Synergistic-Inhibition Mechanism of MgO and ${\mathrm{Na}}_2{\mathrm{CO}}_3$ as Alkali Activator: Hydration–Hardening Characteristics of $\mathrm{MgO}─{\mathrm{Na}}_2{\mathrm{CO}}_3$–Activated Slag

MgO and ${\mathrm{Na}}_2{\mathrm{CO}}_3$ are used instead of strong alkalis (e.g., NaOH, $\mathrm{NaOH}+{\mathrm{Na}}_2{\mathrm{SiO}}_3$) to prepare alkali-activated slag cement, achieving the promotion and use of green-economic alkali activators. In this study, the hydration reaction kinetics, hydration products, mechanical evolution, and microstructure characteristics of $\mathrm{MgO}\text{-}{\mathrm{Na}}_2{\mathrm{CO}}_3$ activated slag (MNAS) system are studied. The results show that adding ${\mathrm{Na}}_2{\mathrm{CO}}_3(>1\%)$ prolongs the time of the second exothermic diffraction peak, and inhibits the hydration of the MNAS system in the early stage. The addition of ${\mathrm{Na}}_2{\mathrm{CO}}_3$ increases the 28 day compressive strength of the MNAS system. However, at 90 days, with the higher content of ${\mathrm{Na}}_2{\mathrm{CO}}_3$, the SiO4, and AlO4 at the ${\mathrm{Q}}^2$ site are dechained, the peak value at the ${\mathrm{Q}}^0$ site increases, the mean chain length (MCL) of the gel phase decreases, the total pore volume increases, cracks appeared in the internal structure, and the compressive strength decreases. ${\mathrm{Na}}_2{\mathrm{CO}}_3$ mixing of 3%, 5%–8%, and 10% is appropriate when using S-type, M-type, and R-type MgO. The MNAS system is excellent from the aspects of ${\mathrm{CO}}_2$ emission, energy consumption, and cost of the alkali-activated materials.