Introduction

Recent studies of multi-component diffusive transport in porous materials indicate that the formation factor and porosity are the only material parameters required to fully characterize diffusive ionic transport in a nonreactive porous solid, regardless of the number of ionic species present [1,2,3,4]. The formation factor $\Upsilon$ is defined as the ratio of the pore solution electrical conductivity $\sigma_p$_p to the bulk (solid and pore solution) conductivity $\sigma_b$_b [5]:

$$\Upsilon = \frac{\sigma_p}{\sigma_b}$$ (1)

While it has been shown that the bulk conductivity can be measured using readily available laboratory equipment [6], determining the pore solution conductivity is more difficult.

The direct method for determining the electrical conductivity of the pore solution uses pore solution expression [7] to obtain a sample of the pore solution. The sample can then be analyzed using a conductivity meter. Unfortunately, the sample obtained from moderate and low water to cementitious ratio specimens older than 56 d may be exceedingly small, making it difficult to construct a conductivity cell for such a sample. Alternatively, quantitative methods such as ion chromatography can be used to determine the concentration of the ionic species present. Since the conductivity of concentrated electrolytes is not linearly proportional to concentration [8], the conductivity of the cement paste pore solution would have to be estimated from an equation that accounted for the nonlinearity.

In some cases, pore expression is either impractical (virtually no expressed fluid) or impossible (limited concrete accessibility). Under these circumstances, the pore solution conductivity can be estimated from the ion concentration predicted from a model. For example, the model of Taylor [9] predicts the concentration of various ionic species in the pore solution from the cement composition and the degree of hydration, and has been shown to be reasonably accurate [10]. From the estimated concentrations, one could, as in the direct method, estimate the pore solution conductivity using the proposed equation. Presented herein is an equation for estimating the electrical conductivity of a well-hydrated cement paste pore solution. The equation is a function of the ionic strength and requires an empirical coefficient for each ionic species. The model is intentionally simplified to include only a single parameter for each ionic species; interaction terms in the model are excluded. To test the model, laboratory measurements of the electrical conductivity of potassium hydroxide and sodium hydroxide mixtures are compared to the predicted values.