How nano-bubble water and nano-silica affect the air-voids characteristics and freeze-thaw resistance of air-entrained cementitious materials at low atmospheric pressure?

With more infrastructures being built in plateau regions, low atmospheric pressure is put forward as a new factor affecting the mechanical properties and durability of cementitious materials, but its influence is inconclusive, and the failure of freeze-thaw has not been effectively solved. This research explored the influences of both nano-bubble water (NBW) and nano-silica (NS) on airvoids characteristics and freeze-thaw resistance of cementitious materials formed and cured at different air pressure (0.4P, 0.7P, P; P=1atm); and investigated the effects and its mechanisms of low air pressure on performances of cementitious materials. The air-voids structure analyser, SEM, XRD and TG-DSC were employed to evaluate the microstructure and cement hydration of hardened cement paste samples. Results show that low-pressure samples have abnormal air-void parameters and poor bubble stability, which lead to a decrease in freeze-thaw resistance. It is proved that both NBW and NS can improve the freeze-thaw resistance and mechanical strength of samples at low atmospheric pressure by accelerating cement hydration, enhancing bubble stability, and optimizing air-voids parameters. The mechanisms of low pressure, NBW and NS on properties of cementitious materials are expounded. This research proposes new perspectives for improving the durability of air-entrained cementitious materials in plateau and enables us to further comprehend the effects and principles of low air pressure on performances of cementitious materials.