Influence of cation type on chloride binding and phase assemblage evolution in saturated concrete: A reactive transport approach

Chloride-induced corrosion remains one of the main durability concerns for reinforced concrete exposed to marine or de-icing environments. Conventional diffusion-based models often neglect the chemical form of chloride and the role of counter-cations in altering hydrated cement. In practice, chloride transport is a reactive process controlled by simultaneous diffusion, binding, dissolution/precipitation, and pH buffering within the evolving cement matrix. This study investigates how different cations Na⁺, K⁺, Ca²⁺, and Mg²⁺ affect chloride ingress, binding, and hydrate stability in saturated concrete. A reactive transport model is developed that couples diffusion, aqueous speciation, mineral equilibrium, kinetic reactions, and surface complexation on C-S-H. The simulations reproduce and extend the experimental results of literature for four boundary solutions: 0.5 mol/l NaCl, 0.5 mol/l KCl, 0.25 mol/l CaCl₂, and 0.25 mol/l MgCl₂, over exposure periods up to ten years in saturated concrete. Under NaCl and KCl, the pore network remains stable, alkalinity is maintained, and binding is moderate producing deep free-chloride penetration. Under CaCl₂ and MgCl₂, strong near-surface reactions occur: AFm phases convert into Kuzel-type compounds, and portlandite dissolution with C-S-H decalcification produces brucite or M-S-H. These transformations trap chloride near the surface, limit transport, and reduce pH in the outer zone. Consequently, monovalent salts lead to transport-controlled ingress, while divalent salts cause binding/microstructure-controlled accumulation. Reliable prediction of corrosion risk requires evaluating free chloride, total chloride, and alkalinity together. Reactive transport modeling thus provides a physically consistent and predictive framework for performance-based durability design of concrete under diverse chloride environments.