Atomic-level investigation into the transport of NaCl solution in porous cement paste: The effects of pore size and temperature

The durability of concrete is determined by the ion transport within porous calcium silicate hydrate (C–S–H) gels. This study utilized molecular dynamics simulation to examine the impact of pore size (from 35 Å to 95 Å) and temperature (from 275 K to 350 K) on the transport of NaCl solution in Tobermorite nanochannels, a representative model of C–S–H. The findings suggested that the transport rate is positively correlated with both pore size and temperature. In smaller pores, a filtration effect on the ions in NaCl solution was noted. As the pore size expanded, this effect gradually diminished but became more pronounced with an increase in temperature. Following the penetration of NaCl solution, a double ionic layer formed on the C–S–H surfaces due to ionic adsorption. The ionic adsorption was enhanced by larger pore sizes and higher temperatures. Furthermore, the increase in pore size and temperature expedited the dissociation of Ca²⁺ from the C–S–H surfaces, attributed to the increased adsorption of Na⁺ on the surface, known as “Na–Ca” cation exchange. This study provides insight into the complex atomic-level phenomena and mechanisms involved in the ion transport in porous C–S–H, contributing to a deeper understanding of issues related to concrete durability.