Insights on ions migration in the nanometer channel of calcium silicate hydrate under external electric field

In this study, molecular dynamics is utilized to investigate on the migration of NaCl solution in the nanochannel of calcium silicate hydrate under external electric field ranging from 0 to 0.05 V/angstrom. The generated ionic current and flux are linearly related with the electric field intensity. While the directional movement of Cl- ions accounts for more than 85% of the ion currents, the migration of Na+ ions and Ca2+ contributes little percentage to the ionic current. The little contribution from cations is attributed to that the silicate chains near the C-S-H surface provide plenty of the oxygen sites to associate with the cations by forming Na-O-s and Ca-O-s bond, immobilizing the migration of the cations. However, the external electric field can weaken the bond stability of the Na-O(s )and Ca-O-s bond, which can be proved from the hydration time of the ion-water cluster and the resident time of ions in the silicate chains. Furthermore, the ions accelerating effect induced by the external field is dramatically retarded by the nanoconfinement. The diffusion coefficient of ions ultra-confined in 3.4 nm and 5.6 nm pores is 3 magnitudes lower than the mobility of ions in the bulk solution. Due to the different H-bond and ionic bond restriction from the C-S-H substrate, the mobility of Na, Cl and Ow ranks in the following order: Cl > Ow > Na, unlike the same transport rate of solution species in the bulk solution. The results obtained from this work may help clarify the diffusion mechanism of ions in concrete structures during the electrochemical desalination processes and is helpful for the design to improve durability of materials. (C) 2019 Elsevier Ltd. All rights reserved.