Mechanisms of in-situ polymerization for enhancing washout resistance of cement paste

Conventional concrete is generally unsuitable for underwater construction, typically requiring the addition of anti-washout admixtures (AWAs) to improve its washout resistance. Herein, we demonstrate the enhancement of cement paste washout resistance through the in-situ polymerization of acrylamide (AM) and sodium acrylate (SA) and elucidate the underlying mechanisms. Macroscopic experiments reveal a significant improvement, with washout loss reduced to 12 % and 2 % of that observed in REF at 60 min for cement pastes modified by the in-situ polymerization of AM and SA, respectively. This enhancement is attributed to the formation of a more flocculated microstructure, where smaller flocs agglomerate into larger ones due to increased floc strength induced by the bridging effect of the resultant polymers. Consequently, flocs in cement pastes with in-situ polymerized SA exhibit higher strength and a denser structure, with a fractal dimension ($D_f$) exceeding 2.00, shifting the floc break mode from surface erosion to large-scale fragmentation and thereby improving washout resistance. Nevertheless, the in-situ polymerization of both AM and SA retards cement hydration, albeit through distinct mechanisms: the non-adsorbing PAM molecules primarily hinder the nucleation and formation of hydration products, whereas the adsorbed PAAS molecules predominantly inhibit the dissolution of aqueous species.