Hydration of alkali-activated slag: thermodynamic modelling

The composition of solid and liquid phases of alkali-activated slag systems was investigated by thermodynamic modelling. The calculations predict the formation of C(–A)–S–H, stratlingite and hydrotalcite in different alkali-activated slag systems, which agrees well with experimental findings. In addition traces of FeS and ettringite are predicted to precipitate. The type of alkaline activator (Na-silicate, NaOH) has no influence on the type of hydrates formed but influences the Ca/Si ratio of the C–S–H. The addition of sodium silicate leads to the formation of C–S–H with a higher Ca/Si ratio (1.1–1.2), whereas NaOH results in a Ca/Si ratio of 0.8to1.1. In contrast, the kind and concentration of the activator has strong influence on the composition of the pore solution. Low concentrations of the alkaline activator result in lower Na, Si and Al concentrations and lower pH values. In all pore solutions a high concentration of sulphide (HS^–) is expected to be present.