Layered double hydroxides (LDHs) have emerged as an effective ingredient for enhancing the durability of cement-based materials against chloride-contaminated coastal or marine environments. Yet, the relationship between the surface chemistry and consequential adsorption affinity, which mainly constitutes chloride binding capacity of LDHs, still remain elusive, especially in alkaline cement pore solutions. Herein, we investigate Mg-Al-CO₃-LDHs to demonstrate the cationic-ratio- (M^II/M^III-) regulated surface chloride adsorption in alkaline solutions through a series of progressively in-depth means. The regulatory mechanism of the cationic ratio is microscopically operated through diverse composition and proportion of hydroxylated clusters with varying deprotonation reactivity of bonded hydroxyl on the surface. DFT calculations combined with multiple surface characterization techniques indicate that the varying reactivity is determined by the ionic bonding characteristics of H–O bond and the electrostatic attraction ability of different clusters. The LDH with Mg/Al ratio of 2.0 exhibits the optimal surface chloride adsorption among ratios ranging from 1.6 to 3.8 in alkaline solutions due to the strong resistance to nucleophilic attack from OH⁻ of Mg₂Al-OH cluster while maintaining high electrostatic attraction ability. Our findings advance the comprehension of surface interactions of LDHs with alkaline environments while underscoring the role of cationic ratio in surface chloride adsorption.