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%.