Effect of nano-SiO2 hydrosol on size distribution, coalescence and collapse of entrained air bubbles in fresh cement mortar

Abstract

In the fresh state of cement-based composite materials, entrained air bubbles change dynamically due to the continuous coalesce and collapse caused by physical and chemical factors. These processes affect the pore structure formed in the later stage. However, only few studies emphasized the process of bubble size refinement and pore evolution. In this paper, attempts have been made to optimize the size distribution of entrained air bubble and prevent the coalescence of fine entrained air bubbles in the fresh air-entraining cement mortar contained nano-SiO2 hydrosol (NSH). Two different kinds of commercial air entraining agents (AEAs) were utilized. The fluidity and initial air content of each mortar set were controlled during the test to prevent the influence on the behavior of entrained bubbles. An Air Void Analyzer (AVA) was used to determine the air size distribution of mortar. The change of fine bubble distribution within 60 min was analyzed to evaluate the degree of coalescence of fine bubbles. By involving a novel method to prepare bubble shell samples, the mechanism of the influence of NSH on the size distribution and trends of entrained air bubble over time was analyzed via micro measurements including X-Ray Diffraction (XRD), Energy-Dispersive X-ray Spectroscopy (EDS), and Scanning Electron Microscope (SEM). Results suggest that the proportion of fine bubbles less than 300 μm in mortar prepared with non-ionic air entraining agent increases with adding NSH, which effectively fined the size distribution of bubbles, while the trend of mortar prepared with anionic air entraining agent is not obvious. The coalescence degree of fine bubbles is effectively inhibited by adding nano-SiO2 into air-entraining mortar, and the inhibition effect decreases with the increase of the content of nano-SiO2. XRD, EDS, and SEM data analysis proved the adsorption of nano-SiO2 on the bubble shell entrained by AEAs. The composition of air bubble shell has a great influence on the adsorption and effect of nano-SiO2. Nano-SiO2 adsorbed on the surface of bubble shell can enhancing mechanical strength of bubble shell and accelerating the hydration of tricalcium silicate at the same time, thus affecting the behavior of the bubble. At last, the method of optimizing air bubble distribution by incorporating nano-SiO2 is suggested for air-entraining cement-based composites mixed with non-ionic air entraining agent.