Insights on the chloride adsorption stability in cement mortar under current field and sulfate attack: From experiments to molecular dynamics simulation

The destabilization together with boundary chloride ions releasing leads to the corrosion of steel bars, posing nonnegligible risks to the durability of structures. This work attempted to design the typical pore solution of underground constructions via methods of combining NaCl with Na2SO4 along with conducting a stray current continuously. The bound chloride stability of mortar under the action of electric current was systematically determined. The reaction mechanisms were tested by quantitative XRD, TG-DTG, FTIR, FESEM, and X-CT. Molecular dynamics was utilized to simulate the physical binding processes of C-S-H of Cl-/SO42- and their interaction under the actions of electric field by comparing the ions displacement trajectory, radial distribution function, and moving ability. The experimental results showed that the SO42- weakened the chemical chloride binding capacity with the Friedel's salt, portlandite, and calcium silicate decomposed to produce gypsum or ettringite. The electric field demonstrated less impact on the stabilities of chemical-bound Cl-, but promoted SO42redistributions, causing substantial pore coarsening. The simulation revealed that the physical binding contained short-range ion Coulomb-interactions and long-range van der Waals interactions. The SO42- inhibited the binding of the C-S-H on Cl- by fact that the deposition large cluster of Na-SO4 blocked the nano-meter channels and competitive adsorption occurred at the SiOCa+ sites, while the introduction of electric field weakened the dominating effects of Coulomb-interactions and improved the ions desorption gradually. Hopefully, the transport and adsorption mechanisms of the Cl- in the cement-based materials can help guide the underground concrete with durability.