Molecular insights into sulfate-induced chloride release from Friedel's salt

Sulfate attack can impair the chloride-binding capacity of Friedel's salt (FS) in cementitious materials by inducing the release of its bound chloride ions (Cl⁻). However, the atomic-level understanding of this reaction remains unclear. In this study, ab initio molecular dynamics (AIMD) is employed to elucidate the molecular-scale exchange kinetics of SO₄²⁻/Cl⁻ at the FS/sulfate solution interface. The results demonstrate that SO₄²⁻ weakens both the electrostatic binding of Cl⁻ to the [Ca₂Al(OH)₆]⁺ layer and its hydrogen bonding with water in the [Cl·2H₂O]⁻ layer through charge redistribution effect, thereby facilitating Cl⁻ release and subsequent SO₄²⁻ substitution. Furthermore, the dynamic reconstruction of hydrogen bond (H-bond) network within the FS anion-binding layer is a critical mechanism for interfacial restabilization after anion exchange. First-principles calculations confirm the significant thermodynamic spontaneity of the SO₄²⁻/Cl⁻ exchange, highlighting both the ionic competitive advantage and binding stability of SO₄²⁻.