Reactions between cement components and U(VI) oxide

Reactions between CaO and SiO2, representing the principal components of cement, and schoepite, UO3•2H2O, have been studied in aqueous suspensions at 85°C to evaluate the solubility of uranium at high pH, which is a principal control of the immobilisation potential. Supplementary experiments with preformed C---z.sbnd;S---z.sbnd;H (calcium silicate hydrogel) are also reported. U loadings are in the range 10–80 mol%. Study of the reaction products of 46 compositions has led to the identification of phase compatibility regions. Several solubility-limiting but hitherto uncharacterised CaO---z.sbnd;UO3---z.sbnd;SiO2---z.sbnd;H2O phases are described; their X-ray powder diffraction data are given. The synthetic phases are compared with natural analogues and it is concluded that similarities exist: weeksite- and becquerelite-like structures are probably developed in the synthetics. With formation of crystalline phases, U solubilities decline to 108-10−9 mol/l. Solubilities correlate well with the observed compatibility regions, with lowest solubilities obtained among the uranium-containing phases which are also compatible with C---z.sbnd;S---z.sbnd;H gel. Some slight U sorption into the gel may occur and is postulated to inhibit its crystallisation at 55–85°C. The solubility values are, in general, less than in previous studies which, by omitting SiO2 from simulants and using relatively brief reaction times, fail to achieve the characteristic immobilisation potential.