We measure the average electromagnetic absorption cross section of building materials in the millimeter frequency range between 50 GHz and 68 GHz, through the reverberation chamber technique. In the context of telecommunications, it is important to characterize the material behavior at those frequencies for future electromagnetics applications oriented to 5G mobile networks. A reverberation chamber is operated at mm frequency and a differential measurement of the quality factor - with and without the material - is performed in order to estimate the average power dissipated through the material sample. Several building materials including travertine, concrete, brick etc have been characterized and the absorption compared. We find that Travertine shows the highest value of absorption cross section