pH-dependent chloride desorption isotherms of Portland cement paste

Carbonation, sulfate, and acid attack can lead to the release of bound chlorides, increasing the concentration of free chlorides in concrete pore solutions. The objective of this study was to develop pH-dependent chloride desorption isotherms for OPC paste exposed to MgCl₂, CaCl₂, and NaCl solutions. This study showed that the Langmuir isotherm suits the chloride binding of ground OPC paste the best, and the amount of bound chloride is predominantly influenced by the cation-type of chloride in the decreasing order CaCl₂ > MgCl₂ > NaCl. The results of the thermogravimetric analysis also confirmed this. The mass loss of Friedel's salt apparent in DTG curves indicated that more chlorides from CaCl₂ salt may bind with cement hydrates. The phase assemblages predicted by thermodynamic modeling were found to be in good agreement with the experimental results. The drop in the pH of the exposure solution, achieved by adding nitric acid to the exposure solution, initially resulted in the total dissociation of the bound chlorides from the MgCl₂ and NaCl salts. However, only 58% of the bound chlorides from the CaCl₂ solution could be dissociated as a result of the reduced pH of the exposure solution. Moreover, the dissociation of bound chlorides resulted in the removal of Friedel's salt and the formation of calcium nitrate. The further addition of nitric acid resulted in the formation of a calcium-rich hydrogel capable of absorbing free chlorides.