Molecular dynamics study on coupled ion transport in aluminum-doped cement-based materials

Abstract

Molecular dynamics simulations were performed to investigate coupled ion transport in the two main hydration products of aluminum-doped cement-based materials, including calcium aluminate-silicate hydrate (C-A-S-H) gel and calcium hydroxide (CH). Each material was simulated in three solutions (1 mol/L NaCl, 0.5 mol/L NaCl + 0.5 mol/L Na2SO4, and 1 mol/L Na2SO4). Sulfate ions were found to promote the aggregation of other ions in solution to form ion clusters that block the materials’ nanopores to some degree, slowing their erosion. The magnitude of this reduction is significantly greater for the C-A-S-H than for CH because the ion clusters in the C-A-S-H gel are adsorbed at the interface where they cause “necking”, whereas in CH they move with the solution and are not adsorbed at the material’s surface. Sodium ions adsorb more strongly to C-A-S-H gels than chloride ions, but the opposite is true for CH. The two hydration products thus have markedly different effects on ions transport.