Study of CO2 Injection Timing within the Mixing Process of Ready-Mix Concrete for Win–Win Improvements of Mechanical Properties and CO2 Sequestration

The optimal CO2 injection timing is a critical factor in CO2 utilization and sequestration during the mixing process of ready-mix concrete. In this study, the optimal CO2 injection timing in fresh cement paste was evaluated based on the compressive strength and environmental benefits. X-ray diffraction (XRD), thermal gravimetric (TG), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) measurements were performed to explore the corresponding changes in carbonation ready-mix concrete (CRC) during the injection of CO2. The beneficial impacts on the compressive strength were observed, which included the refinement effect of nano-CaCO3 formation and the enhancement effect of vanished CO2 bubbles. The early hydration reaction was promoted by CO2 injection while the long-term hydration reaction was hindered, as noted from the corresponding degree of hydration. Furthermore, the win–win improvements of mechanical properties and CO2 sequestration were assessed using the CO2 index. The optimal CO2 injection timing, which involved injecting CO2 during the mixing process in the mixer, was determined based on the minimal CO2 index.