Aluminosilicate Gel Formation in Alkali-Activated Fly Ash: An NMR Spectroscopic Study

This study investigates the impact of a CO2-rich environment on the aluminosilicate gel in alkali-activated fly ash (AAAFA) using NMR spectroscopy. After 24 h of curing at 80°C, the AAAFA samples were exposed to a 10% ambient CO2 concentration. Highly reactive elements such as Na and Al fully reacted within the initial hours and remained unaffected by carbonation under high CO2 levels. However, Si, which reacts more slowly, exhibited distinct behaviour. Monomeric silicates rapidly polymerized despite the carbonated sample showing a lower overall reaction extent. This indicates the formation of a binding gel phase due to intense CO2 exposure, leading to an increased gel content in the carbonated sample. These findings enhance the understanding of aluminosilicate gel chemistry and behavior under elevated CO2 concentrations, providing insights into the long-term performance of alkali-activated materials in carbon-rich environments.