Pore solution of concrete: The equilibrium of bound and free chloride

摘要

The chloride threshold values of concrete specified in the regulations are based on the total chloride content of the concrete. As only the chloride dissolved in the pore solution can initiate corrosion, the investigations focused on the chloride concentration of the pore solution. It was further investigated if the answer to the question - why chloride is taken up by concrete exposed to de-icing salts relatively quickly but is only slowly 'washed out' during summer - can be found in the chloride/cement-binding interactions. The results showed that the residual chloride concentration depends in particular on the OH_ concentration of the pore solution. The results further showed that the same binding mechanisms exist regardless of whether the chloride is mixed to the cement or added subsequently via diffusion. Other results suggested that one of the causes why chloride is taken up faster than it is washed out from concrete by rainwater are the binding interactions as well. As regards the assessment of the corrosion risk, the results showed that both the Cl~- concentration and the Cl~-/OH~- ratio of the pore solution varied between samples made of the same cement and containing the same total chloride content. These results illustrate very well that a certain total chloride content alone is not a suitable criterion for the assessment of corrosion risk and that other factors of influence must be taken into account as well.