New insights into interactions of mixed surfactant system for air entrainment in different media: from solution, suspension, paste to concrete

As a promising strategy for enhancing surface activity, mixed surfactants have a synergistic effect on air entrainment in cement-based materials, but the results are often inconsistent. In this study, the interactions of a binary system of sodium lauryl ether sulfate (AES) and lauryl dimethyl amine oxide (OA12) were investigated by measuring surface tension, contact angle, foam properties of the solution (or suspension), rheological behaviour of fresh cement paste and void parameters of hardened concrete. The air-entrained performances across solutions, suspensions, pastes and concrete were compared using radar plots. The results confirmed that the mixed surfactant system (MSS) could yield lower surface tension at the critical micelle concentration, minimum area per molecule, contact angle, and higher pc20 (=-lgc20) and surface excess than a single surfactant. The MSS achieved higher foaming capability at αOA12 = 41–62% (αOA12 is the mole fraction of OA12 in the MSS), which also led to higher air contents in the cement paste and the concrete. The AES–OA12 system led to a higher percentage of smaller air voids less than 200 μm and decreased the bubble spacing coefficients, suggesting better frost resistance. The thermodynamic parameters (standard Gibbs energy of adsorption, standard Gibbs free energy of micellisation and minimum free energy of given surface at maximum adsorption) of the MSS indicated the addition of OA12 enhanced the favourability of micellisation and a more stable surface was formed. These findings could offer a more comprehensive understanding of the interaction effects of mixed surfactants, contributing to the design of composite surfactants and concrete durability.