Improving the carbonation resistance of alkali-activated slag mortars by MgO with different reactivity

Alkali-activated slag (AAS) is a low-carbon construction material and exhibits high mechanical strength and good fire resistance. However, compared to ordinary Portland cement, AAS has a greater problem in terms of resistance to carbonation. In this study, three reactive MgO and Mg(OH)2 were used to enhance the carbonation resistance of AAS mortars. It was found that both reactive MgO and Mg(OH)2 were able to improve the carbonation resistance of the AAS mortar, and the reactive MgO was more effective than Mg(OH)2. Increasing the reactivity and dosage of MgO can significantly enhance the carbonation resistance of AAS mortar, resulting in a 70.5% reduction in carbonation depth. The compressive strength, phase composition, and microstructure before and after carbonation were tested, which showed that the highly reactive MgO had a greater accelerating effect on the hydration of AAS. It can not only significantly improve the compressive strength of AAS mortar but also limit the diffusion of CO2 into the mortar. Meanwhile, the introduction of higher reactive MgO produced more hydrotalcite with a laminar structure, which could absorb a large amount of CO32-. In addition, the incompletely hydrated MgO could react directly with CO2 during the carbonation process to achieve the purpose of carbon sequestration.