The present invention is directed to a pozzolanic mixture used as a supplementary cementitious material (SCM) comprising an activated clay, a limestone and a setting regulator. Moreover, it is also directed to a cementitious composition comprising said pozzolanic mixture, a clinker and a setting regulator. Finally, the invention is also directed to a concrete comprising the cementitious composition described above, fine aggregate, coarse aggregate and water, optionally the concrete may have other additives.