Conclusions

The electrical conductivity of cement pastes and mortars was measured as a function of volume fraction of sand and degree of hydration (hydration time) using impedance spectroscopy. These results were compared to theoretical models which were developed independently, but used experimental inputs such as the particle size distribution of the aggregate and the mean cement-grain diameter. The results indicate:

• The presence of the ITZ does not significantly enhance the overall conductivity of portland cement mortars. At low and high degrees of hydration, conductivity versus sand volume fraction follows the conventional Bruggeman-Hanai law, (1−V_f,sand) ^3/2 when the conductivity of the mortar is normalized by the conductivity in the nominal paste matrix.
• There are two competing effects associated with the presence of aggregate that tend to counteract each other. The first is the creation of the porous, conductive ITZ in proportion to the aggregate surface area, and the second is the reduction of water/cement ratio in the matrix paste.
• The ratio of the ITZ and matrix paste conductivities (σ_ITZ/σ_matrix) starts out low (≈2) at early hydration, increases through a maximum (≈7), and then decreases to a low value (≈2). The maximum occurs at a degree of hydration between 0.5 and 0.8 (72 to 336 h) at room temperature.
• The contrast between the conductivities of the ITZ and matrix paste, σ_ITZ / σ_matrix, is low enough that the effect of the insulating aggregate dominates over the effect of the higher ITZ conductivity. As sand is added, the overall mortar conductivity always decreases at a fixed degree of hydration. This is not the case for other transport properties, e.g., fluid permeability.