The starting point was an uncomfortable truth about steel slag. This calcium-rich by-product of steelmaking looks ideal on paper, packed with lime, calcium silicates and iron-bearing phases, yet it hydrates painfully slowly at ambient temperature. Earlier long-term measurements showed that pure steel slag pastes reached only around 0.6 to 1.0 megapascals of compressive strength after seven days and just 2.8 to 3.1 megapascals after twenty-eight days, needing a full year to climb above twelve megapascals. The reactive ingredients are there, but they linger largely unconverted, leaving calcium-rich phases stranded instead of knitting them into the continuous calcium-silicate-hydrate gel that gives concrete its strength. Simply dumping steel slag into a cement mix, in other words, mostly dilutes it.

The researchers’ strategy was chemical bookkeeping. Steel slag supplies calcium and an alkaline environment, but it is starved of reactive silicon and aluminium. Phosphorus slag, a glassy discard from yellow phosphorus furnaces that is rich in both lime and silica, can donate the missing silicate species. Spontaneously combusted coal gangue — mining waste that has accidentally baked itself in self-sustaining fires — contains disordered, partially dehydroxylated clay minerals brimming with reactive silicon and aluminium that can consume calcium hydroxide and form calcium-aluminosilicate-hydrate gels. But each helper carries a penalty: phosphorus slag is notorious for delaying setting, with prior studies recording initial setting times stretched by up to 49 percent, while coal gangue’s porous, water-hungry particles sap the workability of fresh mortar.

To find the sweet spot, the team ran a progressive single, binary and ternary mixing programme, systematically varying both the steel slag fraction and the ratio of phosphorus slag to coal gangue, with no chemical activators added. The results were striking. Replacing 30 percent of cement with steel slag alone dropped the 28-day compressive strength from 48.99 to 32.39 megapascals, an activity index of just 66 percent. Binary blends with either phosphorus slag or coal gangue lifted the strength to roughly 40 megapascals. But the ternary blend with a steel slag to phosphorus slag to coal gangue ratio of 7:1:2 — mixture P-12 — reached 44.89 megapascals, an activity index of 91.63 percent and a full 38.59 percent stronger than the single-slag mixture.

Tellingly, the strength did not rise monotonically with any single ingredient. At the same 70 percent steel slag level, mixtures with phosphorus-to-gangue ratios of 2:1 and 1:1 both landed near 39.5 megapascals while the 1:2 ratio soared to 44.89. Too much phosphorus slag prolonged setting — mixture P-13, with the highest phosphorus content at that level, took 1,050 minutes to set initially, nearly six hours longer than P-12 — while too much coal gangue dragged down flowability through the water absorbed by its rough particles. The composition window mattered more than the quantity of any individual waste, and P-12 threaded it neatly, holding a respectable flow of 195 millimetres alongside its strength record.

The microstructural detective work, combining thermogravimetric analysis, X-ray diffraction with Rietveld refinement and scanning electron microscopy, produced perhaps the study’s most counterintuitive insight: the strongest mixture was not the one that consumed the most calcium hydroxide or produced the most bound water. The coal-gangue binary blend actually showed greater portlandite consumption and slightly more bound water than P-12, yet it was 13.79 percent weaker. Its hydration products clustered unevenly, leaving open pores and weakly connected zones between particle clusters.

P-12, by contrast, built a denser, more continuous microstructure. Electron images revealed gel-like calcium-silicate-hydrate, needle-like ettringite crystals and aggregated hydrates interwoven around residual particles, with several grains partly coated and bridged by reaction products. Energy-dispersive spectroscopy confirmed the coexistence of calcium, silicon, aluminium and sulphur in the hydrate regions — exactly the elemental recipe for mixed calcium-silicate-hydrate and calcium-aluminosilicate-hydrate gels together with sulphate-bearing phases. The message is one cement chemists are increasingly voicing: strength depends not on maximizing any single hydration indicator but on how effectively the hydrates are distributed through the pore space, coating particles and knitting them into a load-bearing network.

The team also pushed the winning recipe harder, stretching cement replacement to 35 and 40 percent. Strength fell to 35.85 and 30.52 megapascals respectively, and cement efficiency — strength delivered per gram of cement — declined by about 14 and 21 percent. The reason is structural: clinker hydration creates the early skeleton and supplies the calcium hydroxide that drives secondary reactions in the wastes. Starve the system of clinker and even a perfectly balanced waste blend cannot fully compensate. Notably, even at 40 percent replacement the cement efficiency remained slightly above that of plain cement, showing the wastes were still pulling chemical weight, but the numbers mark 30 percent as the practical ceiling for this composition.

The sustainability arithmetic drives the point home. Replacing cement cuts unit-mass emissions and cost — every blended mixture emitted roughly 540 kilograms of CO2 per tonne against 735 for plain cement — but the single-slag mixture’s strength collapse actually raised its carbon and cost per megapascal above the ordinary cement benchmark, a vivid demonstration that green concrete is worthless if you need more of it. P-12 avoided that trap entirely, posting the lowest strength-normalized global warming potential, 12.04 kilograms of CO2 per tonne per megapascal, and the lowest strength-normalized cost, 6.14 yuan per tonne per megapascal. Its material cost was just 0.77 percent higher than the single-slag mix, yet it delivered 38.59 percent more strength.

The broader lesson reaches beyond one laboratory recipe. As cement producers worldwide hunt for clinker substitutions, the temptation is to maximize waste loading and celebrate the reduced emissions on paper. This study shows why that logic fails and how to fix it: wastes are not interchangeable fillers but chemically distinct participants whose calcium, silica and alumina budgets must be jointly tuned so their hydration products interlock rather than segregate. The authors caution that long-term durability, volume stability, leaching behavior and field-scale trials remain to be demonstrated, and that raw-material variability across slag sources will require ongoing optimization. But as a proof of concept, turning steel-furnace residue, phosphorus discard and self-burned mining rubble into a binder that rivals conventional cement — at a quarter of the cost savings and more than nine-tenths of the strength — is a compelling glimpse of how construction’s raw materials could come from its own refuse heap.

Subject of Research: Steel slag-based low-cement binders improved by ternary blending with phosphorus slag and spontaneously combusted coal gangue

Article Title: Improving steel slag-based low-cement binders with phosphorus slag and spontaneously combusted coal gangue: mechanical performance and hydration microstructure

Article References: Shang, Y., Zhang, Y., Wang, Q., Nehdi, M. L., Zhang, L., & Liu, W. (2026). Improving steel slag-based low-cement binders with phosphorus slag and spontaneously combusted coal gangue: mechanical performance and hydration microstructure. Case Studies in Construction Materials, 25, Article e06625. https://doi.org/10.1016/j.cscm.2026.e06625

Image Credits: AI Generated

DOI: Not provided

Keywords: steel slag, phosphorus slag, coal gangue, low-cement binder, cement replacement, compressive strength, hydration, microstructure, calcium-silicate-hydrate, CO2 emissions, sustainable construction, supplementary cementitious materials