Elucidating factors on carbonation behaviour of hardened cement fines in dripping technique

This study investigates the carbonation behavior of hardened cement fines (HCF) using a dripping technique, a low-energy acceleration method designed to boost the carbonation of waste concrete under atmospheric CO 2 concentrations. The dripping technique subjects HCF to periodic water supply followed by drying phases, creating wet-dry cycle conditions that promote dissolution of calcium-bearing hydrates and advective ion transport. In this study, HCF with water-to-cement (w/c) ratios of 0.5 and 0.6 were exposed to wet–dry cycles at 40%, 60%, and 85% relative humidity (RH) and compared with conventional semi-dry carbonation at 60% RH. The results demonstrate that the dripping technique significantly improves carbonation efficiency, particularly at 40% RH with a 24-hour dripping interval, where the degree of carbonation (DoC) exceeded 60% within 28 days, compared to approximately 45% for the semi-dry 60% RH control. Low RH environment established a capillary pump mechanism via an evaporation-induced wicking effect, which promoted internal carbonation by continuously drawing ions to the surface. In contrast, high RH conditions promoted the formation of a thick, dense CaCO 3 surface layer, leading to reaction stagnation. Kinetic analysis using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model revealed that the dripping process at 40% RH shifts the reaction from diffusion-controlled to phase-boundary-controlled kinetics, effectively neutralizing transport resistance. The dripping solution promoted dissolution–reprecipitation into stable calcite and generated microcracking and pore coarsening, preserving a porous interior. These findings provide a mechanistic basis for adopting low-energy, dripping-based carbonation as a practical industrial method for recycling concrete fines.