Synergistic-Inhibition Mechanism of MgO and Na2CO3 as Alkali Activator: Hydration–Hardening Characteristics of MgO─Na2CO3–Activated Slag

Abstract

MgO and Na2CO3 are used instead of strong alkalis (e.g., NaOH, NaOH+Na2SiO3) 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 MgO-Na2CO3 activated slag (MNAS) system are studied. The results show that adding Na2CO3(>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 Na2CO3 increases the 28 day compressive strength of the MNAS system. However, at 90 days, with the higher content of Na2CO3, the SiO4, and AlO4 at the Q2 site are dechained, the peak value at the Q0 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. Na2CO3 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 CO2 emission, energy consumption, and cost of the alkali-activated materials.