Carbonation of alkali activated binders and comparison with the performance of ordinary Portland cement and blended cement binders

Alkali activated binders are sustainable alternatives to carbon-intensive conventional Portland cements. Although earlier studies focussed on the mechanical properties of alkali-activated binders, studies on their carbonation resistance are limited compared to Portland cement systems. Hence, the present study focuses on a systematic review of the carbonation performance of different types of alkali-activated binders, considering all the influencing parameters such as activator modulus, concentration, and the nature of the precursor etc. This study also presents a comparison of the carbonation of alkali-activated binders with ordinary Portland cement and blended cement binders. The observations on carbonation resistance are presented in terms of carbonation depth, pH change, compressive strength and corrosion resistance. Moreover, the carbonation products formed in different alkali-activated binder systems and their mineralogical, as well as morphological characteristics are discussed based on the results from the thermogravimetric analysis, X-ray diffraction analysis, and scanning electron microscopy. While different polymorphs of calcium carbonate are reported as the carbonated phases in conventional cementitious systems and slag-based alkali-activated binders, nahcolite was found to be the predominant carbonation product when fly ash was used as the precursor. It has also been reported that the intensity of CO2 exposure can result in different forms of carbonate phases in alkali-activated slag, with lower CO2 exposure favouring calcite formation and higher concentrations favouring the formation of vaterite. Matrix densification and crack healing due to carbonation product formation was also observed from the micrographs of carbonated alkali-activated binders, similar to what is reported for conventional cementitious systems. The differential thermogravimetric curves show wide bands in the calcium hydroxide decomposition range in alkali-activated slag binders as opposed to the narrow and distinct lime decomposition peaks in OPC and blended cement systems. The carbonation products are found to change with respect to the carbonation conditions of alkali-activated systems as well as with the type of the precursor.