The relationship between the formation factor and the diffusion coefficient of porous materials saturated with concentrated electrolytes : Theoretical and experimental considerations

The relationship between the formation factor and the diffusion coefficient of porous materials saturated with concentrated electrolytes: Theoretical and experimental considerations Abstract: It has been proposed previously that the formation factor, in conjunction with the self-diffusion coefficient, can be used to determine the apparent diffusion coefficient. Strictly speaking, this application is incorrect. The formation factor is equal to the ratio of the self-diffusion coefficient to the microstructural diffusion coefficient, which is a quantity that characterizes the pore structure and is independent of the pore solution electro-chemistry. The origin of this relationship will be shown using both the electro-diffusion transport equation and the definition of the formation factor. In practice, service life models that solve the electro-diffusion transport equation as a function of time require the formation factor in order to calculate the microstructural diffusion coefficient; the effects of the pore solution chemistry are then calculated independently. To use such service life models, a method is needed to estimate the formation factor from diffusion data in order to apply the service life model to a particular material. An experiment on a model porous material is used to demonstrate one method for determining the formation factor from divided cell diffusion data. Differences among the self-diffusion coefficients of the various diffusing species accentuates the difference between the microstructural and the apparent diffusion coefficients. The significance of this result to cementitious systems is discussed.