Understanding the uneven phase distribution and multi-step reaction mechanism of carbonated γ-C2S-based foam concrete

γ-C₂S has been attracting much attention as the role of exclusive or primary binder to fabricate carbonated materials due to its high carbonation reactivity. However, the very limited hydration reactivity of γ-C₂S makes it difficult in the production of casting-formed materials such as foam concrete, and this is exacerbated by the presence of bursting-prone foams in the mixture. Given the highly cementitious property of Portland cement (PC), 10 wt% of PC was added to γ-C₂S-based foam concrete (CFC) as the supplemented binder to maintain its cellular structure while providing demoulding strength. The compressive strength of the CFC (600 kg/m³), carbonated for 2 h at ambient conditions, impressively peaks at 4.49 MPa, comparable to that of autoclaved aerated concrete with the same density grade, and is three times higher than the standard strength of foam concrete. This is partly attributed to the more uniform air-void size distribution formed by the enrichment of cement hydration products on the void-wall. Furthermore, the presence of cement hydration products positively promotes the dissolution of calcium ions from γ-C₂S, forming a mixture of calcium and silicon products. This paper aims to understand the carbonation mechanisms of composite CFC, and also provide guidance for further realizing the reaction process associated with multiple carbonatable phases.