As an aerobic biological treatment, the composting process is influenced by the physical properties of organic waste. On one hand, material settlement, air permeability and free air space affect the aeration efficiency and the composting performance. On the other hand, the organic matter biodegradation during the process leads to a variation of material physical properties. The aim of this study was to investigate the time variation and the spatial distribution of the physical properties of a sludge and bulking agent mixture during its composting treatment. Composting was carried out in 300L pilot-scale reactors with forced aeration. In order to follow the spatial distribution in each reactor, the material was divided in three layers. Reactors were respectively stopped after 1.5, 3, 7, 14 and 31 composting days, and each material layer was characterized for moisture content, bulk density, free air space (FAS) and air permeability (K). The results showed that the changes in material properties were different according to the considered layer. The deepest layer, impacted by the air inlet in the reactor, had the highest decrease of moisture content and bulk density, a steady K and the highest FAS increase, whereas the surface layer, impacted by a condensate dropout, showed a slight increase of moisture content, steady density and FAS, and a very strong increase of K. Then, a physical structural reorganization of material occurred during the process. In the future, the obtained results could help to improve composting models, taking into account the temporal and spatial distribution of physical properties in the porous media.