Surface modification strategy for controlling wettability and ionic diffusion behaviors of calcium silicate hydrate

Solid substrates of cementitious composites served in high salinity and humidity environments are invariably covered by a layer of fluid water. Thus wetting behavior of cement hydrates is critical to their durability due to calcium leaching. Meanwhile, chloride ingress phenomena are highly dependent on it. These effects severely aggravate the microstructural degradation of calcium silicate hydrate (C-S-H), with mechanisms that are pronounced at molecular level. Here, we investigate here the nanoscale wetting behaviors of C-S-H and report a surface modification strategy to control its hydrophobicity. Molecular dynamic simulation results reveal that the surfactant, fluoroalkylsilane (FAS), furnishes superhydrophobic surfaces, which hinders ionic interactions and stabilizes the interlayer calcium to eliminate calcium leaching in C-S-H. Further, FAS layer provides a rougher and highly electronegative surface, blocking the chloride adsorption and invasion. This work portrays atomistic insights in C-S-H surface properties, improving the chemical and physical stability of cementitious composites in saline solutions and providing strategies for their surface modifications.