Optimization effects and mechanisms of multi-admixture solid wastes such as fly ash and granulated blast furnace slag on the properties of cement mortar

To promote the synergistic utilization of mountain sand resources with solid wastes such as fly ash (FA), ground granulated blast-furnace slag (GGBFS), and silica fume (SF) in cement-based materials, this study incorporates mountain sand (including stone powder) into an integrated composite binder system of “cement + FA + GGBFS + SF + mountain sand.” The effects of single-, binary-, and ternary-admixture scenarios on the workability, compressive strength, and chloride-ion penetrability of cement mortars were systematically investigated. In addition, a synergy index for multi-admixture systems (SCI) and an equivalent activity coefficient (k) were proposed to quantitatively identify synergistic and inhibiting regimes. The results indicate that the mountain sand replacement ratio exhibits a distinct “optimum range”: at 20%–40% replacement, the compressive strength is essentially maintained or slightly improved, while the charge passed decreases from 2800 to ∼2500 C; excessive replacement leads to deterioration in both fluidity and durability. In the single-admixture systems, FA markedly increases workability but reduces early-age strength; GGBFS at 5%–15% simultaneously improves strength at all curing ages and lowers the charge passed; SF at 3%–5% enhances both strength and durability at the expense of some fluidity. In the multi-admixture systems, the ternary blend of FA + GGBFS + SF at a total replacement level of 30% delivers the best overall performance in terms of workability, strength development over the full age range, and resistance to chloride-ion penetration, demonstrating a pronounced nonlinear synergistic effect. The SCI–k relationship further shows that k = 0.235–0.240 corresponds to the optimal synergy-enhancement interval for comprehensive performance, whereas higher k values induce inhibitory effects on later-age strength and workability.