Drying dewatered sludge leads to a complex three-dimensional porous structure. Moreover, this operation is dependent on the way the material is processed. In this study, textural changes of sludge extrudates submitted to convective drying are followed by a 3D image analysis of reconstructed X-ray microtomograms. To achieve this goal, two different wastewater sludges collected in wastewater treatment plant after the thickening step and dewatered in the laboratory are used. It is showed that the evolution of the 3D-crack ratio vs. the residual water content evolves following a hyperbolic law. The 3D opening crack size distribution reveals two different types of pore development, i.e. a continuous pore size evolution for one sludge and the sudden appearance of cracks for the other sludge.