Synthesis and Property Characterization of Low-Activity Waste-Derived Quaternary Cementitious Materials

The environmental risks associated with industrial solid wastes—fly ash (FA), red mud (RM), carbide slag (CS), and steel slag (SS)—are amplified by their massive global accumulation. This study developed a quaternary cementitious system using low-activity industrial wastes—FA, RM, CS, and SS—as alternatives to high-reactivity ground granulated blast furnace slag. The hydration behavior, mechanical properties, and microstructure were investigated, along with the effects of Ca(ClO)2 and Ca3(PO4)2 as calcium additives. Fresh properties (fluidity, pH, and electrical conductivity), compressive strength, and drying shrinkage were evaluated, while SEM-EDS, XRD, FTIR, and TG-DSC analyzed microstructural evolution. The results show that FA-RM alone failed to solidify, but CS enhanced hydration, reducing fluidity and increasing strength, while SS improved thermal stability as a micro-aggregate. The optimized FA-RM-CS-SS system achieved 16.7 MPa at 90 days. Ca(ClO)2 accelerated C-S-H gel formation, whereas Ca3(PO4)2 stabilized the matrix via hydroxyapatite precipitation, mitigating shrinkage. This approach enables simultaneous waste utilization, along with Cl- and P-containing pollutant immobilization, offering a sustainable strategy for eco-friendly construction materials.