Immobilization of heavy metals by calcium sulfoaluminate cement

Two types of calcium sulfoaluminate cement containing 20% and 30% phosphogypsum, respectively, were investigated for their ability in hazardous waste stabilization. Fourteen series of pastes were prepared, each containing the following soluble salt: Na2CrO4·4H2O; Na2Cr2O7·2H2O; CrCl3·6H2O; Pb(NO3)2; Zn(NO3)2·6H2O; ZnSO4·7H2O; and CdCl2·5H2O. The level of pollution was 0.069 mol of heavy metal per Kg of cement.The study has been carried out by means of X-ray diffraction, thermal analysis, scanning electron microscopy, mercury intrusion porosimetry, electrical conductivity, and leaching tests. Very good retention of lead, cadmium, zinc and trivalent chromium is observed. The retention of hexavalent chromium depends upon the nature of the binder: the cement containing 20% gypsum develops the best behaviour. This is explained by the microstructure of the hydrated paste: in the paste containing 30% gypsum, delayed ettringite precipitates and damages the hardened paste.