Insights into the synergistic action of initial hydration and subsequent carbonation of Portland cement

Pretreatment-induced initial hydration would significantly influence subsequent carbonation. However, the evolution of microstructure and performance resulting from the synergistic action of hydration and carbonation remains systematically unexplored. This study investigates carbonation kinetics, microstructure and micro/ macro mechanical properties of carbonated cement pastes (CCPs) under the synergistic action of initial hydration and subsequent carbonation, while elucidating the underlying mechanisms. The results revealed that unhydrated cement exhibited a peak carbonation rate of 0.65 W/g, increasing by approximately 83 % when the cement underwent an 8 h of initial curing, demonstrating the enhancement in the carbonation reactivity due to initial hydration. However, the carbonation efficiency of CCPs increased initially and then decreased as initial hydration extended. This trend emerged because initial hydration enhanced carbonation reactivity, whereas excessive hydration concurrently obstructed CO2 transport. Furthermore, optimal initial hydration was essential for the synergistic interaction between hydration and carbonation, resulting in reduced porosity and a more homogeneous microstructure, as well as improved mechanical properties. These findings underscore the need to carefully consider the synergistic action of initial hydration and subsequent carbonation when designing pretreatment protocols.