The use of supplementary cementitious materials is the most common approach to reduce environmental impact of concrete production. However, the total replacement of cement is not possible because of the dramatically reduction of the mechanical properties of the concrete. The paper presents an experimental study aimed at comparing the mechanical properties (compressive, bending strength and shrinkage) of ordinary concrete with those of blended concretes (with and without steel fibres) with partial cement replacement with Limestone Calcinated Clay (LC3). The results show that the partial replacement with LC3 leads to a different development of the mechanical properties with respect to ordinary concretes. A brief discussion on the structural implications of this behaviour is also presented.