Valorization of phosphogypsum and silica fume in sustainable cementitious materials: Multi-objective optimization and mechanistic insights

The massive global stockpile of phosphogypsum (PG), an industrial solid waste, exceeds 6 billion tons and continues to grow, posing significant ecological risks due to land occupation and contaminant leaching. To address these challenges, overcome the poor mechanical strength and water resistance of β-hemihydrate phosphogypsum (β-HPG), this study developed a novel silica fume-modified phosphogypsum-based cementitious material (SF-PCMS) using industrial solid wastes. Silica fume (SF, micro-filler/pozzolan) was introduced in synergy with ground granulated blast furnace slag (GGBS) and carbide slag (CS, CaO-rich alkaline activator) to enhance mechanical strength and water resistance, evaluated by compressive strength, water absorption, and the softening coefficient. Given the complex interactions in this multi-component system, Response Surface Methodology (RSM) was employed to optimize the mix proportions, establishing high-precision predictive models (R² > 0.97) for 28-day compressive strength, saturated water absorption, and the 7-day softening coefficient. Multi-objective optimization yielded the optimal mix (GGBS:CS:SF:β-HPG = 20:16.69:10:53.31; error <4%). Compared with the SF-free control (M15), MY increased compressive strength from 21.75 to 35.53 MPa while reducing saturated water absorption from 16.23% to 8.48% and improving the 7-day softening coefficient from 0.66 to 0.81. Furthermore, XRD and SEM analyses revealed that SF enhances performance through a triple mechanism: physical filling by its micro-fine particles, activation of hydration reactivity promoting the formation of structurally reinforcing C-(A)-S-H gel, and regulation of ettringite (AFt) formation. This study provides a viable pathway for the high-value utilization of PG through performance-oriented mix optimization and mechanistic understanding of silica fume in solid-waste-based binders.