Natural carbonation of concretes with supplementary cementitious materials in a Brazilian tropical climate: behavioural and predictive models after 21 years of exposure

Abstract

Partial replacement of clinker by supplementary cementitious materials (SCMs) modifies the microstructure of concrete, reducing capillary porosity but compromising the alkaline reserve. The effect of SCMs on concrete carbonation depends on the type and content of replacement, regulated by the water/binder ratio and curing, which makes it difficult to predict carbonation resistance. Thus, long-term durability studies are essential to understand the phenomena and to establish behavioural patterns of carbonation as a function of time. This paper, therefore, discusses the carbonation behaviour of 18 concrete families (6 compositions with SCMs and 3 water/binder ratios) under two extreme curing conditions (air curing and curing for 28 days in a humid chamber with RH above 95 %), after 21 years of natural exposure. Considering a wide range of concretes (36 individual analysis situations) and a strong data robustness, accompanied by statistical analyses, the Smolczyk and Tuutti models produced from experimental data of carbonated fronts over time are presented and discussed. With a view to predictive analysis of carbonation, with potential applicability in the field of service life prediction of concrete structures, linearized Tuutti models (xc = k . √t) are presented for the 36 analysis situations, which were very well fitted, with R² values ranging from 0.92 to 0.99. From the overall analysis, it was found that the absence of curing significantly increased carbonation rates in the first year, with irreversible effects (although attenuated after seven years), and that the water/binder ratio and SCMs showed a synergistic effect. In addition, the w/CaOreactive ratio resulted in a significant indicator of durability in the context of carbonation, while the concrete's compressive strength showed limitations.