Studies of phase relations and AFm solid solution formations in the system CaO-Al2O3-CaCl2-CaCrO4-H2O

- A solid solution-aqueous solution (SSAS) model for a binary system of Cl-AFm and CrO4-AFm is proposed.
- The chromate concentrations in aqueous greatly declines as the formation of the solid solution.
- The solubility constants (pK) are 27.09 and 29.07 for pure Cl-AFm and Cr-AFm, respectively.

The Friedel's salt (Cl-AFm) as a major hydration phase in chlorinated cement-based materials plays an important role in the immobilization of chromate through ion exchange with chloride. To study the solid solutions of Ca4[Al(OH)6]2CrO4·nH2O (CrO4-AFm) and the composition of coexisting aqueous phase, AFm phases containing Cl− and CrO2- 4 were synthesized and characterized with XRD, SEM, FTIR, FAAS and Ion chromatography (IC).Based on the determined total solubility of products and ion concentrations in solution, a solid solution-aqueous solution (SSAS) model for binary mixing of Cl-AFm and CrO4-AFm phases was established to predict aqueous solubility of Ca4[Al(OH)6]2[(CrO4)x(Cl)2-2x]
- nH2O as a function of total Cl/Cr ratio in solid solution system. The solubility constant (pK) of pure Cl-AFm is 27.09 and its free energy of formation (ΔGo f) is −7047.80 kJ mol−1 at 27 °C. pK of pure CrO4-AFm is 29.07 and its ΔGo f is −8238.66 kJ mol−1 at 27 °C. Both the experimental and modeled data showed that the formation of solid solutions between Cl-AFm and CrO4-AFm could greatly lower the chromate concentration in aqueous solution. Therefore, this study suggested that (Cl, CrO4)-AFm compounds could efficiently immobilize the trace metal CrO42- in cement-stabilized wastes.

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