Synergistic utilization of bauxite residue (red mud) and multiple solid wastes for low carbon precast concrete materials: Mechanical performance, mechanism and sustainability assessment

This study investigates the performance and mechanism of bauxite residue (red mud) and multiple solid wastes as active cement replacements in precast concrete. The effects of mixing ratio, water-to-binder ratio, and steam-curing temperature on mechanical properties and microstructural evolution were systematically evaluated. Results showed that incorporating 10–20% red mud significantly accelerates early hydration under 45–60 °C steam curing, enabling demoulding strength above 15 MPa within 8 h. Microstructural characterization reveals that the enhancement originates from the alkali–thermal synergistic mechanism: alkali released from red mud promote dissolution of fly ash glass phases and formation of C–(A)–S–H gels, which interweave with AFt to generate a dense microstructure. A suitable steam curing temperature will further enhance this synergistic effect. This synergistic hydration not only improves early mechanical performance but also supports stable long-term strength development. Sustainability assessment further indicated that the optimized mix design reduces carbon emissions by 46.1% and lowers material cost by 40.8% compared with the pure cement system. Overall, this study clarifies the alkali-thermal synergistic hydration mechanism in solid-waste-based precast concrete and demonstrates an effective pathway for large-scale synergistic utilization of industrial solid wastes such as red mud. • Reveals alkali–thermal synergistic hydration enabling efficient solid wastes utilization. • Synergistic effect accelerates hydration achieving 8 h demoulding strength exceeding 15 MPa. • Adding 10–20% red mud can balance early strength gain and long-term strength development. • Cuts carbon emissions by 46.1% and reduces material cost by 40.8% versus pure cement.