Synthesis of Cr(VI)-Ettringite in Portlandite Suspensions as a Function of pH

The synthesis of Cr(VI)-ettringite was attempted in portlandite suspensions for the immobilization of hazardous hexavalent Cr. The reagents NaAlO2 and CrO3 were used by dissolving them in deionized water to obtain 0.1 mol/L solutions, which were mixed with portlandite suspensions and stirred for 3 h at room temperature in CO2-free synthetic air. Ca/Al atomic ratios were varied from 1.0 to 5.0 against the Cr/Al atomic ratios 1.0, 1.5 and 2.0. Run products were evaluated by XRD, XRF and FE-SEM. The results showed that Cr(VI)-ettringite formed under the conditions of pH 10.9 and more always associates with calcite. The calcite was also formed due to the carbonation of the remaining portlandite Ca(OH)2 during drying in ambient air. The calculated yields of Cr(VI)-ettringite generally exceeded 80 mass%, and 85.6 mass% was reached for the stoichiometric mixture. In the run products obtained at pH lower than 10.9, no Cr(VI)-ettringite was identified, and the XRF analysis showed free-CaO and free-Al2O3 other than free-CrO3 potentials each corresponding to calcite, ill-crystallized gibbsite (gibbsite gels) and the soluble chromatite CaCrO4 adsorbed and trapped in the jelly gibbsite gels.