Carbon sequestration mechanisms and performance evolution of alkali-activated slag during the mixing process

This study investigated the direct incorporation of carbon dioxide (CO₂) during the mixing of alkali-activated ground granulated blast furnace slag as an alternative to traditional post-hardening carbonation. CO₂ was introduced during the final 30 s of mixing. The effects of alkali content and carbonation on CO₂ uptake, compressive strength, water absorption, and microstructure were evaluated. The results showed that CO₂ uptake increased with alkali content and exposure duration, with a maximum uptake of approximately 1.9% after 300 s of continuous CO₂ exposure. The gravimetric and thermogravimetric measurements showed strong agreement, with an R ² of 0.99. For the selected mortar mixtures, 30 s of CO₂ exposure resulted in a 15–25% increase in 28-day compressive strength compared with non-carbonated specimens. However, water absorption showed a slight reduction. The microstructural analysis indicated carbonate-rich precipitates in pore and interparticle regions. The results demonstrate that introducing CO₂ during alkali activation can promote in situ carbonate formation while improving the later-age mechanical performance of GGBFS-based alkali-activated materials.