Impact of SCMs on Alkali Concentration in Pore Solution

The alkali metal concentration and pH values within the pore solution of a concrete strongly influence the occurrence and the extent of alkali-silica reaction (ASR). At (Na + K) concentrations <300 mmol/L, no significant ASR expansion is observed. The alkali metal and hydroxide concentration can be lowered by using either a low alkali cement or by blending Portland cement with silica-rich supplementary cementitious materials (SCMs) such as silica fume, fly ash, calcined clays or blast-furnace slag.

The reaction of silica-rich SCMs lowers the Ca/Si-ratio in C-A-S-H, which increases the uptake of Na and K by C-A-S-H and lowers both alkali concentrations and pH values in the pore solution, efficiently preventing ASR.

Thermodynamic modelling and the empirical Taylor model were used to predict the changes in the pore solution composition as a function of the amount of the respective SCM added to the concrete. Comparison with literature data confirms that the models can be used to predict the trends in alkali concentrations.

The literature data and thermodynamic modelling indicate that the replacement of Portland cement with ≥ 20 wt.% silica fume or metakaolin, ≥ 50 wt.% silica-rich fly ash or ≥ 65 wt.% blast-furnace slag can suppress ASR expansion in concrete. The study was conducted within the framework of working group 2 of RILEM TC 301 ASR.