Mechanisms of air bubble rise in cement suspensions studied by X-ray analysis

The de-airing of fresh concrete is a crucial step in ensuring sustainable and durable concrete structures. Currently, proper de-airing is ensured by adjusting the fresh concrete consistency i.e. its rheological properties, without any knowledge whether this will actually guarantee sufficient de-airing. The exact relationship between the rheological properties and air bubble dynamics in concrete is not yet understood. In this paper, a detailed study was carried out in which air bubbles of different size and volume were injected in fresh cement suspensions with varying solid volume fractions (i.e. varying rheological behaviour). The air bubbles were visualised by means of X-ray radiography and the rise behaviour was evaluated with image based algorithms. It was found that in more viscous suspensions (i.e. higher particle contents) the bubble speed increases with decreasing suspension viscosity. For suspensions with higher flowability (i.e. less particle content), the inertial forces dominate the viscous forces, with the viscosity playing a subordinate role. The shear rate induced by an ascending air bubble is sufficient to locally reduce the dynamic viscosity of the suspension. In addition, it leads to shear-induced particle migration, with less concentrated regions in the ascending channel. Consequently, subsequent bubbles are influenced by changes of viscosity in this channel, rising faster and having different bubble shapes. • X-ray transmission is a tool for investigating the air bubble rise in cement paste. • The bubble speed is influenced by the bubble volume and the viscosity of the paste. • Subsequent bubbles follow a trajectory, rise faster, and with increased aspect ratio. • Bubble rise leads to particle migration and channeling effects in the rise path. • Particle migration results in less concentrated regions in the rise path.