Synthesis and Performance Evaluation of Anti-Washout Admixtures for Underwater Non-Dispersive Concrete Based on Nanosilica

Anti-washout admixtures (AWAs) are a unique component of underwater non-dispersive concrete (UNDC), which gives the concrete the ability to remain undispersed in water. On some special occasions, freshly mixed underwater non-dispersive concrete is exposed to the erosion of moving water, and conventional acrylamide-based AWAs are only suitable for static water or the water flow rate is small. In this study, the inorganic component nanosilica (NS) is modified, treated, and copolymerized with the organic components acrylamide (AM) and acrylic acid (AA) to form an inorganic–organic hybrid polymer with a hyperbranched structure, which changes the linear structure of the original polyacrylamide molecule, and we optimize the synthesis process. The polymers are characterized at the microscopic level and their compatibility with polycarboxylic acid water-reducing agents (SP) is investigated. In addition, the polymers are compared and evaluated with commonly used PAM in terms of their working performance. The experimental results indicated that under specific process conditions, polymers endow cement mortar with good resistance to water erosion. At the same time, the polymers’ three-dimensional network structure is prominent, with good compatibility with SP and better anti-dispersity. The microstructure of the cement paste with added polymers is dense and flat, but its flowability and setting time are slightly worse. This study provides a new development direction for the development of AWAs under a dynamic water environment, which has specific engineering significance.