Research on the Toughening Mechanism of Cementitious Composites Reinforced with SCA-Modified Kevlar Fibers

To mitigate the intrinsic brittleness, poor toughness, and weak fiber-matrix interfacial bonding of cementitious materials, Kevlar fibers were surface-modified with a silane coupling agent (SCA, KH550) to prepare SCA-modified Kevlar fiber-reinforced cementitious composites (SKFRCC). The effects of fiber length, ranging from 1 to 12 mm, on mechanical properties, impact resistance, and interfacial toughening mechanisms were systematically investigated. Results showed that the optimal fiber length depended on the loading mode. At a fiber content of 0.5%, KFRCC with 3 mm fibers achieved a compressive strength of 56.22 MPa, 11.61% higher than the reference group; after SCA modification, SKFRCC reached 59.38 MPa, corresponding to a 17.89% increase. The best impact resistance was obtained with 6 mm fibers, where the first-crack and failure impact energies of KFRCC reached 1398.76 J and 1907.40 J, respectively, and increased to 1494.13J and 2004.36 J after modification. For flexural behavior, 12 mm fibers showed the best performance, with ultimate flexural strength and maximum deflection increasing from 9.91 MPa and 8.04 mm to 10.20 MPa and 8.50 mm after SCA modification. FTIR, XRD, and XPS confirmed that KH550 introduced Si–O–Si and Si–O structures without damaging the intrinsic Kevlar structure. SEM/EDS further revealed Ca - and Si - rich hydration products on modified fibers, forming a denser “Fiber-SCA-C-S-H” interfacial layer. This layer enhanced physical anchoring, chemical bridging, load transfer, crack bridging, and energy dissipation.