Mechanisms of Air Entraining of Proteins in Cementitious Materials

While few prior studies examined the air-entraining properties of proteins in cementitious materials, the underlying mechanisms of proteins' air entraining and the interactions between proteins and cement have not been studied in the past. The significance of this article is to address this knowledge gap by investigating the effect of proteins on relevant factors that affect air entraining in cement paste and establishing an understanding of the mechanism of air entrainment with proteins. These factors include the surface tension of pore solution, protein adsorption on cement particles, cement paste hydrophobization, and flow of fresh paste. Thirteen different proteins were used to investigate the effect of a wide range of protein characteristics on air entraining. Proteins decreased the pore solution surface tension to different degrees. At low concentrations, the adsorption of proteins on cement particles slightly affected the pore solution surface tension. Protein adsorption on cement particles showed a wide range of adsorption isotherms. Proteins generally increased the flow of paste due to electrostatic repulsion between cement particles because of the adsorption of negatively charged proteins on cement particles, as well as the ball-bearing effect of bubbles in fresh paste. The surface hydrophobicity was increased in pastes with proteins. A detailed microcomputed tomography (micro-CT) analysis showed very different air void microstructures in pastes with various proteins. While a relatively strong correlation was observed between air void porosity and surface hydrophobicity, the correlation between air void porosity and the surface tension of pore solution was weak. This indicates that the accumulation of hydrophobized cement particles on the air bubble in the fresh paste, refered to as the Pickering effect, is the main mechanism of air entraining of proteins in the paste. It was shown that a high air void porosity occurs in an intermediate range of flow.