Utilisation of Bayer red mud for high-performance geopolymer: Competitive roles of different activators

This study investigates the synergistic effects of anions in a ternary alkaline activator composed of NaOH (X 1 ), Na 2 O•nSiO 2 (X 2 ), and Na 2 CO 3 (X 3 ) on the workability, setting time, and mechanical properties of Bayer red mud (RM)-based geopolymer. A simplex-centroid design was used to determine the optimal proportion of the available raw materials in the fabrication of the RM-based geopolymers with optimal comprehensive performance and to reduce number of tests. The hydration products and the microstructure of the RM-based geopolymers were characterized using XRD, FTIR, and SEM-EDS analysis to reveal the reaction mechanism. This study demonstrates that the high reactivity of OH⁻ in X 1 accelerates the dissolution of the precursor, while X 2 improves the fresh properties of the paste by providing [SiO 4 ] 4- , which forms oligomers. Additionally, CO 3 2- in X 3 delays setting time by buffering the alkalinity. Microscopic analysis reveals that OH⁻ preferentially dissolves RM and GGBS, leading to the release of [SiO 4 ] 4- and [AlO 4 ] 5- , which then act as monomers to facilitate the formation of a silica-aluminate network through [SiO 4 ] 4- . Notably, the interaction between OH⁻ and [SiO 4 ] 4- enhances the densification of hydration products, whereas the combination of OH⁻ and CO 3 2- induces a competition between sodium salt crystallization. CO 3 2- competes with [SiO 4 ] 4- for binding sites and preferentially forms CaCO 3 , thereby inhibiting strength development in the later stages of hydration. These results provide experimental and theoretical basis for the properties of RM-based geopolymers under diverse alkaline activation conditions.