Measurements of chloride activity coefficients in real portland cement paste pore solutions

The study of chemical and electrochemical equilibria between concrete or embedded steel and ionic species in concrete's pore solution requires the knowledge of ion activities (free concentrations in solution). Recently, a method has been developed for measuring chloride-ion (Cl-) activities in simplified synthetic concrete pore solutions using calibrated Cl- selective electrodes. Cl- activities also have been calculated using Pitzer's specific ionic-interaction model. In this work, we have applied both of these methods, experimental and theoretical, for obtaining Cl- activities in solutions obtained using the technique of "pore pressing," which has been applied to cement pastes with varied admixed Cl- concentrations. A reasonably good agreement has been found between experimental and theoretical results, but the presence of hydroxide ions in these highly alkaline electrolytes interferes with the measurements at low Cl- concentrations.