UTILIZING CARBONATED WASTEWATER FROM WET CARBONATION OF RECYCLED CONCRETE POWDER IN CEMENTITIOUS SYSTEMS

The studies assessed whether aging waste carbonated water (WCW) from wet carbonation of recycled concrete powder (RCP) affects cement hydration and performance. WCW was produced using MgCl2 solutions of 0.1, 0.2, 0.4, and 0.8 M, with carbonation times of 30, 90, 180, and 300 minutes. It was reused as mixing water immediately after carbonation (t₀) or after four months of storage (t₄ₘ), while Portland cement was partially replaced with 10% non-carbonated RCP. Isothermal calorimetry (≥72 h) measured induction, peak behavior, and total heat release. Heatmap analysis revealed that aging reversed the response surface: the cumulative heat optimum shifted from 0.1 M–300 min (12,500) at t₀ to 0.4 M–300 min (12,999) at t₄ₘ, with increases at 0.4–0.8 M (+1,399 at 0.4 M–300 min) and consistent decreases at 0.2 M (to −992 at 300 min). Therefore, aged WCW enhanced mixtures with higher Mg and longer carbonation times but reduced hydration at lower Mg levels. These trends were interpreted as storage-driven changes in ion species, including precipitation of Mg-bearing phases, CO₂ re-equilibration, and altered chloride activity. Future research will evaluate the practical implications by creating UHPC mixes with 10–30% RCP using aged WCW for mechanical properties, supporting a circular-water strategy that lowers freshwater demand while maintaining performance.