Structure–performance synergy of AFt–C(N)(–A)–S–H in red mud–based all-solid-waste binders: Mechanistic insights and quantitative evaluation

The large-scale utilization of red mud (RM) as a cementitious binder is constrained by its low reactivity and the reliance on commercial alkali activators, which limit RM dosage and environmental benefits. This study aims to develop a commercial-alkali-free, high-volume RM-based all-solid-waste binder (RM ≥ 50 wt%) and to establish a quantitative framework for evaluating multi-solid-waste synergy beyond strength-based assessment. Binary, ternary, and quaternary binder systems were designed using RM, ground granulated blast-furnace slag, carbide slag, and phosphogypsum. Mechanical performance, pore-solution chemistry, and heavy-metal immobilization were evaluated, while hydration mechanisms were systematically investigated using XRD, FTIR, TG–DTG, LF NMR, and SEM–EDS, with quantitative characterization of C(N)(–A)–S–H gels and ettringite (AFt). Results demonstrate that high RM utilization without commercial alkalis is achievable through rational multi-solid-waste synergy. Mechanical performance is governed by the cooperative interaction between AFt and C(N)(–A)–S–H gels, forming a reinforced-concrete-like microstructure in which AFt provides a load-bearing skeleton and gels ensure encapsulation and stress transfer. This synergy is highly sensitive to AFt crystal size, with optimal performance obtained at a median AFt diameter of ∼0.24 μm (Q1–Q3: 0.17–0.32 μm); excessively fine or coarse AFt results in insufficient skeletal support or stress-induced cracking. To quantitatively distinguish true synergy from apparent strength enhancement, a Composite Synergy Index (CSI) was proposed by integrating excess-over-additivity, statistical interaction, and independence-based models. Unlike conventional strength comparisons, CSI provides a generalizable and transferable metric for identifying, comparing, and optimizing synergistic interactions in complex multi-component solid-waste binder systems. This study provides new insights into high-volume RM valorization and offers predictive design principles for sustainable, low-carbon multi-solid-waste binders.