Internal carbonation-hydration mechanisms of CO₂ foamed cement pastes

CO₂ foamed cement (CFC) offers promising potential for low-carbon construction, yet the internal carbonation–hydration mechanisms remain poorly understood. This study aims to clarify the internal carbonation-hydration mechanisms induced by CO₂ foam within the CFC system. A series of hydroxypropyl methylcellulose (HPMC) stabilized CFC pastes with varying densities was prepared. In-situ formation of CaCO₃ adhering to the foam bubble walls confirmed the occurrence of internal carbonation. A four-stage evolution framework was proposed based on time-dependent experimental observations and molecular dynamics simulations: Pre-stage—CO₂ dissolution and initial CaCO₃ generation during mixing; Stage I—kinetically constrained carbonation during the early hydration induction period, where Ca²⁺ chelation and limited gas transport, and HPMC-modified foam films slowed down the carbonation; Stage II—accelerated carbonation driven by Ca²⁺ accumulation from the acceleration period of cement hydration, and feedback-enhanced C-S-H growth; and Stage III—diffusion-controlled plateau, as CO₂ became depleted and ionic mobility decreased.