Nanoscale toughening strategy for colloidal calcium silicate hydrate/crystalline calcium hydroxide interface in cementitious composites via molecular bridging in protrusion-induced gap

The interplay between amorphous colloid, crystalline phase, and their interface critically impacts the physics and mechanical behavior of multiphase composites. Colloidal calcium silicate hydrate (C-S-H) and crystalline calcium hydroxide (CH) are primary hydration products of cementitious composite, and the design of multiphase composites prefers C-S-H for mechanical properties. Yet, the significant performance gap between C-S-H and CH makes the C-S-H/CH interface a critical weak link affecting material behavior, and we propose a novel reinforcing strategy redistributing CH within the interface. An upscaling methodology is developed through remapping coarse-grained models to all-atom models, which overcomes barriers in realistic modeling of C-S-H/CH interface and identifying interactions between C-S-H and CH. This study reveals the impact of surface-layer configuration of CH, C-S-H roughness, and water on interfacial properties. Molecular dynamics simulations show that nanoscale protrusion of C-S-H/CH interface is primary contributor to poor toughness. Pore water bridges void and improves interfacial integrity, and embedding residual CH in interface region enhances toughness by over 525%, accompanied with shift of failure mode from interface failure to interlayer failure of CH. Such reinforcing mechanism fills protrusion-induced gap with ions or nanoparticles featuring specific binding, promoting atomic-level understanding of the interface between colloid and crystal, paving the way for in-situ interface-targeted regulation in advanced cementitious composites design.