Underwater carbonation of alkali-activated slag pastes

Carbonation of alkali-activated slag (AAS) materials has been primarily concerned in atmospheres with gaseous CO 2 . This study, by contrast, highlights that AAS pastes would also be carbonated under tap water immersion. Calcite is the main CO 2 -bear phase in both sodium hydroxide- and sodium silicate-activated AAS pastes, and the paste pre-cured for a longer curing period shows more severe carbonation. Additionally, calcium carbonate can densify the deteriorated microstructure of sodium hydroxide-activated paste caused by long-term leaching. The indentation modulus of pastes subjected to tap water immersion is higher than those under deionized water immersion. The uptake of CO 3 2- by hydrotalcite (Ht) and gels is also detected, resulting in the formation of Ht-CO 3 and decalcification of gels. Due to the synergistic effect of leaching and carbonation, a characteristic layered distribution of pastes close to the exposure front is observed, comprising the carbonated layer, transitional (carbonated + leached) layer, and leached layer, progressing from the outermost to the inner regions. Eventually, the kinetics of underwater carbonation, as well as the discrepancy between dry and underwater carbonation, is revealed. • Alkali-activated slag can experience underwater carbonation when subjected to tap water immersion. • Calcite is the predominant CO 2 -bearing phase upon underwater carbonation. • Sodium hydroxide-activated slag is more vulnerable to underwater carbonation than sodium silicate-activated slag. • A representative layered distribution is formed near the exposure front, including the “carbonated layer”, “carbonated + leached layer” and “leached layer”. • Underwater carbonation of AAS subjected to tap water immersion is milder than natural carbonation.