Carbonation of concrete: the role of CO2 concentration, relative humidity and CO2 buffer capacity

In this study, the effect of CO2 concentration and ambient relative humidity ( RH) on accelerated and natural carbonation of 18 concrete mixtures produced with nine different cement types is investigated. Increasing the CO2 concentration from 0.045 to 1 and 4 % at 57 % RH does not alter the relative carbonation resistance between the concrete mixtures. The increase of RH from 57 to 70 and 80 % RH at 4 % CO2 shows a water-to-cement ratio and cement-specific effect that affects the relative carbonation resistance between the concrete mixtures. The carbonation resistance at 4 % CO2 and 57 % RH allows assessing the carbonation resistance of concrete in sheltered and with restrictions in unsheltered outdoor exposure. The carbonation resistance below 70 % RH is mainly governed by the CO2 buffer capacity. However, in the accelerated tests at 80 % RH and in the unsheltered outdoor exposure capillary condensation is of increased importance.