Ingress of NaCl in concrete with alkali reactive aggregate: effect on silicon solubility
Enhanced damage due to the alkali silica reaction (ASR) in concrete exposed to deicing salt (NaCl) is usually attributed to binding of chloride ions in the hydration products of cement. To balance charge, OH ions are released into the concrete pore solution which increases alkalinity. However, during NaCl ingress a decrease in the OH concentration of the concrete pore solution due to potassium leaching would reduce SiO2 solubility and therefore ASR damage. The present work combines expansion measurements with pore solution analysis by ICP OES and XRD measurements on concretes and hydrated cement pastes. Solubility equilibria calculations were performed with the hydrogeochemical simulation program PHREEQC. The investigations show that the OH concentration of the pore solution is mainly lowered by potassium leaching during NaCl ingress. The OH concentration also decreases owing to the formation of Friedel's salt from ettringite which is associated with the release of sulphate. Although the OH concentration with NaCl is lower, ASR damage is intensified and the silicon concentration in the pore solution is higher. Higher silicon solubility is explained by the higher total alkali concentration which increases surface silicate solubility, the formation of an aqueous complex NaHSiO30 and a higher ionic strength. These effects promote the sensitivity of silicate minerals to ASR, the formation of alkali silica gel and finally ASR damage.