Early-age hydration of cement paste prepared with CO2 mixing under varying pressures

This paper presents an innovative approach to enhancing the early-age performance of cement paste through controlled CO2 injection during mixing at varying pressures. By employing isothermal calorimetry, inductively coupled plasma-optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), thermogravimetric analysis (TG), Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (1H NMR), and mercury intrusion porosimetry (MIP), this study unveils the intricate mechanisms underpinning hydration and microstructural evolution. The findings reveal that CO2 injection accelerates clinker dissolution with Ca2+ being consumed to form CaCO3. Elevated CO2 pressures enhance AFt formation and delay the end of pre-induction period of hydration heat flow. However, carbonation reduces CH content, delays the acceleration stage, and diminishes the second exothermic peak. From the deceleration period onward, nano-carbonates promote C-S-H gel growth, increasing cumulative heat release and improving compressive strength. Optimal performance is achieved at CO2 pressures between 0.5 MPa and 1.0 MPa, while higher pressures reduce strength due to excessive carbonation. These findings elucidate the pressure-dependent effects of CO2 injection on hydration dynamics, microstructural evolution, and early-age strength, offering valuable insights for optimizing CO2-enhanced cement formulations to improve performance.