Concrete is the most widely material used in the construction industry. However, it has a great environmental impact, mainly because of cement manufacture and the inherent CO2 emissions into the atmosphere. This research intends to contribute to the reduction of the environmental impact of the concrete industry through the uptake of CO2 during the concrete production phase. The few research works on this issue report contradictory results regarding the impact of CO2 when added during the mixing phase. These results report an acceleration of the reactions [1], a reduction of the time setting [2] and either an increase or a decrease in the amount of hydration products, depending on the CO2 amount [3, 1]. Thus, this research aims at understanding the impact of CO2 amount on the hydration reactions of cement in order to enable its adoption as a component mixture. To achieve this purpose, 4 cement pastes were produced with different amounts of CO2. The pH of the mixture was measured to evaluate the impact of CO2 on its alkalinity reduction. Compressive strength, XRD and SEM analysis were also performed to assess the influence on the hydration reactions and also on the degree of carbonation. Results suggest that CO2 can increase the mechanical resistance at early ages if used in a little amount. However, the reduction in pH due to CO2 appears to compromise this performance at longer ages.