THz radiation can be employed to characterize the dielectric constant and loss tangent of ceramic materials widely used for industrial applications. However, a key parameter in ceramic material production concerns the control of the porosity since it is directly related to the ceramic properties. The evaluation of the ceramic porosity can be achieved by mercury injection. However, THz radiation can propose an alternative effective contact-less and non-destructive approach. In this communication, we focus our attention on alumina (Al2O3), the most important, widely used and cost effective oxide ceramic material. We investigate the THz dielectric properties of alumina ceramic samples with porosity fractions from 0% to 20%. Sample porosity was first characterized by the mercury porosimetry technique. Then, THz spectra were obtained with a commercial THz time-domain spectrometer. Using the effective medium theory with most common models, we predict the sample permittivity for all porosity fractions from 0% to 20% with a good agreement with the experiment. We will also demonstrate that THz time-domain spectroscopy can differentiate alumina samples fabricated with different grain sizes, or reveal a small fraction of impurities, such as 5% ZrO2, embedded in non-porous alumina. These results demonstrated the potential of THz radiation for non-destructive characterization of low loss high dielectric constant.