Molecular dynamics study of the inhibition effect of the hexanoate hydrophobic agent on the transport of the NaCl solution in the pores of C-S-H: A case study of tobermorite

The inherent porosity of cement-based materials renders them susceptible to the ingress of aggressive ions, which leads to the deterioration of engineering structures. Hydrophobic modification of the matrix is regarded as a crucial technique to address permeability issues, and carboxylates have been proven to act as novel and efficient matrix hydrophobizing agents. This study employed molecular dynamics simulations to investigate the inhibitory effect of sodium hexanoate on water molecules, Na + and Cl - ions transport in C-S-H nanopores. The simulation results revealed that sodium hexanoate molecules are adsorbed on the pore walls primarily through strong chemical coordination between their carboxyl groups and calcium ions on the C-S-H surface. While this adsorption can form a hydrophobic surface to delay water ingress, sodium hexanoate molecules tend to desorb easily. At a low coverage of 0.8 nm −2 , the accumulation of sodium hexanoate after minor desorption is insufficient to completely block water transport, resulting in the inability of the C-S-H surface with such low coverage to stably inhibit water molecule diffusion. In contrast, at high coverages (1.6 nm −2 and 2.4 nm −2 ), although sodium hexanoate still desorbs, the large quantity of desorbed sodium hexanoate that accumulates at the front of the transported liquid surface forms a U-shaped distribution together with the sodium hexanoate adsorbed on the pore walls. This distribution exerts resistance on the liquid surface, thereby stably preventing the transport of corrosive solutions. This finding provides important microscopic insights and a theoretical basis for the development of novel surface modifiers to enhance the durability of cement-based materials.