The Use of MSWI Fly Ash in Promoting Low-Titanium Slag Activation for Use in Low-Carbon Cementitious Materials

A quaternary solid-waste-based binder was prepared from low-titanium slag, municipal solid waste incineration (MSWI) fly ash, steel slag, and flue-gas desulfurization gypsum (FGDG) to clarify the activating effect of MSWI fly ash on low-titanium slag and its influence on hydrate evolution. Unlike conventional solid-waste-based binders in which MSWI fly ash is mainly regarded as a hazardous residue requiring stabilization, this study demonstrates its specific role as a Ca-rich alkaline activator for promoting low-titanium slag depolymerization and coordinated hydrate formation. The results showed that the compressive strength first increased and then decreased with increasing MSWI fly ash content. Considering both strength development and MSWI fly ash utilization, the optimum mixture was identified as low-titanium slag:MSWI fly ash:steel slag:FGDG = 43.0:17.2:25.8:14.0, with compressive strengths of 9.51 and 46.32 MPa at 3 and 90 d, respectively. These values corresponded to 5.66 and 1.04 times those of the reference mixture without MSWI fly ash, respectively. Ettringite and C-(A)-S-H gel were the main strength-contributing hydration products, while Friedel’s salt was identified as a chloride-bearing AFm phase. Moderate MSWI fly ash addition promoted alkaline activation and low-titanium slag depolymerization, leading to increased formation of ettringite, C-(A)-S-H gel, and Friedel’s salt, which contributed to improved compressive strength. In contrast, excessive MSWI fly ash disturbed the Ca-Si-Al balance and inhibited effective hydrate formation. These results demonstrate that MSWI fly ash can serve as an effective Ca-rich activator for low-titanium-slag-based low-carbon cementitious materials and provide a feasible route for the synergistic utilization of multiple solid wastes.