Sulfuric acid degradation of alkali-activated metakaolin cements supplemented with brucite

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 highalkali 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.

Automated summary

This study investigated the impact of alkali content and brucite mineral addition on the acid resistance of alkali-activated metakaolin. Results showed that higher alkali content and brucite addition improved the acid resistance of sodium-stabilized aluminosilicate hydrate framework, primarily through increased Mg+2 retention and Mg-Al interaction. Additionally, brucite addition reduced micro-scale porosity and increased gel pore proportion, indicating potential benefits for Mg+2 retention and Mg-Al coupling.