Concrete durability is a subject of considerable interest, especially with the use of cement based materials on structures increasingly demanding on term of sustainability and resistance to aggressive ions penetration or radionuclide release. Diffusion is considered as one of the main transport phenomena that cause migration of aggressive solutes and radionuclide in a porous media according to most studies. In order to enable more effective prediction of structures service life, the understanding of the link between cement based materials microstructure and transport macro properties needed to be enhanced. In this context, the present study is undertaken to enhance our understanding of the links between microstructure and tritiated water diffusivity in saturated mortars. The effect of aggregates via the ITZ (Interfacial Transition Zone) on transport properties and materials durability is studied. To this end, several series of tritiated water diffusion tests were conducted on mortars with various water-to-cement ratio, sand volume fractions and particle size. Materials microstructure was also characterized by water porosimetry, mercury porosimetry and by backscattered electron microscopy associated to images analysis. In particular, an image analysis protocol was developed to quantify the porosity in the vicinity of aggregates. The relationship between microstructure and transport properties was then examined. For this, tritiated water (HTO) diffusion tests were conducted and correlations between microstructure parameters and transport properties were made. Finally, in order to identify the role of mesoscopic phases (matrix / aggregate / ITZ) in diffusion mechanism, a 3D model was developed and calculations of equivalent diffusivities were made. The study showed that a porous interface exists in the vicinity of grains of silica sand. It has a thickness ranging from 10 to 20 µm and its porosity is approximately three times higher than the cement matrix one. Below 55% of standard sand, the effect of this interface on macroscopic transport properties is weak. In fact, the effect of dilution and tortuosity remains dominant. Therefore, data acquired across cement paste remain valid and are extrapolated across mortars. These results were confirmed also by analytical and numerical calculations of the diffusivity. Beyond 55% of standard sand, other effects related to the large number of sand grains appear such as air voids and porous patches mainly due to the difficulty of obtaining well-compacted materials. This makes these formulations extreme ones and not allows us to consider them to improve our understanding of the relationship between microstructure and transport properties
