Modelling of chloride-binding isotherm by multi-species approach in cement mortars submitted to migration test

Based on a multi-species approach, a new model for evaluating free chloride content in cementitious materials is proposed. To validate the model, migration tests were carried out on cement mortars with several NaCl concentrations. The total chloride was measured with the potentiometric method. The subtraction of the free chloride from total chloride results in bound chloride. Thus, a chloride-binding isotherm is obtained. First, it is shown that free chloride depends on the concentration of anions and cations contained in the pore solution. Second, the chloride-binding isotherm obtained (linear isotherm) shows that the interactions between chloride and hydrated products of the tested mortars are stronger than those (non-linear isotherm) quantified by the method based on water extracting. Finally, the electrical current is shown to be dependent on the binding phenomenon during the unsteady state. In fact, in a steady state, the sites that can react with chlorides are saturated; hence the binding isotherms do not affect the transport process.