Design of a novel in-situ wet carbonated magnesium slag binder based on sodium carbonate activation: mechanical properties, phase transformation and microstructural evolution

To enhance the low hydration activity and cementitious potential of magnesium slag (MS), this study developed an in-situ wet carbonated MS binder activated by sodium carbonate. Mechanical properties, reaction kinetics, phase assemblage, pore structure and microstructure were investigated to clarify mechanism-microstructure-performance relationships. Results reveal a concentration-dependent balance between hydration and carbonation. At moderate sodium carbonate contents, alkalinity promotes γ-C2S dissolution and C-S-H gel formation, while carbonate induces synchronous calcite precipitation, refining pores and producing a dense composite microstructure of interwoven C-S-H and rhombohedral calcite. This synergy increases compressive strength by about 103% relative to water curing. In contrast, excessive sodium carbonate causes rapid early calcite precipitation, forming surface encrustations that consume Ca2+ and hinder subsequent hydration, leading to discontinuous gel formation and limited strength gain despite pore refinement. Without sodium carbonate, both hydration and carbonation are kinetically limited. Controlled sodium carbonate activation therefore unlocks MS reactivity and transforms this industrial residue into a stronger, denser cementitious binder.