Carbonation‐Driven Microstructural Transformation in Alkali‑Activated Binders: Coupled Multiscale Characterization

ABSTRACT

CO ₂ mineralization offers a viable pathway to reduce the carbon footprint of cementitious materials. Alkali‑activated binders (AABs) can chemically bind and mineralize CO ₂ within their microstructure, yet the conditions leading to beneficial densification remain unclear. We investigated the impact of carbonation on one‐part AAB, focusing on the (C,N)‐A‐S‐H gel, by integrating chemical, morphological, and mechanical characteristics across length scales in both paste and mortar. Two formulations, 100% slag (AAS) and a 50%–50% slag–fly ash blend (AASF), were tested under different CO ₂ curing levels. In situ SEM micropillar compression and nanoindentation measured small‐scale mechanical properties, linked to other morphological and chemical characteristics. Carbonation significantly affected mortar compressive strength, with trends varying by CO ₂ curing levels. Nanoindentation showed that (C,N)‐A‐S‐H gel stiffness was largely preserved, while micropillar compression revealed increased compressive strength in both binders. EDS analysis indicated that the apparent Ca/Si ratio of the (C,N)‐A‐S‐H increased after carbonation, which may be linked to calcium carbonate precipitation and could potentially contribute to the observed strength improvement by reinforcing the matrix. After 28 days of CO ₂ curing, AASF formed about 30% calcium carbonate by mass, compared to 23% in AAS. Overall, carbonation favored strengthening at the paste level, whereas mortar strength depended on CO ₂ concentration and decreased under certain curing conditions.