A hybrid carbonation system for enhancing recycled aggregate performance through particle size and spatial distribution

Recycled aggregates (RA) are vital for sustainable construction but have poor mechanical properties. This study investigates carbonation treatment by exposing RA of three size ranges (0.6–5.0 mm, 5.0–16.0 mm, 16.0–31.5 mm) to a 20 % CO₂ unidirectional flow. Smaller particles rapidly absorb CO₂ (99.6 % in 3 h) but show decline over time, while larger particles absorb more slowly yet sustain carbonation, with mass gain increasing by 329 % over 7 days. Upstream sections capture CO₂ quickly, while downstream sections utilize residual CO₂ for prolonged carbonation. Strength was highest near the CO₂ inlet (+19 %), decreasing downstream due to reduced CaCO₃ formation. All mixes showed brittle failure, with consistent ductility (0.9–1.3 mm). A hybrid RA system—small particles upstream and large downstream—may improve long-term CO₂ sequestration and mechanical performance, supporting low-carbon concrete applications.