Investigation of Physico-Chemical Aspects of Air Entrainment in Cementitious Systems

Abstract

A method for measuring air entrainment in cement pastes was developed and validated against mortars and concrete air entraining protocols with commercially available air-entraining admixtures (AEA). The paste method was found to adequately reflect variations in air content as a function of AEA dosage observed in mortars and concrete; it thus provides a useful means for extensive studies of the relative performance of AEA's, or of the air entrainment mechanism using a series of reference surfactants. The paste method was employed to investigate air entrainment by several homologous surfactants in the series: n-alkylcarboxylates (n-RCOO-); n-alkysulfonates (n-RSO3-); and n-alkylsulfates (n-RSO4-). Sodium salts of the latter, with hydrocarbon chain lengths between C7 and C12 were used to investigate the influence of anionic head group and hydrocarbon chain length on the performance of these surfactants as AEA. Parallel measurements were also performed on the solubility of these surfactants in various systems, namely lime solutions, dilute cement slurries, cement pastes and solution pre-extracted from cement slurries. The study included three different cements. The combined results provide a basis for understanding the mode of action of surfactants in terms of a balance between the inherent surface activity (ability to reduce the surface tension of water) and the solubility of the surfactant homologs in alkaline solutions containing Ca ions.