Functionality‐Tailored Silane Grafting Unlocks Strength–Ductility Synergy and Long‐Term Corrosion Barrier in Alkali‐Activated Geopolymer‐Reinforced Epoxy/Polyurethane Adhesives

ABSTRACT Simultaneously strengthening and toughening epoxy/polyurethane (EPU) structural adhesives while ensuring long‐term corrosion resistance remains challenging. Here, we develop a functionality‐tailored silane modification strategy that reinforces EPU with alkali‐activated metakaolin‐based geopolymer (GP). Amino‐functional KH550 and epoxy‐functional KH560 are grafted onto the GP framework via Si–O–M (M = Si, Al) bonds, while –NH₂ or epoxy groups further react with the EPU network, constructing chemically bonded organic–inorganic interfacial layers. At an optimal GP loading of 0.75 wt%, the KH560‐GP/EPU composite exhibits a ~21.6% increase in tensile strength, a 64.2% increase in elongation at break, and a lap‐shear strength of 13.5 MPa, achieving a favorable strength–ductility synergy. Flexural and compressive tests, together with scanning electron microscopy (SEM)/energy‐dispersive spectroscopy (EDS), reveal that silane modification—especially KH560—suppresses GP agglomeration and forms a continuous, compact interfacial transition zone, markedly enhancing flexural load‐bearing capacity with minimal impact on compressive strength. TGA/DSC and contact angle measurements further demonstrate higher thermal stability and pronounced surface hydrophobicity (water contact angle up to ~118°). Electrochemical impedance spectroscopy confirms superior long‐term barrier performance, with |Z|(0.01 Hz) remaining as high as 6.0 × 10 8 Ω·cm 2 after 14 days in 3.5 wt% NaCl. These results establish silane‐modified GP as a powerful platform for designing high‐performance EPU‐based structural adhesives and protective coatings.