Unraveling nanoscale chemo-mechanical interlocking in organic-inorganic sol-gel functionalized steel fiber toward ductile cementitious composite

Low ductility of concrete makes it vulnerable to catastrophic failure via uncontrolled crack propagation. Steel fiber can suppress crack through bridging effect, while the chemically inert surface weakens interfacial bonding and limits energy dissipation. This study develops an organic-inorganic sol-gel coating to functionalize fiber, aiming to strengthen interfacial bonding with cementitious matrix and unlock toughening capability. The bond strength with cementitious matrix increased by 97.7%, enabling one dominant localized crack with secondary cracking of cementitious composite under tensile loading. Microscale characterization and molecular dynamics simulation reveal that modified fiber features nanoscale protrusions, enhancing surface roughness and providing additional nucleation sites for SiOCa coordination interactions with matrix. Moreover, the hydrophilic coating promotes dense calcium silicate hydrate growth. Combined mechanical interlocking, chemical bridging, and interfacial microstructural densification facilitate robust bonding for stress transfer. This study presents a chemical coating to elevate energy dissipation, offering a novel and effective interfacial functionalization tactic for tough and durable construction material.