In this paper, a modified alkali-activated slag cementitious material was proposed by applying calcium and alumina mineral phases (Ca(OH)2 and γ-Al2O3) as partial replacement of slag, aiming at promoting the formation of Friedel’s salt (F salt) in the matrix when in the presence of chloride, and improving the chloride binding capacity. The effects of calcium and aluminum mineral phase content on reaction products, chloride binding capacity and mechanical properties were discussed. The results show that the addition of calcium and aluminum mineral phases improves the chloride binding capacity of alkali-activated slag, and the mass ratio of Ca(OH)2 to γ-Al2O3 is highly related to this capacity. XRD analyses show that there exists unreacted Ca(OH)2 within the reaction products, and all those Ca(OH)2 transform into F salt or other phases after chloride attack. It is demonstrated that the enhancement of chloride binding capacity is attributed to the formation of F salt. The compressive strength results indicate that the addition of Ca(OH)2 exhibits a negative impact, while the γ-Al2O3 can compensate for the strength loss due to the Ca(OH)2 addition.