REACTIONS BETWEEN CEMENT COMPONENTS AND U(VI) OXIDE

Reactions between CaO and SiO2, representing the principal components of cement, and schoepite, UO3 . 2H(2)O, have been studied in aqueous suspensions at 85 degrees C to evaluate the solubility of uranium at high pH, which is a principal control of the immobilisation potential. Supplementary experiments with preformed C-S-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-UO3-SiO2-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 10(-8)-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-S-H gel. Some slight U sorption into the gel may occur and is postulated to inhibit its crystallisation at 55-85 degrees 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.