文摘
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 °C, a solubility product log KS0 of −40.2 ± 0.4 was calculated: log KCrO4−ettringite = 6log{Ca2+} + 2log{Al(OH)class="stack">4class="stack">−} + 3log{CrOclass="stack">4class="stack">2−} + 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 (a0) 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.