pH-dependence of chloride binding in ordinary Portland cement

Abstract

A new method for artificially leaching hydrated cement paste and studying how lowered pH in the pore solution affects chloride binding has been developed. Hydrated cement paste was exposed to a NaCl-solution before being acidified by adding small volumes of HCl in steps over several days, gradually lowering the pH. A pH-development curve was established, determining how the pH in the pore solution responded to increasing amounts of acid being added. The chloride binding of the cement paste as a function of the pH and free chloride concentration was then determined. For the range of pH from 12 to 13, decreasing pH increased chloride binding. At a pH of 11, the cement paste showed almost no chloride binding. The mechanisms of changes in chloride binding were investigated using TGA, XRD, SEM-EDS, ICP-MS and thermodynamic modelling. One of the main chloride binding phases, Friedel s salt, could not be detected with the applied techniques due to the cement paste being cured at 60 °C, therefore its influence could not be verified. Thermodynamic modelling does however confirm the possibility of Friedel s salt decomposing at pH 11. The increase in chloride binding from pH 13 to 12 is most likely related to the changes in the main hydrate phase C-S-H. The effect of pH on chloride binding should be accounted for in service life prediction models for concrete structures exposed to chlorides.