Concentration dependence of diffusion and migration of chloride ions: Part 1. Theoretical considerations

Abstract

This paper series includes two parts: (1) theoretical considerations and (2) experimental evaluations. In Part 1, the chloride ion diffusion and migration processes in concentrated solutions are theoretically analyzed. The quantitative relationship between diffusion coefficient and migration coefficient is established. Two important parameters that are inherent in the cement based materials (i.e., ion-selective semipermeability and chloride binding) are discussed. The theoretical analysis shows that the “effective” chloride diffusion coefficient determined from the conventional diffusion or migration tests is not a constant, but rather is a complicated function of concentration. This function contains four main factors: (1) the parameter K τ, describing the effect of counter-electrical potential; (2) the ratio of cation velocity to anion velocity β v ; (3) the friction coefficient f, which reflects the ionic interaction; and (4) the activity coefficient γ, which is also a function of concentration.