CO2 binding capacity of alkali-activated fly ash and slag pastes

Quantification of the CO2 binding capacity of reinforced concrete is of high importance for predicting the carbonation potential and service life of concrete structures. Such information is still not available for alkali activated materials that have received extensive attention as a sustainable substitute for ordinary Portland cement (OPC)-based concrete. To address this gap, this paper evaluates the CO2 binding capacity of ground powders of alkali activated fly ash (FA) and ground granulated blast furnace slag (GBFS) pastes under accelerated carbonation conditions (1% v/v CO2, 60% RH, 20 degrees C) for up to 180 days. The CO2 binding capacity, the gel phase changes, and the carbonate phases are investigated with complementary TG-DTG-MS, FT-IR and QXRD techniques. Five mixtures with different FA/GBFS ratio are considered. CEM I and CEM III/B pastes are also studied to provide a baseline for comparisons. The results showed that the alkali-activated pastes have a lower CO2 binding capacity in comparison to cement-based pastes. Furthermore, alkali-activated pastes have similar CO2 binding capacity regardless of the FA/GBFS ratio. It was observed that the silicate functional groups corresponding to the reaction products in the pastes were progressively changing during the first 7 days, after which only carbonate groups changed. It was also found that the CO2 bound in the alkali-activated pastes occurs to a substantial extent in amorphous form.