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

The slow ambient reactivity of steel slag limits its use in low-cement construction binders because Ca-rich phases are not efficiently converted into continuous hydration products. This study designed a steel slag–phosphorus slag–spontaneously combusted coal gangue (SS–PS–SCG) binder by combining Ca-rich, siliceous and aluminosilicate solid-waste components without additional chemical activation. Single, binary and ternary blends were compared at 30% cement replacement, and the optimized composition was further evaluated at 35% and 40% replacement. Replacing 30% cement with steel slag alone reduced the 28 d compressive strength to 32.39 MPa, corresponding to an activity index of 66.12%. In contrast, the ternary blend with SS:PS:SCG = 7:1:2 reached 44.89 MPa and an activity index of 91.63%, while retaining a flowability of 195 mm and initial/final setting times of 922/990 min. Increasing the replacement level to 35% and 40% decreased the strength to 35.85 and 30.52 MPa, respectively, indicating that strength retention rather than maximum cement substitution governed the viable design window. TG-DTG, XRD and SEM-EDS results showed that the highest-strength mixture did not exhibit the maximum low-temperature mass loss or the greatest CH consumption, while its microstructural characteristics were consistent with a more continuous distribution of hydration products. Carbon–cost assessment further showed that the optimized ternary binder reduced the strength-normalized global warming potential and material cost to 12.04 kg CO₂·t -1 ·MPa -1 and 6.14 CNY·t -1 ·MPa -1 . The results demonstrate that composition-window regulation can convert steel slag-based low-cement binders from a dilution-dominated system into a strength-retentive and carbon–cost-efficient low-cement binder.