Delayed Ettringite Formation (DEF), is one of the different Internal Sulfate Attack (ISA), is a slow chemical reaction that can be responsible for the degradation of cementitious materials, through the swelling of the material followed by crack diffusion in the structure. The objective of this study is the experimental characterization at the local scale (10 x 10 x 30 mm 3 samples) of the concrete reached by DEF. The composite samples, consist of a 15 mm thick cement paste placed in contact with a siliceous aggregate. The shape of these samples makes it possible to study the mechanical behavior of the interface between the cement paste and the aggregate. This zone is identified as a privileged zone of the development of DEF. Given the slowness of this pathology, the experimental setting in the laboratory is accelerated by choosing conditions favoring the appearance of DEF. The tests are exploited at the local scale by measuring expansions by image correlation of the degraded samples and by scanning electron microscope observations. The results showed a higher expansion at the cement paste-aggregate interface compared to the cement paste. A tensile test performed at local scale allowed to characterize the impact of DEF on mechanical properties of the cement paste-aggregate interface. A drop in strength was observed with heterogeneous formation and localization of ettringite inside the interface.