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The RILEM TC 281–CCC "Carbonation of concrete with supplementary cementitious materials" is studying the effect of supplementary cementitious materials (SCMs) on natural and accelerated carbonation of blended cements. In this context a large database with carbonation depths over time has been set-up with currently 910 concrete and mortar mixes, which have been tested at different curing conditions. The collected data includes information on chemical composition and physical properties of the raw materials, on mix-design, compressive strength, curing and carbonation testing conditions. Data for a large variety of binders (with up to 94 % SCMs) and mix designs (with w/b ratios ranging from 0.26 to 0.75) were collected from the literature and unpublished material testing projects. Natural carbonation was recorded for several years in many cases, and indoor as well as outdoor results are included. However, in this contribution only lab conditions are considered. Carbonation kinetics are investigated considering the binder composition and mix design. Furthermore, the accuracy of accelerated carbonation testing is evaluated. As described in literature, the w/CaOreactive-ratio is a decisive factor for carbonation resistance but curing and exposure conditions influence the results as well. Even with identical test conditions, the scatter is very large.
The current ability to predict the carbonation resistance of alkali-activated materials (AAMs) is incomplete, partly because of widely varying AAM chemistries and variable testing conditions. To identify general correlations between mix design parameters and the carbonation rate of AAMs, RILEM TC 281-CCC Working Group 6 compiled and analysed carbonation data for alkali-activated concretes and mortars from the literature. For comparison purposes, data for blended Portland cement-based concretes with a high percentage of SCMs (≥66% of the binder) were also included in the database. The results show that the water/CaO ratio is not a reliable indicator of the carbonation rate of AAMs. A better indicator of the carbonation rate of AAMs under conditions approximating natural carbonation is their water/(CaO + MgOeq + Na2Oeq + K2Oeq) ratio, where the index ‘eq’ indicates an equivalent amount based on molar masses. This finding can be explained by the CO2 binding capacity of alkaline-earth and alkali metal ions; the obtained correlation also indicates an influence of the space-filling capability of the binding phases of AAMs, as for conventional cements. However, this ratio can serve only as an approximate indicator of carbonation resistance, as other parameters also affect the carbonation resistance of alkali-activated concretes. In addition, the analysis of the dataset revealed peculiarities of accelerated tests using elevated CO2 concentrations for low-Ca AAMs, indicating that even at the relatively modest concentration of 1% CO2, accelerated testing may lead to inaccurate predictions of their carbonation resistance under natural exposure conditions.