Unlocking the hydration-carbonation mechanism and Nano-CaCO₃ crystallization in cement paste governed by CO₂ nanobubble water

Nanomaterials like nano-CaCO₃ can accelerate cement hydration by nucleation effects and increase C–S–H gel formation, thereby enhancing concrete strength. However, the complex preparation process and high cost of nanomaterials, coupled with their difficult dispersion in concrete, restrict their wider application. This work proposed a novel pathway by incorporating CO₂ nanobubble water into concrete to facilitate the in-situ formation of nano-CaCO₃ which acted as nucleation sites to promote cement hydration. The hydration and carbonation coupled mechanisms were elaborated. CO₂ nanobubbles in cement paste produced nano-sized CaCO₃ (<100 nm) in situ, increasing the amount of total CaCO₃ by 36.7–62.2 %, crystalline CaCO₃ by 52.4–95.2 %, calcium hydroxide by 25.9–37.0 %, and non-evaporable water content by 33.8–47.5 %, compared to the control group. The paste mixed with CO₂ nanobubble water showed a CO₂ sequestration capacity of 24 g CO₂/kg cement paste. The nano-CaCO₃ particles induced by CO₂ nanobubbles were efficient as nucleating agents for calcium silicate hydrate (C–S–H), thereby increasing the 28 d compressive strength of cement pastes by 8.2–20.8 %. The CO₂ nanobubble technology provides a novel carbon sequestration pathway for construction industry, while enhancing the mechanical strength of cement-based materials.