Enhancement in stability of air bubbles in mortar at fresh state with different SCMs

The stability of air bubbles is a crucial factor in determining the workability, strength, durability and surface quality of concrete. There is a growing interest in the regulation of air bubble stability in concrete industry. This study examines the influence of various supplementary cementing materials (SCMs, 20% by weight in replacement of cement) on the foam/air bubble stability. The time-dependent evolutions of foaming height and air bubble size distribution were tested, which reflected the trend of the influence of different SCMs on the foam/air bubble stability in solutions or mortars. The air bubble size distribution in mortar was tested using AVA (air void analyzer) from 5 minutes to 60 minutes and X-CT from 60 minutes to 180 minutes after the mixture were prepared. The results demonstrated that over time, the number of small bubbles decreased, while the number of large bubbles increased. The primary change occurred within the initial 60 minutes. The results of the wettability test were combined with those of the X-ray diffraction (XRD) analysis to identify a correlation between the stability of air bubbles and the wetting angle of the supplementary cementitious material (SCM). The wetting angle of the SCM was found to be within 90° in cases where the air bubbles were more stable. Furthermore, the XRD patterns revealed significant differences in the mineral compositions between the air bubble shells and the screened pastes from fresh mortar. The presence of more SCMs and hydration products on the air bubble shells than in the paste was identified as a potential reason for the observed differences in air bubble stability. The utilization of specific SCMs has the potential to enhance the air bubble stability in the context of concrete construction engineering, in addition to chemical admixtures.