Résumé
Chromate is a toxic contaminant: of potential concern, as it is quite soluble in the alkaline pH range and could be released to the environment In cementitous systems, CrO42- is thought to be incorporated as a solid solution with SO42- in ettringite. The formation of a solid solution (SS) could lower the soluble CrO42- concentrations.
Ettringite containing SO42- or CrO42- and mixtures thereof have been synthesized. The resulting solids and their solubility after an equilibration time of 3 months have been characterized. For CrO4-ettringite at 25 degrees C, a solubility product log K-SO of -40.2 +/- 0.4 was calculated: log KCrO4-attringite = 6log{Ca2+} + 2log{Al(OH)(4)(-)} + 3log{CrO42- + 4log{OH- + 26log{H2O}. X-ray diffraction and the analysis of the solution indicated the formation of a regular solid solution between SO4- and CrO4-ettringite with a miscibility gap between 0.4 <= XCrO4 <= 0.6. The miscibility gap of the SO4- and CrO4-ettringite solid solution could be reproduced with a dimensionless Guggenheim fitting parameter (a(0)) of 2.03.
The presence of a solid solution between SO4- and CrO4-ettringite results in a stabilization of the solids compared to the pure ettringites and thus in an increased uptake of CrO42- in cementitious systems.