Hydration kinetics and modified model for effective water to cement ratio control in recycled aggregate concrete

The rapid urbanization surge has intensified the dual challenges of natural aggregate depletion and construction waste accumulation, necessitating sustainable solutions like recycled aggregate concrete (RAC). However, the higher water absorption of recycled aggregate (RA) disrupts the effective water-to-cement (w/c) ratio, critically impacting RAC performance. While prior research focused on RA water absorption ratio or empirical w/c ratio correlations of RAC, the interplay between RA moisture dynamics and hydration kinetics remained unexplored. This study bridges this gap by systematically investigating how RA initial moisture degree (D_im) and additional water ratio (R_aw) influence hydration behavior through isothermal calorimetry. A modified Krstulovic-Dabic hydration kinetics model, incorporating D_im and R_aw dependent correction coefficients (p_i, q_i), was developed to elucidate hydration mechanisms. Key findings reveal that RA with D_im ≤ 0.5 reduced cumulative hydration heat by up to 11.8 % due to water absorption, while D_im ≥ 0.75 enhanced heat release by up to 14.3 % via internal curing. The duration of the interactions at phase boundaries process gradually shortened and even disappeared with the increase of R_aw under low D_im (≤ 0.5). A critical D_im threshold of 0.65 balanced water transport equilibrium, aligning effective and nominal w/c ratios. By linking RA moisture states to hydration kinetics, this study provides a framework for optimizing RAC mix designs with controllable effective w/c ratio, advancing sustainable construction practices.