Neglected carbonation effect on moisture profiles in hardened cement paste during short-term drying

Abstract

This study investigates the influence of atmospheric CO₂ on moisture transport during drying in cementitious pastes with different microstructures, namely ordinary Portland cement (OPC), volcanic glass powder (VGP), and limestone-calcined clay cement (LC 3 ) pastes. To isolate the effect of atmospheric CO₂, the specimens were exposed either to pure drying conditions (PD, CO₂-free) or naturally carbonating drying conditions (NCD) under identical temperature and relative humidity (RH). Water content distributions along the drying direction were measured using single-sided 1 H NMR relaxometry. Compared with PD, NCD resulted in lower water contents not only within the carbonated zone but also in adjacent non‑carbonated regions, indicating that carbonation-induced microstructural changes influence water transport beyond the carbonation front. Matano analysis further revealed significant differences in the water diffusion coefficient (D(w)) between PD and NCD, particularly at low water contents. The effect was material-dependent, increasing in the order of OPC < VGP < LC 3 , with maximum deviations reaching approximately 6–10 fold. These findings demonstrate that even the limited carbonation occurring under atmospheric CO₂ concentrations can substantially influence experimentally derived water transport properties during conventional laboratory drying experiments. The results suggest that the effect of natural carbonation may have been unintentionally incorporated into previously reported water transport data and should therefore be explicitly considered in drying experiments and transport-related analyses, particularly for modern cementitious materials containing supplementary cementitious materials.