Effect of non-deformable and deformable bubbles on static yield stress of cement mortar

While extensive research has delved into the impact of single size distribution and volume fraction of bubbles on the static yield stress of cement mortar, this investigation pertains to previously underexplored variables. These encompass the ramifications of multi-size bubble distributions, variations in bubble geometries, disparities in airliquid interfacial characteristics, and surfactant concentrations. In order to scrutinize these factors, bubbles exhibiting distinct volume fractions and size distributions were deliberately induced through the admixture of varying concentrations of nonionic surfactants, while maintaining a water-to-cement ratio (w/c) of 0.42 under controlled conditions at a temperature of 20 degrees C. Experimental testing involved the analysis of bubble deformation during the yield process of cement mortar, including a surface tension tester, micro-CT, and numerical simulations. Furthermore, the study investigated the influence of nonionic surfactants on the interaction forces between cement particles, employing comprehensive analysis methods such as a total organic carbon analyzer, Zeta potential measurements, and contact angle tests. The consolidated outcomes from experimental trials and numerical simulations substantiate that augmenting the concentration of nonionic surfactants does not amplify the interparticle interaction forces within cement particles. Rather, it solely leads to an escalation in the volume fraction of bubbles. During the cement mortar yielding process, bubbles exhibit two distinct states: deformable and non-deformable, characterized by the Bingham capillary number. Notably, the research reveals that the effect of increasing the volume fraction of non-deformable bubbles on the static yield stress is more significant than the impact of these bubbles on reducing maximum packing density. This effect is attributed to the increased number of contacts between particles. Additionally, the larger and flatter surface area of deformable bubbles promotes the orderly arrangement of cement particles, thereby reducing local shear dissipation. Consequently, an increase in the volume fraction of deformable bubbles and the Bingham capillary number contributes to a reduction in the static yield stress. In consideration of these discoveries, this study introduces a qualitative approach for characterizing the static yield stress of aerated cement mortar, leveraging the volume fractions of deformable and non-deformable bubbles, in conjunction with the maximum packing density and Bingham capillary number.