The mechanism of limited inhibition by fly ash on expansion due to alkali–silica reaction at the pessimum proportion

The inhibitory role of fly ash (FA) on alkali–silica reaction (ASR) expansion is drastically reduced at the pessimum proportion relative to its role at full proportion. The main contribution of FA to inhibiting ASR expansion is to reduce the alkalinity of the pore solution and it is dependent on the thermodynamic equilibrium between the pore solution and the calcium-silicate-hydrate (C-S-H) gel. C-S-H gel with low Ca/Si ratio can bind a large amount of alkalis. However, when the concentration is higher than the threshold concentration for reaction, C-S-H gel plays a role in buffering the alkali concentration in the pore solution by supplying alkali. Thus, the inhibitory role of FA on ASR expansion decreases at the pessimum proportion because the aggregate can react with alkali even at low alkali concentrations. Therefore, replacing cement with a larger amount of fly ash is essential to reaching the threshold for reaction.