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Relationship Between the Formation Factor and the Diffusion Coefficient of Porous Materials Saturated With Concentrated Electrolytes: Theoretical and Experimental Considerations.


pdf icon Relationship Between the Formation Factor and the Diffusion Coefficient of Porous Materials Saturated With Concentrated Electrolytes: Theoretical and Experimental Considerations. (1140 K)
Snyder, K. A.

Concrete Science and Engienering, Vol. 3, No. 12, 216-224, December 2001.

Keywords:

porous materials; diffusion; concretes; porous media; transport; formation factor

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. A method is needed to estimate the formation factor from either diffusion or conductivity data so that service life models can be applied 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. The estimated formation factor is then compared to results from conductivity measurements. 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.