Immobilization mechanism of Cr in alkali‐activated red mud based cementitious materials

Abstract Alkali‐activated red mud based cementitious materials (AARMCs) offer a potential route for immobilizing Cr, but the relationship between Cr loading, phase evolution, and Cr valence transformation remains insufficiently clarified. In this study, red mud (RM) and ground granulated blast furnace slag were used to prepare alkali‐activated binders. The Cr immobilization performance was evaluated by combining leaching analysis, multi‐scale characterization, and molecular simulation. The binder achieved a Cr immobilization efficiency of 98.44% at 28 d under the tested conditions. Cr immobilization follows a three‐stage pattern. Kinetic fitting showed that the pseudo‐second‐order model described the immobilization process better than the pseudo‐first‐order model. Multi‐scale characterization showed that Na 2 CrO 4 addition affected the hydration process, phase assemblage, gel formation, and pore structure of the binder. The progressive development of C/N‐A‐S‐H type gels and pore refinement reduced Cr mobility by limiting connected transport pathways. XPS analysis revealed a decrease in Cr(VI) and Fe(II) contents, accompanied by an increase in Cr(III) and Fe(III), indicating Fe‐assisted Cr(VI)‐to‐Cr(III) transformation during curing. Molecular simulations further showed that CrO 4 2− substitution into silicate or aluminate tetrahedral units was energetically unfavorable.