Dielectric properties of cement pastes are measured using impedance spectroscopy, and the effective dielectric constants of the low frequency bulk arcs are reported. The unusually high values thereby obtained, and their dependence on reaction time and water:cement ratio, are explained by the presence of microstructural features that serve to amplify the dielectric constants of the individual material phases. The dielectric properties of three-dimensional cement paste models and of simple two-dimensional models of the hypothesized microstructural features are analysed. The model results provide insight into the proposed dielectric amplification mechanism in real cement paste.