Thermal Self-Curing Mechanism of All-Solid-Waste Cementitious Materials Based on Circulating Fluidized Bed Fly Ash, Steel Slag, and Red Mud for Mine Backfill

This study documents the development of a clinker-free cementitious material based on circulating fluidized bed fly ash (CFBFA), red mud (RM), and steel slag (SS) without external alkaline activators. The thermal self-curing mechanism was elucidated through hydration heat–temperature field coupling simulations and microstructural characterization. Results indicate that the CFBFA-RM-SS ternary system achieved a 28-day compressive strength of 7.65 MPa (meeting the design strength of 3–5 MPa for the filling design area of Jinxinda Coal Industry in the article background) under a water-to-binder ratio of 0.6 by mass, along with a certain degree of fluidity (slump of 215 mm) and reasonable setting times (initial setting time of 12.1 h, final setting time of 14.0 h). Thermal self-curing significantly enhanced early strength, yielding a 1-day strength of 1.88 MPa with a 1837.5% improvement over ambient curing. Numerical simulations based on a coupled hydration heat–temperature field model (MIDAS Civil) revealed that backfill volumes ≥ 1 m3 can sustain a core temperature of 40–60 °C for over 72 h. This elevated temperature self-curing mainly accelerates early hydration and promotes faster formation of binding hydration products (hydrated aluminosilicate gels and ettringite), leading to a denser microstructure and improved early strength; this trend is supported by XRD, FTIR, and SEM-EDS observations. This work provides theoretical and technical foundations for large-scale utilization of industrial solid waste in mine backfill engineering.