Multiple ions transport and interaction in calcium silicate hydrate gel nanopores: Effects of saturation and tortuosity

Abstract

The durability of cement-based materials is significantly influenced by the transport ability of multiple ions in gel nanopores of calcium silicate hydrate (C-S-H) that is directly associated with the degree of water saturation. In this study, the mechanism of ions transport in unsaturated C-S-H gel pores was revealed by the molecular dynamics simulation at the nanometre scale. The ions transport behaviour in gel pores was investigated via the establishment of different types of simulation models, which had different initial saturations, pore sizes and bending numbers of gel pores. The thickness of the electrical double layer was determined by the analysis of the distribution of ions, and the ions transport in C-S-H gel pores was affected by the electrical double layer. The percolation effect would appear in gel pores during the simulation from the dry state to saturated state, and it could promote the transport ability of ions significantly. However, it affected the extreme value of ion diffusion coefficient rather than the maximum value. Another result is that the bending numbers of gel pores would hurt the transport of ions. It is noted that the incorporation of Ca2+ will weaken the diffusion capacity of ions in the NaCl solution, while the reverse phenomenon is observed when K+ is incorporated.