Hydration Mechanisms and Microstructural Evolution of MgO–Metakaolin–Slag Cementitious Materials

Abstract This study aims to investigate the hydration process and mechanism of MgO–metakaolin–slag (MMKS) ternary cementitious materials. The effects of different slag (SG) contents (0–40%) and MgO/metakaolin (MK) mass ratios (4:6 and 6:4) on the properties of MMKS cementitious materials were examined. XRD, TG/DTG, IR, solid-state NMR, SEM, and mercury intrusion porosimetry (MIP) were employed to systematically characterize the phase composition, morphological features, and microstructures of the hydration products. The results indicate that when the MgO/MK mass ratio is 6:4 and the SG content is 20%, both the fluidity and compressive strength of the MMKS cementitious paste are significantly improved. Mechanistic analysis shows that SG can be effectively activated in the MMKS system with a high MgO/MK ratio, which not only promotes the polymerization of magnesium silicate hydrate (M–S–H) or magnesium aluminosilicate hydrate (M–A–S–H) gel phases but also facilitates the assembly of active Al3+ to form hydrotalcite-like (LDH) phases. These phases act synergistically with the gel phases, thereby markedly enhancing the mechanical performance of the MMKS cementitious materials. Pore structure analysis reveals that an appropriate SG dosage refines the pore size distribution of hardened MMKS cementitious materials, whereas excessive SG incorporation (e.g., 40%) leads to pore coarsening and strength reduction due to a dilution effect. This study elucidates the pathways by which SG enhances the performance of the MgO–MK systems. Through the synergistic design of mixed proportions and dosages, hydration products can be effectively regulated, providing key theoretical and experimental support for the development of high-performance MMKS ternary systems.