COMPUTATION OF DIFFUSIVITY

An electrical analogy is used to compute the relative diffusivity of the composite media¹⁶, where relative diffusivity is the ratio of the diffusivity of an ion in the composite media relative to its value in bulk water (proportional to the inverse of the formation factor). Conductivities are assigned to each phase comprising the microstructure and the resultant composite relative conductivity is computed¹⁷ and related to a relative diffusivity using the Nernst-Einstein relation¹⁶:

$$\frac{D}{D_0}~=~\frac{\sigma}{\sigma_0}$$ (1)

where σ / σ₀ is the computed relative conductivity and D/D₀ is the relative diffusivity for the random microstructure. For this study, capillary pores (Φ) are assigned a relative diffusivity of 1.0, while the C-S-H gel is assigned a relative diffusivity of 0.0025 (1/400)¹⁷.

Previous studies have indicated that diffusivities computed using these relative values compare favorably to those measured experimentally^17-18. For each hydrated/leached microstructure, the diffusivity was computed in each of the three principal directions and the average value reported.