Matrix stability and leaching behaviour in ettringite-based stabilization systems doped with heavy metals

A stabilizing matrix able to generate ettringite from calcium sulphoaluminate in the presence of CaSO4•2H2O and Ca(OH)2 has been studied. Ten series of stabilized samples have been produced, each containing 10% of one of the following heavy metal salts: Cd(NO3)2, Cr(NO3)3 Cu(NO3)2, Fe(NO3)3, Mn(NO3)3, Ni(NO3)2, Pb(NO3)2, Zn(NO3)2, K2CrO4 and K2MoO4. The study has been directed towards the matrix stability and metal release behaviour in three different dynamic leaching tests carried out with the following media: distilled water, pH 4 HNO3 solution and pH 4.74 acetic acid/sodium acetate buffer solution. It has been found that the matrix stability and leaching behaviour strongly depend on the nature of the dopant salt, as well as on the nature of the leaching medium. In many cases the leaching medium attack causes disaggregation of the matrix but this does not imply complete dissolution of the dopant metal. When the leaching medium is the acetate buffer the chemical resistance to dissolution increases as the physico-mechanical resistance to disgregation decreases. This apparently contradictory result has been explained in terms of microstructure of the stabilized samples.