Accelerated carbonation fronts in cement pastes: Mechanistic insights and simplified modeling

This study investigated the carbonation behavior of ordinary Portland cement (OPC), volcanic glass powder (VGP), and limestone-calcined clay cement (LC3) pastes, focusing on the water content distribution, phase assemblages, and microstructural evolution. Spatiotemporal changes in water and mineral phases were tracked using proton nuclear magnetic resonance relaxometry and micro-X-ray diffraction, respectively. A simplified model reproduced the drying front, defined by a material-specific threshold water content, which aligned closely with the experimentally measured carbonation front. This confirmed that carbonation progression is governed by water diffusion, referred to as the “plugging effect”. Furthermore, carbonation advanced until empty space increased to approximately 18%, resulting from the decalcification-induced agglomeration of calcium (alumino)silicate hydrate (C-(A)-S-H). This limit was consistently observed across pastes with varying Ca/(Si+Al) ratios. These findings provide mechanistic insight into carbonation front development and offer a physically grounded criterion for predicting carbonation depth and assessing CO₂ uptake in sustainable cementitious materials.