impurities and lack the reactive phases that make calcined clays useful binders.

This is where red mud enters the picture. Red mud is the caustic slurry left behind when bauxite ore is processed with sodium hydroxide to extract alumina, the feedstock for aluminum metal. It is produced at a rate of well over 150 million tonnes per year worldwide, and it accumulates in vast holding ponds that pose both environmental hazards and disposal costs for the aluminum industry. Chemically, however, red mud contains oxides of iron, aluminum, and silicon, along with residual alkaline compounds, several of which are precisely the kinds of constituents that can contribute to cementitious reactions if the material is properly activated. The Concordia researchers hypothesized that red mud could compensate for the deficiencies of low-quality dredged sediment, supplying the reactive components that the sediment lacks.

To test that hypothesis, the team first subjected both the red mud and the dredged sediment to a thermal treatment, heating them at 800 degrees Celsius. Calcination at this temperature drives off bound water and restructures the mineral phases, breaking down crystalline structures and creating amorphous, chemically hungry material that can dissolve and react in the alkaline environment of a hydrating cement paste. The researchers varied the duration of the heating and found that one hour produced the best overall results, maximizing the reactivity of the two materials without wasting energy on longer treatment. That finding matters for practical deployment, because the energy cost of the calcination step directly affects both the economics and the carbon accounting of the final cement.

With the activated materials in hand, the team prepared a series of blended cement formulations in which the treated red mud and dredged sediment replaced varying proportions of Portland cement. The mixtures were then evaluated for their physico-chemical performance, including the formation of hydration products and the mechanical strength of the resulting pastes and mortars. The results revealed a clear synergistic effect: red mud could offset some of the inherent limitations of the dredged sediment, promoting the formation of the compounds that give cement its strength. In several formulations, the combination of the two waste materials produced stronger products than mixtures containing the dredged sediment on its own, demonstrating that the pairing is more than the sum of its parts.

Equally important was the finding that dosage matters. The researchers observed that excessive amounts of red mud could reduce strength in some mixtures, a consequence likely tied to the material’s iron-rich composition and its residual alkalinity, which can interfere with normal hydration chemistry when present in high concentrations. At moderate levels, however, the red mud improved the structure and performance of the blended binder, refining the microstructure and supporting the development of strength-giving phases. This dose-response behaviour underscores a recurring theme in cement chemistry: supplementary cementitious materials are not interchangeable fillers but reactive components whose optimal proportions must be carefully calibrated for each combination of source materials.

The implications extend beyond the laboratory. Dredged sediment is generated continuously by ports, waterways, and harbours around the world, and disposing of it is a persistent logistical and environmental challenge, since contaminated sediments cannot simply be dumped back into ecosystems. Red mud, meanwhile, sits in accumulating stockpiles that occupy land and carry risks of alkaline leaching. If even a fraction of these two streams could be diverted into cement production, the construction industry would gain a low-cost, locally available source of binder materials while two waste-management problems would be partially solved at once. The emissions benefit compounds the appeal: because the waste materials displace both clinker and virgin raw materials, the approach attacks the carbon problem from two directions simultaneously, reducing the need for limestone calcination and for quarrying fresh clay.

The study, titled “Physico-chemical performance of red mud/dredged-based LC3,” was published on 12 September 2026 and received funding from the Natural Sciences and Engineering Research Council of Canada, the Fonds de Recherche du Québec – Nature et Technologies, and the Canadian Standards Association. The authors declared no known competing financial interests. As with any experimental cement research, the path from laboratory formulations to structural concrete will require further validation, including long-term durability testing, work on workability and setting behaviour, and assessment of how variability in waste-stream composition affects performance at industrial scale. But the core result stands: materials that industries currently pay to dispose of can be transformed, with a modest input of heat, into ingredients that make cement both stronger and substantially cleaner. In a sector where even single-digit percentage reductions in emissions are celebrated, a formulation capable of cutting greenhouse gases by up to half represents the kind of advance that could reshape how the world builds.

Subject of Research: Using red mud and dredged sediment as supplementary materials in low-carbon limestone calcined clay cement

Article Title: Research: Industrial waste can be a key ingredient in low-carbon cement, new Concordia study shows

Article References: Research: Industrial waste can be a key ingredient in low-carbon cement, new Concordia study shows. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cement, red mud, dredged sediment, LC3, clinker, carbon emissions, Concordia University, construction materials, aluminum industry, waste valorization, calcination, sustainable building

The research team, working with basic oxygen furnace (BOF) slag from a steel plant in Xinjiang, started from an observation that seems obvious in hindsight but has been overlooked in industrial practice: steel slag particles are not uniformly hard. When the researchers ran coarse slag through a planetary ball mill under deliberately mild conditions, which they call light ball milling, the material split into two distinct populations. A fraction of the particles crumbled readily into powder fine enough to pass through a 75-micrometer sieve, while the remaining hard particles were only reduced from large chunks to smaller ones without ever fragmenting into ultrafine material. By adjusting grinding time and energy input, the team could separate these two fractions and study each in detail.

The numbers behind this separation are striking. Light grinding for just ten minutes, using iron balls as the grinding medium so they could later be pulled away with an electromagnet, revealed that the easily pulverizable component makes up roughly 30 percent of the slag by mass. Within the 0.5 to 1 millimeter size range, more than 90 percent of the material is easily pulverizable, while larger particles from 1 to 20 millimeters still contain 12 to 25 percent of the soft fraction. The resulting powder is extraordinarily fine, with a median particle size between roughly 4 and 10 micrometers and a specific surface area approaching 2900 square meters per kilogram, comparable to ultrafine cements that normally demand intensive, energy-hungry grinding.

Why do some particles yield so easily while others resist? The answer lies in mineralogy, which the researchers probed with quantitative X-ray diffraction, X-ray photoelectron spectroscopy, and thermal analysis. The easily pulverizable fraction is dominated by dicalcium silicate, tricalcium silicate, tetracalcium aluminoferrite, magnetite, and magnesium oxide. The hard fraction, by contrast, is rich in the same calcium silicates but also contains free calcium oxide and iron(II) oxide, and it completely lacks detectable magnesium oxide. Spectroscopy of the iron states told the same story from a different angle: iron in the soft fraction is predominantly trivalent, at about 64.6 percent, while iron in the hard fraction is mostly divalent, at 63.4 percent. The team attributes this divide to the oxidation conditions during slag cooling. An oxidizing environment converts Fe(II) to magnetite, which suppresses the association between magnesium oxide and iron(II) oxide; under reducing conditions those phases combine into a rigid, poorly grindable framework, and free calcium oxide becomes encapsulated between silicate crystals.

The carbonation chemistry at the heart of this work exploits the fact that calcium silicate minerals react spontaneously with carbon dioxide and moisture, forming calcium carbonate and silica gel that bind particles together into a solid mass. Unlike ordinary hydration, carbonation of steel slag proceeds faster, delivers higher early strength, permanently sequesters CO2, and reduces alkalinity. Previous studies have used this reaction to make artificial gravel, sulfate-resistant blocks, plant-friendly porous concrete for ecological slope protection, and even materials that immobilize lead and zinc in contaminated soil. The obstacle has always been grinding: the very minerals that resist fragmentation, particularly dicalcium silicate, are locked inside hard particles, so producing fine slag powder conventionally means grinding everything, including the refractory majority, at enormous energy cost.

To compare performance, the researchers pressed each fraction into cylindrical specimens with water, compacted them at 2 megapascals, and cured them in a chamber filled with 99.9 percent CO2 at 0.1 megapascal. The hard fraction powder was prepared for fair comparison by intensive grinding to a specific surface area of about 105 square meters per kilogram, matching powders used in earlier studies. Strength measurements showed that both fractions follow the classic carbonation pattern in which strength rises with moisture content to a peak and then falls, but the optima differ sharply. The easily pulverizable powder reaches maximum carbonation strength at 16 percent moisture, substantially higher than the 10 percent optimum of the hard powder, a difference the authors attribute mainly to its much greater fineness, with mineralogy a possible secondary factor.

The performance gap that emerged was dramatic. Specimens made from the easily pulverizable fraction gained strength rapidly within the first three hours of carbonation and stabilized at 73.9 megapascals after 24 hours, roughly 1.5 times the 50.7 megapascals achieved by the hard fraction, despite actually containing a lower proportion of carbonation-active calcium silicates. Carbon dioxide uptake followed the same trend, with the soft fraction absorbing more CO2 at every curing duration. Thermal gravimetric analysis quantified the point: after just 3 hours of carbonation, the easily pulverizable fraction had already produced as much calcium carbonate, about 16.4 percent by mass, as the hard fraction generated after a full 24 hours. Electron microscopy explained the mechanical advantage. The calcium carbonate product in the soft fraction forms spindle-shaped crystals about 0.1 micrometers long, some 30 times smaller than the roughly 3-micrometer crystals in the hard fraction, and these fine products fill micropores more effectively, yielding a total porosity of only 17.5 percent versus 32.4 percent for the carbonated hard fraction.

The broader implication is a smarter, cheaper route to carbon-negative construction materials. Instead of grinding the entire slag mass, which risks over-grinding the hard particles into fragments that actually coarsen the powder and raise the carbon footprint, processors could lightly mill the slag, magnetically remove the grinding media, and sieve out the roughly 30 percent soft fraction as a ready-made ultrafine feedstock for carbonated cements. The team also showed that over-grinding can be detected by monitoring the growth rate of fine powder or the specific surface area as energy accumulates, giving operators a practical warning signal. The hard remainder, meanwhile, could find use as asphalt aggregate, where the authors note its selective removal may actually improve performance.

The researchers are careful about limits. Their slag came from a single BOF source, and they recommend validating the approach across different plants and slag chemistries. They also note that the study cannot fully disentangle how much of the soft fraction’s advantage comes from fineness versus mineral composition, and that definitive criteria for identifying over-grinding remain to be established, with mineralogical analysis or iron valence measurements proposed as future diagnostic tools. Still, the core message stands: steel slag is heterogeneous, that heterogeneity is exploitable with minimal energy, and exploiting it could turn one of the steel industry’s biggest waste streams into a workhorse for carbon sequestration in the built environment, a small but meaningful step toward China’s dual carbon goals and the global effort to decarbonize construction.

Subject of Research: Selective light ball milling of BOF steel slag to separate easily pulverizable, carbonation-active components from hard particles for low-carbon cementitious materials

Article Title: Enhanced carbonation performance of pulverizable components in BOF steel slag separated by light ball milling: Comparative analysis of physical properties and process implications

Article References: Yu, C., Zheng, G., Han, D., Cui, J., Chen, W., Ran, W., Cheng, L., & Wu, Z. (2026). Enhanced carbonation performance of pulverizable components in BOF steel slag separated by light ball milling: Comparative analysis of physical properties and process implications. Case Studies in Construction Materials, 25, Article e06501. https://doi.org/10.1016/j.cscm.2026.e06501

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06501

Keywords: steel slag, carbonation, ball milling, CO2 sequestration, low-carbon cement, calcium carbonate, mineralogy, construction materials, basic oxygen furnace slag, grinding energy, calcium silicate, sustainable building