Organosilicon functionality modification mechanisms in geopolymers and the improvement of carbonation resistance

Abstract

This study explores a route for improving carbonation resistance of geopolymers by CO₂–H₂O–gel interface modification using organosilicon functionality. In this study, three organosilicon modifiers with distinct non-hydrolysable groups, including PMS, S823, and KH570, were added into geopolymers. The carbonation behavior of the geopolymers was comparatively evaluated under different drying conditions. Fresh-state properties, wettability, carbonation depth, and compressive strength were evaluated. To clarify the modification mechanism, gel structure evolution, carbonation products, and water-state changes were characterized by XRD, FTIR, TGA, LF ¹H NMR, and MIP. The results showed that the carbonation resistance of the modified geopolymers was strongly dependent on organosilicon functionality. The water contact angle increased markedly from 36.52° for G0 to 117.87° for GS5. After 7 d of carbonation without pre-drying, the carbonation depth decreased from 30.25 mm for G0 to 15.31 mm for GS5, which was a 50% reduction. The residual compressive strengths of carbonated G0 and GS5 were 37.6 MPa and 36.7 MPa, respectively. Mechanistically, the long hydrophobic alkyl group of S823 promoted the formation of a low-surface-energy interfacial environment and disrupted the effective CO₂–H₂O–gel reaction interface, thereby most effectively suppressing carbonation. By contrast, KH570 mainly influenced local crystallization and carbonate phase evolution through its polar methacrylate functionality. PMS, containing only a small methyl group, showed limited improvement because it primarily participated in aluminosilicate network reorganization rather than establishing a strongly hydrophobic interfacial environment. This work provides a basis for the functionality-guided design of organosilicon-modified geopolymers with enhanced carbonation resistance, and contributes to their practical development for durable applications in CO₂-rich environments.