Optimizing Wet Carbonation Conditions to Improve the Reactivity of Recycled Concrete Powder in Sustainable UHPC

The drive toward sustainable construction materials has encouraged the partial replacement of cement with Recycled Concrete Powder (RCP) in Ultra-High-Performance Concrete (UHPC). This study presents a preliminary evaluation of UHPC incorporating 10%, 20%, and 30% RCP by mass as a cement substitute. Compressive strength tests revealed a ∼40% reduction at 10% RCP, ∼60% at 20%, and over 75% at 30%, relative to the RCP0 control mix. To enhance the reactivity of RCP and offset these performance losses, a wet carbonation treatment was conducted using magnesium chloride (MgCl2) solutions at 0.1, 0.2, 0.4, and 0.8 M for durations of 30 minutes, 1.5 hours, 3 hours, and 5 hours. The heat of hydration of a UHPC mix with 10% wet-carbonated RCP was measured via isothermal calorimetry. Results showed that shorter carbonation conditions (e.g., 0.4 M MgCl2 for 30 min) increased cumulative heat release by 16% compared to untreated RCP, indicating improved pozzolanic reactivity. Conversely, higher concentrations and longer durations resulted in a decrease of up to 5%, suggesting inhibitory effects from salt deposition or pore blocking. These findings support the feasibility of optimizing carbonation conditions to improve the performance of RCP in UHPC, enabling more sustainable and durable concrete mix designs.