This study describes the mineralogical transformation of ferrihydrite (Fh) coated on grains of pozzolana into mixed FeII–FeIII sulphated green rust {GR( )}. The formed GR( ) coating is intended to be used for the reduction of nitrate in batch and column experiments. Raman and X-ray photoelectron spectroscopies (XPS) are used to characterize the pozzolana substrate and the coatings and proved that in the presence of sulphated anions and FeII cations, Fh is transformed into chemically stable sulphated green rust. The Raman spectra exhibit the presence of characteristic bands of sulphated green rust, spectra on which we rely on to interpret the O 1s XPS high resolution spectra which are characterized by a major contribution located around 531.5 eV, in good agreement with the formation of OH− coordinated to FeII and FeIII ions in brucite-like layers of GR. The reduction of nitrate by GR( ) coatings is then investigated from combined solution and solid phases analyses. In agreement with previous studies carried out on bulk mineral, GR( ) coating reduces nitrate into ammonium and is mainly transformed into magnetite.