The influence of lightweight expanded clay and shale aggregates on the mechanical performances of concrete is studied through their volume fractions and aggregate qualities. A very thorough characterisation of a series of LWAs is completed. The impact of aggregates is not only related to its particle density but also to the shell thickness, the percentage of large pores and broken grains. A reverse determination of the elastic modulus of LWA is carried out based to the experimental data. The comparison with values measured ultrasonically confirms the part played by the aggregate stiffer shell in mechanical properties of LWA.