From inhibition to enhancement: Time-staged complementary roles of sugarcane bagasse ash and red mud in sustainable cementitious composites

Functionalizing industrial and agricultural wastes, such as sugarcane bagasse ash (SCBA) and red mud (RM), is imperative for developing sustainable cementitious materials. While binary OPC systems with high dosages of either this waste underperformed, due to the low reactivity of SCBA or the inhibitory effect of RM, ternary OPC-SCBA-RM blends demonstrated a promising synergistic enhancement. To offer optimization strategies and mechanistic insights, this study investigated ternary OPC-SCBA-RM systems using response surface methodology, isothermal calorimetry and multi-scale characterizations. The results indicate that SCBA and RM functioned via distinct yet complementary mechanisms. SCBA primarily reduced total porosity and elongated the silicate chain length in C-S-H gels. Differentially, RM preferentially refined harmful macropores and the polycondensation degree of reaction products, evidenced by the increased partial correlation coefficient, the presence of Al-coordinated Q 2 (2Al) and Q 3 (3Al) units alongside the significantly higher MCL value than plain mixture (7.52 vs. 4.54). The interactive synergy in ternary cement-based mixtures mitigated the RM-induced retardation upon phase boundary reaction and diffusion kinetics. This synergy represented not an additive but a competitive interaction, shifting the hydration pathway toward more favourable late-stage kinetics. A promising formulation for the ternary composite of 76.3 % OPC, 20.7 % RM, and 3 % SCBA was identified to exhibit equivalent mechanical performance to the pure OPC control. Moreover, a 28 % cement replacement can be achieved with ≤5 % strength loss. This meaningfully presents a viable route for high-volume waste utilization in low-carbon cementitious composites.