Assessment of the carbonation resistance of alkali-activated concretes by spraying with phenolphthalein. The colouring of the complete fracture surface demonstrates that significant carbonation has not yet taken place.
Source: BAM, Division 7.4 Technology of Construction Materials
Alkali-activated concretes are materials that are produced without conventional cement, leading to a much lower CO2 footprint of these materials than that of conventional concretes. However, before alkali-activated concretes can be applied with confidence in infrastructural and other buildings, their durability under various conditions needs to be understood in more detail than is currently the case. A particularly important aspect of their durability is their resistance against carbonation, i.e., against the reaction with atmospheric CO2 and the associated modifications of the microstructure and chemistry of the materials and their pore solutions, which can lead to corrosion of embedded steel reinforcement.
The current ability to predict the carbonation resistance of alkali-activated concretes is incomplete, in large part because of widely varying binder chemistries of these concretes and variable testing conditions. To identify general correlations between mix design parameters and the carbonation rate of alkali-activated concretes and mortars, carbonation data for these and related materials were compiled and the resulting dataset analyzed. The analysis showed that, contrary to what is observed for conventional concretes, the water/CaO ratio of the binder paste is not a reliable indicator of the carbonation rate of alkali-activated concretes. A better indicator of their carbonation rate was found to be the water/(CaO + MgOeq + Na2Oeq + K2Oeq) ratio of the binder paste. The index ‘eq’ for some of the oxides in the formula indicates an equivalent amount, calculated to account for the differing CO2 binding capacities of the oxides. However, it was also found that this ratio can serve only as an approximate indicator of carbonation resistance, demonstrating that other parameters also significantly affect the carbonation resistance of alkali-activated concretes.
In addition, the analysis of the dataset revealed peculiarities of widely used, accelerated tests of the carbonation resistance for a subset of alkali-activated concretes, viz., concretes made from binders with low CaO contents. This indicates that even at the relatively modest concentration of 1 % CO2, accelerated testing may lead to inaccurate predictions of the carbonation resistance of these concretes under natural exposure conditions.
Gregor GluthORCiD, X. Ke, A. Vollpracht, L. Weiler, S. A. Bernal, M. Cyr, K. Dombrowski-Daube, D. A. Geddes, C. Grengg, C. Le Galliard, M. Nedeljkovic, J. L. Provis, L. Valentini, B. Walkley (2022) Carbonation rate of alkali-activated concretes and high-volume SCM concretes: a literature data analysis by RILEM TC 281-CCC. Materials and Structures, Volume 55, Issue 8, pages 1–15.
BAM Department Safety of Structures
BAM Division Technology of Construction Materials