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Increasing attention is concentrated on the alkali-activated slag (AAS) as an alternative binder owing to its low environmental impact and high mechanical strength, whereas the carbonation resistance of AAS can be quite problematic. In this work, calcined Mg/Al layered double hydroxide (CLDH) was synthesized and used to improve the carbonation resistance of AAS mortars activated by NaOH. The effect of CLDH on the carbonation depth, mechanical strength, and microstructure before and after carbonation was investigated. The results show that CLDH reconstructed the lamellar structure and formed a hydrotalcite-like phase because of its memory effect. The hydrotalcite-like phase acted as a buffer system to absorb CO 2 intruding into the AAS mortars from the atmosphere, which retarded the decalcification of C-(A)-S-H. Meanwhile, the porosity decreased and the mortars become denser by the addition of CLDH, which prevented the diffusion of CO 2 into the mortars. The compressive strength of AAS mortars was enhanced by the carbonation regardless of CLDH addition. Calcite and vaterite were the main carbonation products. The addition of CLDH did not significantly change the polymorphs of carbonates but mitigated their production. • The effect of CLDHs on the carbonation depth of AAS mortars is studied. • The effects of CLDHs on the phase composition and microstructure of AAS are discussed. • The CO 3 2− is absorbed by the process of CLDHs recrystallized to LDHs. • The porosity of AAS mortars is significantly reduced by adding CLDHs. • Adding CLDHs effectively reduces the susceptibility of AAS mortars to carbonation. [ABSTRACT FROM AUTHOR]