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

Abstract

The optimal CO 2 injection timing is a critical factor in CO 2 utilization and sequestration during the mixing process of ready-mix concrete. In this study, the optimal CO 2 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 energydispersive spectroscopy (EDS) measurements were performed to explore the corresponding changes in carbonation ready-mix concrete (CRC) during the injection of CO 2 . The beneficial impacts on the compressive strength were observed, which included the refinement effect of nano-CaCO 3 formation and the enhancement effect of vanished CO 2 bubbles. The early hydration reaction was promoted by CO 2 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 CO 2 sequestration were assessed using the CO 2 index. The optimal CO 2 injection timing, which involved injecting CO 2 during the mixing process in the mixer, was determined based on the minimal CO 2 index.