Effect of Speciation on the Apparent Diffusion Coefficient in Nonreactive Porous Systems.
Effect of Speciation on the Apparent Diffusion
Coefficient in Nonreactive Porous Systems.
Snyder, K. A.; Marchand, J.
Cement and Concrete Research, Vol. 31, No. 12,
1837-1845, December 2001.
concretes; diffusion; pore solution; electrochemical
properties; electro-diffusion; service life; speciation;
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.