Resolving rheological dilemma in non-dispersible underwater concrete: Conflict between fluidity and anti-washout

The washout resistance of non-dispersible underwater concrete (NWC) is typically improved at the cost of a significant loss of flowability, which poses challenges in construction processes such as pumping and placing. Herein, we demonstrate a novel strategy to balance washout resistance and fluidity of UWC via in-situ copolymerization of sodium acrylate (SA) and acrylamide (AM), where the copolymers formed at different monomer ratios exhibit different roles due to their inherent properties. Results reveal that, before microstructural `disruption, copolymers with higher charge density preferentially adsorb onto cement particle surfaces, reinforcing the flocculated structure through the bridging effect. After predominant microstructural breakdown, copolymers with longer chain thicken the interstitial solution through chain association and entanglement, potentially forming a dynamic network with cement particles and reducing flowability. The dynamic yield stress of S7A3 remains below 150 Pa at 30 min, indicating favorable flowability during placement. In contrast, its static yield stress exceeds 600 Pa at 60 min, corresponding to a washout loss of only ∼1 %, thereby demonstrating a well-balanced performance between high fluidity and markedly enhanced washout resistance. Our work proposes a novel strategy for designing UWC tailored to practical application scenarios with a balance between washout resistance and fluidity and sheds light on the understanding of mechanisms underlying the rheological manipulation via in-situ copolymerization.