<p>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<i>.</i>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.</p>