Enhancement of Mechanical Performance of Cement Pastes Prepared with Concrete Reclaimed Water Using CO₂ Intermixing

This study investigates the effects of CO₂ intermixing on the fresh and hardened properties of cement pastes made with concrete reclaimed water. The reclaimed water, sourced from a construction site and stored for 30 days, was used to completely replace potable water in the cement paste preparation. The mixing process involved introducing CO₂ in varying amounts (0–1.2% of total cement weight) into cement pastes made with reclaimed water. Five cement paste formulations were studied: a control sample using potable water and four samples using reclaimed water with different CO₂ doses. The fresh properties of the cement pastes were evaluated through flowability and rheological tests, while the hardened properties were assessed by compressive strength and hydration heat measurements. The results showed that using reclaimed water slightly reduced the flowability of cement pastes, with a notable increase in rheological properties. Higher CO₂ doses intensified this effect due to the accelerated hydration kinetics from carbonation reactions that formed CaCO₃. Hydration heat patterns were similar between samples made with potable and reclaimed water, with 0.3% and 0.6% CO₂ significantly increasing hydration heat, while 1.2% CO₂ reduced it. The compressive strength of samples with 0.3% and 0.6% CO₂ was 10% higher than the control at 28 days. However, the 1.2% CO₂ dose caused a 15% decrease in compressive strength, likely due to hydration inhibition. These findings suggest that CO₂ intermixing can enhance the mechanical properties of cement pastes made with reclaimed water, with optimal CO₂ doses around 0.6%.