Carbonation of Concretes with SCMs

Due to the urgent need to reduce CO2 emissions, cement and concrete producers aim to substitute increasing proportions of Portland cement by supplementary cementitious materials (SCMs). However, lowering the clinker content can lead to an increasing risk of neutralisation of the concrete pore solution and reinforcement corrosion due to carbonation. Carbonation mechanisms and kinetics depend on hydrate phase assemblage, pore solution chemistry and porous structure as well as the exposure conditions. To study the effects of mix-design, curing, preconditioning and storage conditions on carbonation resistance, the RILEM TC 281–CCC established a comprehensive database, consisting of 1044 concrete and mortar mixes with their associated carbonation depth data over time. The dataset comprises mix designs with a large variety of binders and includes information on chemical composition and physical properties of the raw materials, compressive strengths, curing and carbonation testing conditions. Both natural and accelerated carbonation with CO2 concentrations of up to10 vol.-% were considered. The analysis of the available data confirmed that the w/CaOreactive ratio is a decisive factor for carbonation resistance, but curing and exposure conditions also influence carbonation. Probabilistic inference suggests that both accelerated and natural carbonation processes follow a square-root-of-time behavior, but direct conversion between accelerated and natural carbonation by solely taking the CO2 concentration into account is not possible. Accelerated carbonation testing and application of the square-root-of-time law yield predicted natural carbonation depths that tend to be lower than the measured values. This effect was particularly noticeable for concretes with a higher SCM fraction (w/CaOreactive > ~1.1).