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This study evaluated the effects of various Mg-based admixtures, including magnesium oxide (MgO), layered double hydroxides (LDH), and calcined layered double hydroxides (CLDH), on the chloride resistance of alkali-activated fly ash-slag (AAFS) mortars. To this end, chloride diffusion tests were conducted on AAFS mortars containing 5 wt% MgO, Mg-Al-CO3 LDH, and CLDH. The results revealed that all Mg-based admixtures investigated in this study enhanced the chloride resistance of AAFS mortars. The inclusion of MgO led to a finer pore structure and promoted the formation of more Friedel's salt, thereby enhancing the chloride resistance performance. Although the inclusion of Mg-Al-CO3 LDH and CLDH resulted in more lager pores in the matrix, leading to slightly lower compressive strength, the overall content of Mg-Al LDH and Friedel's salt formed in the system increased, which led to an improved chloride binding capacity and enhanced chloride resistance.