Synergistic hydration mechanism of composite cement system of calcined coal gangue and recycled powder

To promote the low-carbon development of cement-based materials and the high-value utilization of solid wastes, this study systematically investigates the effects of composite replacement of cement with calcined coal gangue (CCG) and recycled powder (RP) on the hydration behavior, microstructure, mechanical properties, and environmental–economic performance of cementitious systems. Rheological tests and hydration kinetics analyses, combined with multiscale microstructural characterization techniques, were employed to elucidate the hydration reaction characteristics and pore structure evolution under different replacement strategies. In addition, carbon emission and cost indices normalized by the 28 d compressive strength were introduced to comprehensively evaluate the environmental and economic performance of the materials. The results indicate that single incorporation of CCG or RP can significantly reduce the absolute carbon emissions and material costs; however, insufficient mechanical performance compensation is observed at high replacement levels. In contrast, the combined use of CCG and RP exhibits a pronounced synergistic effect. Among the investigated mixtures, the C20R10 system (20% CCG + 10% RP) achieves a 28 d compressive strength comparable to that of ordinary Portland cement (OPC), while markedly reducing carbon emissions and costs per unit strength. This synergistic enhancement is primarily attributed to the filler and nucleation effects of RP, which accelerate early-stage hydration, as well as the sustained pozzolanic reaction of CCG at the middle and later stages, leading to continuous formation of C–(A)–S–H gel. As a result, the pore structure is refined, and the matrix densification is improved. These findings provide valuable guidance for the synergistic utilization of multiple solid wastes and the design of low-carbon, high-performance cementitious materials.