Using modified nano-silica to prevent bubble Ostwald ripening under low atmospheric pressure: From liquid foam to air-entrained cement mortar

Air bubble instability leads to the deterioration of air-void structure and threatens the frost resistance of airentrained concrete, especially in plateau low atmospheric pressure environment. In this study, modified partially hydrophobic nano-silica (NS) particles are used to stabilize entrained air bubbles in cement mortar under low atmospheric pressure. The experimental results indicate that low atmospheric pressure has a limited effect on the bubble size distribution during the bubble generation process. However, the accelerated Ostwald ripening of the bubble system under low atmospheric pressure leads to a significant increase in the mean bubble size and air content loss. Furthermore, the modified NS shows a noticeable bubble stabilization effect. When adding 3% modified nano-silica (NS/MPS), the air-void structure of cement mortar does not show deterioration under 60 kPa. The analysis of bubble shells in foam, fresh and hardened paste system revealed that the NS/MPS with suitable wettability could strongly adsorb at the air-water interface and form adsorption layers around bubbles, thus reduce the gas permeability across the bubble films and prevent the Ostwald ripening of bubbles. This work finally provides a viable method to stabilize air bubbles and improve the air-void structure of concrete in some extreme environments.