Recent results on synthesis, crystal chemistry, structure and stability of ettringite*
, with specific reference to use of this material in hazardous waste immobilization, are presented. Ettringites with selenate, chromate, borate, sulfite and carbonate replacing sulfate have been synthesized and characterized by XRD for phase purity and structure. In X-ray powder diffractograms, selenate and chromate etringites have the normal trigonal P31c
ettringite structure, whereas the borate, sulfite and carbonate ettringites can be indexed on a hexagonal unit cell with a halved c parameter. In excess of 95% of the Se and B in solutions spiked with various concentrations of these elements has been removed by precipitation of ettringite. Ettringites appear to be unstable in solutions outside of the approximate pH range 11.0-12.5, and there is a potential for some oxyanion exchange between ettringites and oxyanion-containing solutions. Buffering of pH, and dissolved Ca, Al and sulfate activities by other components of ettringite-based hazardous waste forms, combined with low-reactivity C-S-H matrices, could largely mitigate these concerns.