Report of RILEM TC 301-ASR: relation between pore solution composition and ASR expansion

An important parameter determining the risk of alkali-silica reaction (ASR) in concrete is the availability of alkali ions as they are closely related to the OH⁻ concentration in the pore solution. Comparison of concrete or mortar expansion with cement and pore solution composition from literature data showed no obvious relationship of the expansion with the total alkali content of the cement. However, the observed expansion was strongly dependent on the alkali and hydroxide concentrations in the pore solution. At a hydroxide concentration greater than 250 mmol/L in the pore solution or at (Na + K) concentrations greater than 300–400 mmol/L, significant ASR expansion took place for the highly reactive aggregates studied in the laboratory samples exposed at near ambient temperatures (20–40 °C). Less reactive aggregates will have a higher “alkali threshold”. At 60 and 80 °C correlations between pore solution and expansion tend to fall apart as temperature influences many factors that can accelerate or slow down ASR. High Al concentration in the pore solution as well as a low undersaturation with respect to silica have been suggested to slow down ASR formation. Neither the Al concentration in the pore solution nor undersaturation with respect to SiO₂ was found to be selective criteria for ASR expansion. The pH measurements are highly dependent on temperature, difficult to measure, and determined using a variety of techniques that are frequently poorly explained. The pore solution’s pH by itself is therefore not a good way to evaluate the risk of ASR expansion, although this study showed that there is a good correlation between pH and the sum of (Na + K) concentration in the paste pore solution. It is recommended that (Na + K) is used as the most reliable and relatively easily accessible parameter to indicate the potential for ASR in concretes or mortars up to 40 °C.