Evaluation of carbonation conversion of recycled concrete fines using high-temperature CO2: Reaction kinetics and statistical method for parameters optimization

Abstract

Accelerated carbonation of recycled concrete fines (RCF) is an eco-friendly technology and has been considered as an effective way to achieve CO2 fixation and improve the cementitious activity of RCF. However, the slow gas-solid reaction under a standard (20 °C) environment is an issue affecting the carbonation performance. On the other hand, the surface water of alkaline solids plays a crucial role as an ionic transferring medium for high-temperature carbonation. This work, therefore, investigates the influences of operating parameters, i.e., temperature (60–180 °C), water content (W/S ratio; 0.1–0.5 ml/g) and carbonation duration (5–50 min) on the reaction of RCF. After that, multiple kinetics models, namely shrinking-core model and surface water coverage model, are presented to elucidate the carbonation kinetics of RCF under high-temperature environments. The maximum diffusivity of CO2 and carbonation rate of RCF are 1.33 × 10−6 cm2/min and 0.052 min−1, respectively at 100 °C, whereas the carbonated particles are surrounded by polyhedral calcite crystals. Kinetics study shows that the activity of calcium ions in water film decreases with higher temperature due to faster evaporation of water and the development of an electrical double layer in the interface particles. In addition, the non-evaporated surface water at high W/S ratios limits the CO2 penetration and reaction rate. Finally, the response surface methodology is employed for statistical prediction of the maximum conversion of RCF. Accordingly, the highest carbonation conversion of RCF is 56.69% for particles wetted with a W/S ratio of 0.3 ml/g and carbonated at 100 °C for 40 min.