Aggregate dissolution kinetics

The alkali–silica reaction (ASR) starts with the dissolution of silica in alkali-reactive aggregates. Besides the thermodynamic basics of silica dissolution, kinetic data are needed to model ASR in order to describe the temporal course of the reaction. Although the dissolution kinetics of silica has been investigated frequently, data for concrete-like environments (pH > 12) and under special influences, such as de-icer impact by sodium chloride (NaCl) or the release of aluminium (Al) from feldspars, are rare or missing. In the present study, long-term (2.5 years) dissolution tests were performed to obtain kinetic data for two types of aggregate (granodiorite, rhyolite) under concrete-like conditions (pH 13.8, with/without calcium hydroxide (Ca(OH)₂) and sodium chloride) at temperatures of 25, 45 and 60°C. The findings show that silica dissolution rates effective for ASR differ up to four orders of magnitude, ranging from (log r₁) −13.13 to −9.96 mol/m²s as a function of aggregate type, temperature, sodium chloride concentration and calcium hydroxide. Besides a predominant influence of temperature and calcium hydroxide, sodium chloride promotes the dissolution of silica and lowers the aluminium concentration, especially at higher temperatures. The activation energy for silica dissolution (84 ± 4 kJ/mol) did not change significantly in the presence of sodium chloride.