Understanding the Toughening Mechanism of Silane Coupling Agents in the Interfacial Bonding in Steel Fiber-Reinforced Cementitious Composites

Interfacial bonding between a fiber and a matrix plays an essential role in composites, especially in fiber-reinforced cementitious composites that are superior forms for bearing flexural and tension load in construction applications. Yet, despite the importance, effective and economic approaches to improve the interfacial bonding between a steel fiber and a cementitious matrix remain unfeasible. Herein, we report a pathway adopting a silane coupling agent (SCA) to modify an interfacial transition zone (ITZ) and enhance interfacial bonding. This approach involves coating a SCA layer onto a steel fiber, where tight physical and chemical bondings (via cross-linking of silicate chains) with a cementitious matrix are formed, leading to an 83.5% increase in pullout energy. Combining nanoindentation and an atomistic force microscope with molecular simulation, we find that SCA increases the surface roughness of the steel fiber, accelerates the hydration reaction of cement clinker, and promotes the volume fraction of the C-S-H phase, inducing a denser and more uniform ITZ with an adequate stress-transfer capability that shifts the mode of failure from interfacial debonding to cement cracking. This work presents an effective and economical approach to improve interfacial bonding, and it enables us to design more durable fiber-reinforced cementitious composites, which can be massively used to build innovative infrastructures.