The stability of a tailored ceramic waste form under hydrothermal conditions was the subject of several previous papers [1,7–10] from this laboratory. One of the results reported in these studies was the apparent dissolution and reprecipitation of the alkaline-earth molybdate phases (scheelite-structure phase). The composition of the scheelite-structure phase after hydrothermal treatment was different from that before treatment (approximately Ca95 Sr5 after and Ca35 Sr40 Ba25 before). A schematic phase diagram was presented at that time to explain the results obtained.
This paper is a report of our attempt to experimentally determine the phase relations in the alkaline-earth molybdate (Ca, Sr, and Ba) and verify our previous interpretation. For this work, we prepared compositions throughout the ternary system and heated them at 1200°C for periods of 2 to 24 hours. The experimental products were characterized by x-ray diffraction and SEM/EDX examination.
At 1200°C, the two-phase solvus is nearly symmetric and extends from near the pure calcium and barium end-members to about 35 mole percent strontium at the critical point of the solvus. The increase in grain size, uniformity of composition within and between grains of the same phase and the approach to “textural equilibrium” in the 1200°C experiments all strongly suggest that equilibrium was attained.
New, calcium-rich compositions are suggested for the scheelite-structure phases in the supercalcine-ceramics based upon the interpretation of data from this study. This observation forces a re-examination of the assumption concerning the partitioning of Sr and Ba amongst the phases in the supercalcine-ceramic.