Carbonation behaviour of concretes with alternative binders at different CO₂ concentrations

In order to reduce CO₂ emissions in the construction sector, alternative binders are attracting more and more attention. Whether these alternative binders contain a lower clinker‐cement ratio or no Portland cement clinker at all, to be truly sustainable they must not compromise durability and must therefore be durable.

The progress of carbonation and the associated reduction of the pH value have a major influence on the corrosion protection of steel reinforcement, especially as such “low carbon” concretes are typically less resistant against CO₂ exposure. Previous research has shown that accelerated tests at elevated CO₂ concentrations can lead to altered carbonation behavior, especially in systems low in Ca(OH)₂.

In this study, hydration products and carbonation resistance of various alternative cements and binders, such as composite cements with calcined clays or modified steel slag, alkali‐activated materials, C$SMA cement and a C‐S‐H binder were investigated. The carbonation was studied on mortar samples stored in three different CO₂ concentrations ‐ atmospheric storage (∼0.04 vol.%), 0.3 vol.% and 1.0 vol.% CO₂. It was found that the binders can be divided into three clusters on the basis of their hydration products and w/CaO or w/(CaO + MgO_eq + Na₂O_eq + K₂O_eq) ratios, which in turn provide information on the carbonation behavior. For each cluster a different CO₂ concentration can be recommended to achieve a realistic carbonation behavior in accelerated test procedures.