Effects of sodium ions on synthesized alkali silica reaction gels

The alkali-silica reaction (ASR), leading to serious structural degradation, is the chemical reaction between reactive silica presenting in aggregates and hydroxyl ions from cement paste or pore solution. Although the chemical mechanism of ASR attack has been well studied for years, the mechanism of micro scale ASR gel formation leading to macro scale expansion is still under debate. The present study aims to illustrate the performance of ASR gel by investigating the interaction between ASR gel and sodium ion from solution. In this study, ASR gels with different calcium silica (Ca/Si) ratios (0.1, 0.5, 1) are synthesized by mixing reagent Ca(OH)2 with silica fume in a sodium hydroxide solution for seven days. Afterwards, the synthesized ASR gel is immersed in sodium hydroxide solutions with different concentrations (0.1mol/L, 0.5mol/L, 1.0mol/L) for seven days. Chemical composition, structure and water content of the ASR gel before and after alkali exposure are studied by XRD, XRF and TGA. The results confirm that an ASR gel with a targeted Ca/Si ratio can be synthesized. In addition, XRD and TGA results show that part of the calcium in the ASR gel is exchanged by sodium, leading to a structural modification. In general, this study will give further comprehension of ASR gel performance under alkaline environment, and provide detailed data to investigate the interaction between ASR gel and calcium ions in an alkaline solution in the future