Molecular Structural Characterization and Bond-Breakage Investigation of Calcium Silicate Hydrate Gel Intercalated by Ionized Surfactants Subjected to Uniaxial Tensile Deformation

The wide application of surfactants in cement-based materials requires the understanding of the influence of surfactants on the structure and properties of C–S–H gel. In this study, cation surfactants (CTAB) and anion surfactants (SDS) in ionized type are successfully intercalated into C–S–H gel, generating different molecular models through molecular dynamics simulation. Systematic analysis and quantitative description were performed on the molecular structures, bonding properties together with deformation and fracture mechanisms. It was found that the majority of ${\mathrm{Br}}^{–}$ ionized from CTAB could interact with Ca_interl, forming ${\mathrm{Br}}^{-}-\mathrm{Ca}\text{\_}\text{interl}$ bonds. Smaller ${\mathrm{Na}}^{+}$ ionized from SDS could bond with both O_bri and O_nbri atoms, while ${\mathrm{O}}^{–}$ and O atoms in SDS anions could interact with Ca_interl atoms as well. It is noteworthy that the intercalation of CTAB into C–S–H gel promotes the diffusion of water molecules, whereas the intercalation of SDS into C–S–H gel inhibits the diffusion of water molecules. Uniaxial tensile simulation results reveal that the breakage of both H_water–O_water hydrogen bonds and Ca_interl–O_water bonds dominates the failure process of CTAB/C–S–H gel, while the breakage of H_water–O_water hydrogen bonds governs the fracture of SDS/C–S–H gel.