3.4. Evaluation of chemical activity coefficients

The models to calculate the chemical activity coefficients are numerous. The first ones developed are the Debye–Hückel and extended Debye–Hückel models [37]. From purely electrostatic considerations, they relate the chemical activity coefficients of ionic species to the ionic strength of a solution. They are valid for ionic strengths up to 10 and 100 mmol/l, respectively.

In cement-based materials, the ionic strength is much higher. To suit this particular situation, a chemical activity relationship was developed recently by Samson et al. [38], which gives good results for highly concentrated solutions: