Effect of air and CO ₂ micro/nano bubbles on the fresh and hardened properties of cement paste

Micro/nano bubble water (MNBW) can introduce additional gas-liquid interfaces and modify early-age cement hydration. The effect of air- and CO2-containing MNBW on fresh and hardened cement paste were characterized by rheometry, compressive strength, nitrogen adsorption and mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) in this work. Results show that air MNBW reduces yield stress and plastic viscosity by 23.6% and 32.1%, versus 4.9% and 24.7% for CO2 MNBW. This difference suggests that CO2-related changes partially offset the bubble-induced lubrication and dispersion effects. Both MNBWs enhance compressive strength, with CO2 MNBW showing larger gains of 8.5% at 24 h and 18.4% at 72 h, coinciding with the lowest cumulative pore volume (∼0.136 mL/g at 72 h). Air MNBW reduces porosity mainly via pore size redistribution, while CO2 MNBW more effectively refines pore connectivity. XRD and TGA results reveal limited phase changes across samples. Overall, both MNBWs combine physical and chemical effects differently, and CO2 MNBW shows potential for further improving early-age performance.