A comprehensive study of calcium dissolution kinetics from recycled concrete fines: Experimental and modeling insights

Abstract

This study quantifies calcium dissolution kinetics from Recycled Concrete Fines (RCFs), a key step in the indirect CO₂ mineral carbonation. RCFs were characterized prior to leaching using XRD, SEM, XRF, BET, and laser diffraction techniques. Batch leaching experiments were performed using a Design of Experiments (DoE) approach to investigate the effects of particle size (0–4 mm), initial pH (2−7), and solid-to-liquid ratio (25–150 g/L) on calcium release kinetics. Calcium dissolution rate increased with decreasing particle size and pH, and with increasing solid-to-liquid ratio. XRD and SEM analyses revealed selective leaching of cementitious phases (portlandite, ettringite, hemicarboaluminate, and amorphous C-S-H), while inert aggregates remained unaffected. A mechanistic core-shell dissolution model was developed, representing RCFs particles as inert cores surrounded by reactive shells composed of hydrated calcium-rich cement phases. The model uses a single apparent rate constant (k_obs) to integrate surface reaction, internal diffusion, and external mass transfer. The reactive surface area was dynamically updated using BET data and the evolving mass of cementitious phases. The model closely reproduced experimental data across all tested conditions (R² > 0.93). The variation of k_obs across operating conditions reflects transitions between different rate-limiting steps, ranging from external mass transfer to surface reaction control.