One approach to improve the carbon footprint of concrete is the use of alternative binders, which can be produced with less CO₂ (Carbon dioxide) emissions and lower energy consumption. These alternative binders either have a reduced clinker to cement ratio or contain no Portland cement clinker at all. The protection of the embedded steel is an important characteristic, which determines the applicability of such binders in reinforced concrete structures. The progress of carbonation (Carbonation is a process that takes place at the nanometric scale) and the associated reduction of the pH value have a major influence in this regard, especially as these binders are typically less resistant against CO₂ exposure. Previous research has shown that accelerated carbonation tests at elevated CO₂ concentrations can lead to altered carbonation behavior, especially in systems low in calcium hydroxide. Accordingly, special consideration must be given to the CO₂ concentration in such accelerated tests and to the interpretation of the resulting carbonation behavior. In this study, the carbonation resistance of various alternative cements and binders, such as composite cements with calcined clays or modified steel slag, alkali-activated materials, CSA cement and a C-S-H binder were investigated on mortar samples stored in three different CO₂ concentration – atmospheric storage (~0.04 vol.%), 0.3 vol.% and 1.0 vol.% CO₂. The results were analyzed with regard to the carbonation progress and the specific carbonation behavior of the binders. It was found, that even within this comparably low range, the binders react very differently to an increased CO₂ concentration. Consequently, binder specific CO₂ concentrations might be necessary, if the results of accelerated tests are intended to reflect the carbonation behavior in atmospheric conditions.

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