Synthesis and hydrothermal stability of U doped zirconolite–sphene composite materials

Two series of U doped zirconolite–sphene composite materials were prepared by solid state reaction method: CaZr(1−m)UmTi2O7−(1−m) Ca(1−x)UxAl2xTi(1−2x)SiO5 (m = 7x) and Ca(1−n)U6nZr(1−5n)Al2nTi(2−2n)O7−(1−5n) Ca(1−y)UyAl2yTi(1−2y)SiO5 (n = 5y/6). The effects of U content on the phase structure of the composite materials were mainly investigated. The results show that the optimal synthesis temperature of the composite material is ∼1230°C. In comparison with the incorporation of U in the Zr site of zirconolite, U incorporation in the Ca site of zirconolite using Al as charge compensating ions was not very efficient. Hydrothermal stability of the U doped zirconolite–sphene composite material was examined by modified product consistency test method at 90°C in deionised water (pH 7). The normalised U leach rate is fairly constant in a low value below 10−5 g m−2 day−1 after 28 days.