This study investigated the effect of alkali content (Na:Al = 0.86 and 1.39) and brucite (i.e., Mg(OH)₂) mineral addition on the sulfuric acid resistance of alkali-activated metakaolin (i.e., geopolymers). Geopolymers consist primarily of a sodium-stabilized aluminosilicate hydrate (N-A-S-H) framework. Results demonstrate that higher alkali contents and brucite addition improve the acid resistance of N-A-S-H, as evidenced by reduced dealumination and Si and Na leaching upon exposure to acid. These results are mechanistically explained by increased retention of Mg⁺² within the microstructure and increased Mg–Al interaction upon acid exposure. Higher Mg⁺² retention and increased Mg–Al coupling together provide empirical evidence of polyvalent cationic stabilization—a mechanism involving polyvalent cations (e.g., Mg⁺²) that stabilize the N-A-S-H binder by arresting acid-induced electrophilic attack. Results further illustrate that brucite addition, especially at high-alkali content formulations, reduces micro-scale porosity while increasing the proportion of gel pores (<5 nm), which suggests that increased tortuosity of gel pores may aid in Mg⁺² retention and promote the increased Mg–Al coupling observed herein.