Natural carbonation-induced phase and molecular evolution of alkali-activated slag: Effect of activator composition and curing temperature

In this work, the evolution of reacted phase assemblage and molecular structure of alkali-activated slag (AAS) exposed to atmospheric natural carbonation is studied. The hardened AAS pastes with various activator types (i.e., sodium hydroxide, sodium carbonate, sodium sulfate, potassium hydroxide, and potassium carbonate solutions) and cured under two temperature conditions (20 degrees C and 80 degrees C) are evaluated. The results show that natural carbonation leads to a strong decalcification and silicate polymerization of calcium-aluminosilicate-hydrate (C-A-S-H) and precipitation of various calcium carbonate polymorphs depending on activator composition. The sulfate-activated slag shows the poorest carbonation resistance likely due to its limited CO2 absorption capacity, manifested with the formation of calcium carbonate polymorphs, gypsum, gibbsite, and calcium-deficit aluminosilicate gels. While high-temperature curing tends to increase the crystallinity level of C-A-S-H, it has little effect on its carbonation resistance in terms of phase and molecular stability. (C) 2020 Elsevier Ltd. All rights reserved.