Understanding the hydration behavior and action mechanism of magnesium slag in the binary and ternary cementitious systems

Magnesium slag (MS) is a burgeoning solid waste with a huge potential to use as a supplementary cementitious material (SCM). However, the fundamental interaction mechanisms between MS and Portland cement (PC) or other SCMs like blast furnace slag (BS) remain insufficiently understood. This study systematically investigates the hydration behavior and reaction mechanisms of high-volume MS in both binary MS-PC and ternary MS-PC-BS systems, integrating hydration kinetics, phase evolution, microstructural characterization, and thermodynamic modelling. Results show that MS significantly alters cement hydration by introducing additional Mg²⁺ ions, which promote the formation of layered double hydroxides (LDHs) and magnesium silicate hydrate (M-S-H) phases. A distinct transformation sequence of ettringite → hemicarbonate → LDHs is identified, accelerating aluminate phase hydration (C₃A and C₄AF) while concurrently suppressing early silicate phase hydration (C₃S and β-C₂S). BS exhibits pronounced synergistic effects with MS. On one hand, the incorporation of BS increases the MS/PC ratio, thereby intensifying the MS-induced promotion of aluminate hydration while aggravating the retardation of silicate hydration. On the other hand, BS provides additional Al and Mg, which facilitates the rapid formation of OH-AFm at early ages and promotes the formation of cordierite at later stages. This also accelerates the phase transformation from ettringite to LDHs, thereby enhancing aluminate hydration. Furthermore, the pozzolanic reaction of BS consumes portlandite, shifts the reaction equilibrium, and promotes silicate hydration, effectively mitigating the MS-induced suppression of silicate reactivity. The development of high-volume MS binary and ternary system holds significant scientific and practical implications for advancing sustainable construction materials.