A Study on the Mechanical Properties of Engineered Geopolymer Composites Based on Red Mud, Fly Ash, and Slag

To promote the valorization of red mud (RM), a high-volume industrial solid waste, engineered geopolymer composites (EGC) were developed using a ternary binder comprising ground granulated blast-furnace slag (GGBS), fly ash (FA), and RM. Ten mixtures were designed using a simplex-centroid method, and the effects of binder composition on fresh-state flowability, axial compressive behavior, and axial tensile performance were systematically investigated. The results showed that GGBS was the dominant factor governing compressive strength. The mixture with the highest GGBS content, G50F50R0, achieved a maximum compressive strength of 111.7 MPa. Although RM incorporation reduced compressive strength, the strength remained approximately 70 MPa at an RM content of 30%, while the post-peak brittleness was noticeably mitigated. In tension, an appropriate RM content of 5–15% effectively reduced matrix toughness, enabling pronounced strain-hardening and multiple-cracking behavior through fiber bridging. Consequently, tensile ductility and strain energy density were substantially enhanced, with the ultimate tensile strain reaching up to 6.9%; however, excessive RM addition led to performance degradation. By overlaying multi-objective performance boundaries, an optimized composition range of 40–45% GGBS, 50–60% FA, and 0–8% RM was identified, enabling the simultaneous achievement of high strength, high toughness, and high energy absorption. These findings demonstrate the technical feasibility of using RM to produce EGC and provide a theoretical basis for the high-value utilization of RM.