Competing mechanisms of cement hydrates and anhydrous phases at ambient and 120 °C carbonation

Abstract

The carbonation of a fresh cement matrix involves several parallel reactions, including the hydration of anhydrous phases and the carbonation of both anhydrous phases and cement hydrates. This study aims to elucidate the competing mechanisms of anhydrous phases and cement hydrates during high-temperature carbonation. We comparatively investigate the behaviors of three representative precursors (a) fresh cement powder (as a composite system), (b) hydrated cement powder (representing cement hydrates), and (c) steel slag powder (representing anhydrous cement phases) under high-temperature (120 °C) carbonation. By differentiating the concurrent reactions occurring in the fresh cement system, the individual contribution of each material can be identified. The results show that carbonation occurs more significantly on cement hydrates than on anhydrous phases at ambient temperatures, but the trend reverses under high-temperature carbonation. Notably, dicalcium silicate (C₂S) directly reacts with CO₂ at 120 °C within the fresh cement matrix, producing calcite and a highly polymerized calcium silicate hydrate (C-S-H) gel similar to that of steel slag. This reaction not only contributes to carbonation but also facilitates hydration through its nucleation effect. In contrast, for tricalcium silicate (C₃S), hydration initiates first, followed by the carbonation of its resultant product, namely portlandite, and subsequently C-S-H.