The material in question is ferrochrome slag, a stony by-product of producing ferrochrome, the chromium-iron alloy that underpins the stainless steel industry. Ferrochrome is made in submerged electric arc furnaces at temperatures between 1500 and 1700 degrees Celsius, and for every ton of alloy produced, between 1.1 and 1.6 tons of molten slag float to the surface and are skimmed off. With global ferrochrome production reaching approximately 18.25 million tons in 2024, that translates into an estimated 20 to 29 million tons of slag generated in a single year. South Africa and China, which hold the largest chromite reserves, together account for more than two-thirds of the world’s ferrochrome slag, while Oman ranks among the top ten producing countries.

Historically, most of this slag has been relegated to use as coarse and fine aggregate in concrete, valued mainly for its physical similarity to natural rock. But the new study, published in Environmental Science and Pollution Research, takes a different approach. Led by Alaa Aref Ahmed Wazwaz, with Kazi Md Abu Sohel, Khalifa Al-Jabri and Sherif El-Gamal, the research team investigated whether finely ground ferrochrome slag could serve as a supplementary cementitious material, or SCM, a partial replacement for Portland cement that reacts chemically to improve the properties of mortar and concrete. The critical question they set out to answer was one that previous studies had never addressed: does it matter whether the molten slag is cooled slowly in air or quenched rapidly in water?

To find out, the researchers obtained both air-cooled and water-cooled ferrochrome slag from the Al Tamman Ferro Alloys plant in Sohar, Oman. Both varieties were dried at 105 degrees Celsius for 24 hours and then ground in a horizontal ball mill at 200 revolutions per minute for two hours, a process the authors describe as mechanical activation. The resulting powders were sieved so that only particles finer than 75 micrometers were used. Scanning electron microscopy combined with ImageJ-based image analysis revealed a striking difference between the two powders: the water-cooled slag had a median particle size of 18.69 micrometers, smaller than the 20.92 micrometers of the cement itself, while the air-cooled slag averaged a coarser 30.00 micrometers. The water-cooled powder also achieved a higher Blaine fineness, meaning a larger reactive surface area.

The reason for this difference lies in the glass chemistry of rapid quenching. X-ray diffraction analysis showed that neither slag is fully amorphous, but the water-cooled sample displayed a far higher degree of amorphous, glassy structure than its air-cooled counterpart. When molten slag is plunged into water, its atoms are frozen in place before they can organize into orderly crystals; air cooling gives them time to crystallize. The dominant minerals in the air-cooled slag were lizardite, magnesium spinel and magnesium olivine, while the water-cooled slag contained chromite, magnesium spinel and magnesium olivine. Importantly, the magnesium in both slags exists in stable crystalline form, which means its use in cement is unlikely to cause soundness problems such as delayed expansion. X-ray fluorescence confirmed that the cooling process does not change the chemical composition at all: both slags are dominated by silicon dioxide, aluminum oxide, magnesium oxide and iron oxide, with the combined silicon, aluminum and iron oxides exceeding 60 percent of the slag mass, comfortably within the 50 to 70 percent range that ASTM C618 requires for pozzolanic materials. In chemical terms, ferrochrome slag closely resembles Class F fly ash, the workhorse SCM of the concrete industry.

Armed with this characterization, the team blended each slag type into mortar at replacement levels of 5 to 35 percent of the cement, in 5 percent increments, following ASTM C109 with a binder-to-sand ratio of 1:2.75 and a water-to-binder ratio of 0.485. They then subjected the fresh and hardened mortars to a battery of tests: flow table workability, fresh and hardened density, compressive and flexural strength, porosity, water absorption, ultrasonic pulse velocity, rapid chloride permeability, thermal conductivity and even exposure to temperatures up to 1000 degrees Celsius. The results painted a clear picture. Early-age strength at 3 and 7 days was slightly lower in the slag blends, a predictable consequence of diluting the cement, but by 28 days the pozzolanic reaction had caught up and, in some cases, overtaken the control.

The standout performer was the mortar containing 10 percent water-cooled slag, which achieved a 28-day compressive strength of 38.68 megapascals, a remarkable 16.26 percent higher than the 33.27 megapascals of the pure cement control mix. The 5 percent water-cooled blend reached 38.04 megapascals, an improvement of 14.34 percent, and even the air-cooled slag delivered gains of 11.81 percent at 5 percent replacement. The mechanism is classic pozzolanic chemistry: over time, the reactive silica and alumina in the slag react with the calcium hydroxide released during cement hydration, forming additional calcium silicate hydrate gel, the glue that binds concrete together. Electron microscopy of the 10 percent water-cooled blend confirmed the formation of extra calcium silicate hydrate along with calcium aluminosilicate hydrate gel, a denser interfacial transition zone between paste and aggregate, and, crucially, no visible unreacted slag particles. Beyond 15 percent replacement, however, the benefits faded as excess slag remained inert, acting merely as filler and weakening the matrix.

Durability results reinforced the case for the water-cooled variety. Mortars with up to 15 percent water-cooled slag showed lower porosity and water absorption than the control, and the rapid chloride permeability test classified the 10 percent blend, at 1939 coulombs, as having low chloride permeability, compared with the moderate rating of 2048 coulombs for the control mortar. This matters enormously for reinforced structures, since chloride ingress is the primary trigger for steel corrosion in concrete. Ultrasonic pulse velocity values remained in the good-quality range of 3500 to 3800 meters per second for all water-cooled blends, while air-cooled mortar degraded to poor quality at 35 percent replacement. Thermal conductivity also increased with slag content, a consequence of the iron oxide content and the chromite phase, which could be advantageous in certain applications. Perhaps most strikingly, when mortar cubes were heated from 100 to 1000 degrees Celsius, all mixes gained strength up to 300 degrees, in some cases reaching 1.31 times their unheated strength, before declining as calcium hydroxide decomposed and the calcium silicate hydrate gel broke down beyond 600 degrees. The 10 percent water-cooled blend consistently retained the highest residual strength at every temperature, suggesting genuinely improved fire performance.

The environmental implications are considerable. Replacing even 10 percent of the world’s cement with a waste material that would otherwise be stockpiled would avoid a substantial fraction of the roughly 0.65 to 0.92 tons of carbon dioxide emitted per ton of cement, while simultaneously addressing a waste management challenge measured in tens of millions of tons annually. Concerns about chromium leaching, raised by the presence of chromate minerals in the slag, appear manageable: prior research cited in the study reported total chromium leaching of just 0.071 parts per million, far below the United States Environmental Protection Agency limit of 5 parts per million, and the cement matrix has been shown to effectively stabilize chromium within the binder. The authors acknowledge that their chemical evaluation of the blended and heat-exposed pastes was limited to scanning electron microscopy and energy-dispersive X-ray analysis, and they call for future work incorporating X-ray fluorescence and diffraction on heated specimens, along with testing against aggressive chemical environments. Still, the core message stands: the same slag, from the same ore, behaves entirely differently depending on whether it is cooled in air or in water, and the water-quenched version, with its glassy structure and finer grind, is a genuinely superior cement replacement. In the quest to decarbonize construction, sometimes the answer is hiding in the waste heap, waiting for the right cooling process to unlock it.

Subject of Research: Use of air-cooled and water-cooled ferrochrome slag as a supplementary cementitious material in mortar

Article Title: The effect of the cooling process of ferrochrome slag waste on its performance as a sustainable supplementary cementitious material

Article References: Wazwaz, A. A. A., Sohel, K. M. A., Al-Jabri, K., & El-Gamal, S. (2026). The effect of the cooling process of ferrochrome slag waste on its performance as a sustainable supplementary cementitious material. Environmental Science and Pollution Research, 33(27), 13967-13988. https://doi.org/10.1007/s11356-026-38174-0

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38174-0

Keywords: ferrochrome slag, supplementary cementitious material, cement mortar, pozzolanic reaction, carbon dioxide emissions, sustainable construction, compressive strength, chloride permeability, elevated temperature resistance, industrial waste recycling, X-ray diffraction, circular economy

The motivation is twofold. Cement production is one of the most energy-intensive industrial processes on the planet, responsible for an estimated 7 to 8 percent of global anthropogenic carbon dioxide emissions, while concrete itself is the most widely consumed manufactured material on Earth, with annual usage exceeding 4 billion metric tons. Replacing even a modest fraction of cement with an industrial by-product therefore carries enormous environmental leverage. At the same time, self-compacting concrete, first developed in Japan in the late 1980s, has become a favored technology for densely reinforced and geometrically complex structures because it flows under its own weight and compacts without vibration. Its precise mix design requirements make it particularly receptive to supplementary cementitious materials such as fly ash, slag, and silica fume, which have long been used to cut cement content. Coal slurry powder, however, has remained largely unexplored, especially at high temperatures.

The raw material in this study was air-dried and ball-milled before use, yielding a powder with a specific surface area of roughly 25 to 30 square meters per gram and a specific gravity of 2.35. X-ray fluorescence analysis revealed a predominantly siliceous-aluminous composition, containing approximately 35 to 45 percent silicon dioxide, 8 to 12 percent alumina, 4 to 6 percent iron oxide, and 2 to 4 percent calcium oxide, with the combined silica, alumina, and iron oxide content exceeding 50 percent. The loss on ignition was 13.9 percent, a figure the author treats cautiously because it may include volatile components beyond residual carbon. Importantly, the study stops short of classifying the powder as a confirmed pozzolan, since a standardized pozzolanic activity index and quantitative mineral-phase analysis were not performed; its contribution is interpreted as physical filler action combined with possible secondary reactions.

The experimental program replaced ordinary Portland cement with coal slurry powder at 0, 10, 20, and 30 percent by mass, keeping the total binder content constant at 450 kilograms per cubic meter and the water content at 180 kilograms per cubic meter, for a constant water-to-binder ratio of 0.40. Because the cement fraction shrank as replacement increased, the effective water-to-cement ratio rose from 0.40 in the control to 0.571 in the 30 percent mixture, a detail that helps explain the strength reductions observed at higher dosages. Fresh concrete properties were assessed with slump-flow, V-funnel, and L-box tests following European guidelines for self-compacting concrete. All mixtures fell within the SF2 slump-flow class, with values declining from 720 millimeters for the control to 660 millimeters at 30 percent replacement, while V-funnel times rose from 8.2 to 10.4 seconds and L-box passing ratios dropped from 0.98 to 0.90, all still within acceptable limits thanks to mixture-specific superplasticizer adjustment.

The hardened results told a nuanced story. At ambient temperature, the control mix reached compressive strengths of 30.3, 45.29, and 57.1 megapascals at 7, 28, and 56 days respectively, while the 30 percent replacement mixture managed only 19.7, 29.44, and 37.1 megapascals at the same ages, reflecting straightforward cement dilution. Splitting tensile strength followed the same pattern, falling from 5.64 megapascals in the control at 28 days to 3.67 megapascals at 30 percent replacement. Yet the study’s statistical analysis, a two-way analysis of variance on 28-day compressive strength, showed that exposure temperature was actually the dominant factor controlling performance, with a partial eta squared of 0.993, compared with 0.964 for replacement level and 0.791 for their interaction. Coefficients of variation remained below 6 percent across all conditions, indicating solid experimental repeatability.

Thermal exposure was conducted in a programmable furnace at a deliberately slow heating rate of 2 degrees Celsius per minute, with four-hour holds at 300, 600, and 900 degrees Celsius followed by gradual cooling. The author is careful to note that this protocol characterizes material-level degradation rather than reproducing a standardized structural fire test such as ISO 834 or ASTM E119. At 300 degrees Celsius, strength losses were modest, around 8.5 to 8.6 percent. At 600 degrees, all mixtures lost roughly 41 to 43 percent of their strength as calcium hydroxide decomposed and the calcium silicate hydrate gel began to break down. At 900 degrees, only about one-third of the original ambient strength remained in every mixture; the 28-day control retained 14.90 megapascals and the 30 percent mix just 9.69. Crucially, the proportional losses were nearly identical across all replacement levels, at roughly 67 percent, meaning the coal slurry powder did not improve percentage strength retention at extreme temperatures.

Where the moderate replacement levels genuinely shone was in durability and microstructure. At ambient conditions, the 20 percent mixture absorbed less water than the control, 3.6 percent versus 4.2 percent, showed lower sorptivity at 0.09 versus 0.12 millimeters per square root of minute, passed a lower chloride charge in the rapid chloride permeability test, 2200 versus 2800 coulombs, and exhibited higher electrical resistivity at 16.8 versus 12.5 kilohm-centimeters. These gains are consistent with improved particle packing and possible secondary reactions that reduced pore connectivity. By contrast, the 30 percent mixture performed worse than the control on every transport measure, confirming that excessive substitution dilutes the cementitious phases. Scanning electron microscopy reinforced the picture: at 300 and 600 degrees, the 10 and 20 percent mixtures showed less cracking and better preserved interfacial transition zones than the control, while the 30 percent mix deteriorated fastest, and at 900 degrees all samples suffered severe damage.

The multi-technique microstructural analysis traced the degradation mechanisms in detail. Energy-dispersive X-ray analysis showed that the control’s calcium-to-silicon mass ratio of about 2.17 at ambient temperature dropped to roughly 0.85 to 0.95 with 10 to 20 percent replacement, reflecting the influx of silica-rich phases, then climbed to above 4 in all mixes at 900 degrees as silicate hydration products were destroyed and calcium-rich decomposition products concentrated. Fourier-transform infrared spectroscopy documented the progressive loss of hydroxyl and water-related bands, while thermogravimetric analysis recorded total mass losses of 6 to 8 percent at low temperatures, 18 to 22 percent after intermediate degradation, and 30 to 40 percent across the full range, with distinct events marking water evaporation below 200 degrees, portlandite dehydroxylation at 400 to 500 degrees, and carbonate decomposition between 650 and 800 degrees. X-ray diffraction confirmed the near-complete disappearance of calcium hydroxide and calcium carbonate peaks and the collapse of the amorphous binding phase at 900 degrees.

The practical takeaway is a carefully bounded one. Within the conditions investigated, 10 to 20 percent coal slurry powder offers a favorable balance: it cuts cement consumption by up to 90 kilograms per cubic meter at the 20 percent level while maintaining acceptable mechanical performance and delivering the clearest durability benefits of any mixture tested. The author emphasizes, however, that these material-level results do not establish a structural fire-resistance rating, and that fire-critical load-bearing applications would require standardized structural fire testing, realistic heating curves, applied loading, and appropriate fire protection measures. The study also flags important caveats for real-world deployment: only a single coal-processing source was examined, slurry composition varies widely with geology and beneficiation methods, and coal-processing residues may contain environmentally relevant trace metals, so total elemental analysis and standardized leaching tests should precede large-scale structural use, particularly where concrete may contact groundwater.

Future work, the study suggests, should tackle long-term durability under chloride exposure, freeze-thaw cycling, and chemical attack, explore blending coal slurry powder with other supplementary materials, apply advanced tools such as nano-indentation and in-situ thermal characterization, and conduct full-scale structural testing and life-cycle assessments that account for drying, grinding, and transportation of the powder. Until then, the research stands as a compelling proof of concept that one of coal mining’s most troublesome waste streams can, at moderate dosages, become a functional ingredient in high-performance concrete, turning an environmental liability into a small but meaningful wedge against the cement industry’s carbon footprint.

Subject of Research: Use of coal slurry powder as a partial cement replacement in self-compacting concrete evaluated for mechanical, durability, and elevated-temperature performance

Article Title: Valorization of coal slurry powder in self-compacting concrete: Mechanical, durability, and elevated-temperature performance

Article References: Almutairi, A. (2026). Valorization of coal slurry powder in self-compacting concrete: Mechanical, durability, and elevated-temperature performance. Case Studies in Construction Materials, 25, Article e06537. https://doi.org/10.1016/j.cscm.2026.e06537

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06537

Keywords: coal slurry powder, self-compacting concrete, cement replacement, sustainable construction, elevated temperature, durability, compressive strength, microstructure, supplementary cementitious materials, thermal degradation, chloride permeability, carbon emissions