Free Chloride Content Determination Using Total and Bound Chloride in Cement Pastes

The need for determining critical chloride concentration threshold (Ccrit) and effective chloride diffusivity in concrete elements exposed to a chloride environment shows the importance of chloride binding and necessitates developing a procedure that can determine the free chloride content in cement paste. The acid-soluble chloride and equilibrium methods were used to determine the free chloride content in cement paste mixtures. Three water/cement (w/c) ratios of 0.6, 0.5 and 0.42 were assessed through acid-soluble chloride, water-soluble chloride, and equilibrium methods. Thin disc samples showed greater chloride binding capacity at higher chloride concentrations than powdered samples. Thermodynamic modeling results obtained from Gibbs free energy minimization software (GEMS 3) were used to determine the amount of chemically bound chloride in the cement systems and further separate the contribution of physically and chemically bound chlorides. Results indicate higher physical binding at lower chloride concentrations and higher chemical binding at high chloride concentrations. A linear relationship between calculated free chloride content and water-soluble chloride was noted. The introduced procedure was used to determine effective chloride diffusivity in cement paste.