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Effect of Speciation on the Apparent Diffusion Coefficient in Nonreactive Porous Systems.


pdf icon Effect of Speciation on the Apparent Diffusion Coefficient in Nonreactive Porous Systems. (1142 K)
Snyder, K. A.; Marchand, J.

Cement and Concrete Research, Vol. 31, No. 12, 1837-1845, December 2001.

Keywords:

concretes; diffusion; pore solution; electrochemical properties; electro-diffusion; service life; speciation; transport properties

Abstract:

A combined theoretical and experimental study is made of the effect that concentration and ionic speciation have on the apparent diffusion coefficient is performed using a nonreactive porous material in a diffusion apparatus. Varying the ionic species concentration over two orders of magnitude changes the apparent diffusion coefficient by no more than 20% for the systems studied. By contrast, at fixed ionic concentration, varying the ionic species changes the initial apparent diffusion coefficient by a factor of two. Over longer periods of time, the apparent diffsuion coefficient varies in time, increasing by a factor of ten or more. For one system, the macroscopic diffusion potential across the specimen induces a transient negative apparent diffusion coefficient; iodide ions are transported from regions of low iodideconcentration to regions of high iodide concentration. The theoretical analysis shows that, in nonreactive porous systems, the behavior of all the concentrations and species studied can be completely characterized by an electro-diffusion system of equations that contain two time-independent constants: the porosity and the formation factor. The relationship between these results and the prediction of concrete performance in the field is discussed.