Reactivity of aqueous carbonated cement pastes: Effect of chemical composition and carbonation conditions

Aqueous carbonation of end-of-life concrete fines is a promising method to alleviate greenhouse gas emissions by CO₂ sequestration from point-source emitters. This produces a valuable material that can be utilized in new cement formulations. This study investigates effects of the composition of cement pastes and of the carbonation conditions on the reactivity and phase assemblage for aqueous carbonated Portland cement pastes incorporating silica fume, fly ash, and blast furnace slag. Results from ²⁷Al and ²⁹Si NMR show that hydration of the carbonated pastes under reactivity test conditions lead to phase assemblages dominated by a C-(A)-S-H phase, with reduced Al/Si ratio, as well as by ettringite and hemi/monocarbonate AFm phases. The results from the reactivity tests demonstrate that the carbonated blended cement pastes exhibit superior reactivity compared to carbonated neat Portland cement paste because of their increased fraction of reactive alumina and silica species. The variations in carbonation conditions (i.e., temperature, CO₂ gas concentration, and solution composition) do not alter significantly the reactivity of the carbonated pastes. These findings demonstrate the robustness of aqueous carbonation of concrete fines and support its wider application as a mean to reduce CO₂ emissions and enhance circularity of cement-based materials.