Influences of composition and reaction conditions on molecular structure, hygroscopicity, and swelling behavior of alkali-silica reaction gels

Abstract

Alkali-silica reaction (ASR) gel, a product of the reactions between the dissolved silica from aggregate and the alkalis from cement, governs the destructiveness of ASR and concrete deterioration. The influences of chemical composition and reaction conditions of ASR gel on its structure and properties remain unclear. In this work, the correlations between the molecular structure, hygroscopicity, and swelling behavior of ASR products were uncovered by investigating six CaO–SiO2-M2O systems with varying alkali/Si and Ca/Si ratios under two reaction conditions. The results indicate that the low-alkali gels have a layered silicate structure dominated by Q1 and Q2 sites similar to calcium silicate hydrate (C–S–H), whereas the high-alkali gels showed coexistence of tobermorite-type C–S–H and alkali-silicate hydrates featured with Q3 polymerization. The 29Si nuclear magnetic resonance (NMR) results showed that the mean chain length (MCL) and degree of polymerization (DP) decrease with alkali/Si and Ca/Si ratios but increase with moisture. Under 97 % RH, decreased crystallinity and enhanced formation of ASH and Q3 sites are obtained along with increased polymerization and d-spacing. The moisture uptake and swelling of the high-alkali gels showed a reverse correlation with the Ca/Si ratio, which confirms the role of calcium in suppressing the formation of ASH and the dominant role of Q3 sites in determining the hygroscopicity of ASR products.