Contaminated sludges from Hamburg harbour have been stabilized by means of lime, calcium hydroxide, calcium carbonate, gypsum, trass, waste kiln dust, cement, coal fly ash, and red mud. Analysis of the solidification products by polarization microscope, electron microscope, x-ray diffraction, electron microprobe, and PIXE microprobe indicate that new minerals have been formed which stabilize the material and change, as confirmed by chemical extraction methods and leaching tests, the phase specific bonding of heavy metals.