Effect of carbonation on bound chlorides in different cementitious systems

Deterioration of reinforced concrete is often considered in idealised scenarios where only one exposure environment is dominant. In de-icing salt environments for example, a cyclic behaviour of wetting and drying is considered, but deterioration by carbonation is not given any emphasis. It is unclear if multiple modes of deterioration in such common environments can be beneficial or detrimental to life of the structure.

This article investigates the influence of carbonation on the bound chlorides in different cementitious pastes. Chloride binding is the capacity of a cement matrix to react (or physically attach) chloride ions to its chemical structure and prevent these ions from diffusing further towards the steel. This article highlights the positive initial effects of carbonation has on the binding behaviour and later the changes that occur in both pH and the bound chloride content for low w/b mixes. Two mathematical expressions are put forward for mapping the (i) changes in apparent pH as a function of the duration of carbonation and mix ingredients and (ii) reduction in the bound chlorides with a proportional reduction in apparent pH. The latter is valuable in quantifying the changes to binding capacity in service life models due to carbonation with the help of a simple pH measurement.