Assessment of the thermodynamics of Na,K-shlykovite as potential alkali-silica reaction products in the (Na,K)2O-CaO–SiO2-H2O system

The thermodynamic description of the alkali-silica reaction is incomplete, due to the lack of thermodynamic data for the alkali-silica reaction products. Here, we estimate the temperature-dependent thermodynamic properties of Na- or K-shlykovite phases which are found to form during alkali-silica reaction in concrete. The thermodynamic properties are then used to produce a series of binary phase diagrams considering different concentrations of alkalis. The results show that these two products start to form at 0.01 mol/kg of alkalis in the CaO–SiO2 binary phase diagrams. Both Na- and K-shlykovite can co-exist with C-S-H at an increased concentration of CaO, consistent with findings that these alkali-silica reaction products may form in real cement systems. However, there is a maximum alkali concentration in the binary phase diagram beyond which shlykovite cannot form, due to limited calcium solubility. This work can help understand and improve alkali-silica testing protocols proposed in standards.