Molecular dynamics study on coupled ion transport in aluminium-doped cement-based materials: effect of concentration

Abstract

The mutual inhibition effect of transport of sulfate and chloride in concrete specimens was determined in a macroscopic experiment. A higher concentration of sulfate has a better inhibition effect on chloride transport and the opposite is also true. In this paper, molecular dynamics (MD) simulation was performed to explore the effect of concentration (0, 0.5, 1.0 mol/l) on the transport of mixed solutions (sodium chloride (NaCl) and sodium sulfate (Na2SO4)) in the main hydration products of aluminium-doped cement-based materials (i.e. calcium–aluminium–silicate–hydrate (C–A–S–H) gel). Sulfate was found to promote the aggregation of other ions to form ion clusters, which can reduce the effective width of the channel entrance and create a ‘necking’ effect, thus reducing the overall transport rate of the solution. With the increase of sodium chloride concentration, sulfate ions in the mixed solution can adsorb more Na+ and Cl− ions, and then form larger ion clusters to block the nanopores. Moreover, with increasing sodium sulfate concentration, a higher amount of sulfate ions existing in the solution makes it possible to form more ion clusters. The results can provide a reasonable nanoscale explanation for macroscopic experiment.