Synergistic mechanisms of magnesium slag on the reaction behavior of alkali-activated blast furnace slag

Abstract

This study elucidates the synergistic mechanisms between magnesium slag (MS) and blast furnace slag (GGBS) in alkali-activated systems. Through an integrated analysis of reaction kinetics, phase assemblage, microstructure evolution and gel physicochemical characteristics, this work establishes the governing mechanism-microstructure-performance relationships for the systems. Results show that the reaction is governed by a composition-dependent, dual-stage interplay of competitive and synergistic pathways. The role of MS transitions from an initial diluent, which retards and diminishes the main GGBS exothermic peak, to an active synergistic partner. In the subsequent synergistic stage, the initial GGBS reaction provides the ion conditions and nucleation sites that trigger the hydration of MS, generating products that in turn accelerate the ongoing GGBS reaction. The addition of MS induces an Al-deficient condition, and triggers a competition where available Al preferentially partitions into the C-N-A-S-H gel, thereby inhibiting LDH formation. This in turn diverts the abundant Mg towards forming Al-independent phases such as M-S-H, brucite and magnesite. At moderate MS levels, a synergistic interplay promotes the co-precipitation of multiple gel and crystalline products, yielding a densified composite matrix. Conversely, excess MS renders the system GGBS-limited, resulting in a weak and porous matrix. Furthermore, MS promotes a dimensionally stable microstructure by diluting the overall shrinkage potential of the binder system, providing an internal restraining skeleton, and offering partial chemical compensation through expansion. The 50 % MS content is identified as optimal because it achieves the lowest strength-normalized cost and carbon footprint without compromising the effective immobilization of heavy metals.