Insights into early-age microstructure evolution of toughness-enhanced cementitious composites by acrylamide in-situ polymerization

The in-situ polymerization of polymers in cement systems significantly affects early hydration and workability, yet its underlying mechanisms were still insufficiently clarified. In this study, the cement-based composites containing different dosages of acrylamide (AM) were prepared to systematically investigate the effects of in-situ polymerization on rheological behavior, setting characteristics, and the morphology and microstructural evolution of hydration products. The results demonstrated that increasing AM dosage markedly reduced fluidity and increased yield stress due to the rapid formation of polyacrylamide (PAM) networks under alkaline conditions. Compared with the control sample, incorporating 5 % AM reduced the fluidity by 49.4 % and increased the yield stress by 140 %. The in-situ polymerized AM inhibited silicate hydration through adsorption and encapsulation effects, extending the initial setting time to over 350 min. Multi-scale characterizations revealed that AM strongly regulates cement hydration and phase evolution. The in-situ polymerized AM adsorbs on clinker surfaces and forms thin films that retard silicate dissolution and delay CH and C–S–H formation. At low dosages, AM promotes a denser and more uniform C–S–H structure through polymer–gel integration, while excessive AM hinders ion migration and markedly suppresses hydration. TEM observations show that the in-situ polymerization of AM induces a transformation of C–S–H from amorphous gel to a locally ordered structure. These findings elucidate the dual regulatory role of AM in cement hydration and microstructural evolution, offering a theoretical basis for optimizing polymer dosage and designing high-toughness, in-situ polymerized cementitious materials. • Revealed the dual regulatory role of acrylamide in-situ polymerization on cement hydration. • AM polymerization forms PAM networks to enhance structural toughness. • Elucidated molecular mechanisms of hydration inhibition and polymer-gel integration. • Provided a multi-scale insight into microstructural evolution and pore refinement behavior.