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