The X-ray powder diffraction patterns for two new synthetic calcium uranium (VI) silicate hydrate phases are reported. Ca1.5U6(OH)(7)O-16. 7H(2)O is orthorhombic, space group P*a*, with unit cell a = 13.8949(14), b = 12.0776(12), c = 15.228(3) Angstrom. The structure appears to be related to that of becquerelite. Ca-2(UO2)(2)(Si2O5)(3) . 10H(2)O was also indexed on an orthorhombic unit cell, a = 12.075(3), b = 15.406(6), c = 26.043(6) Angstrom. The Powder Diffraction File coverage of uranium-containing minerals which could, on the basis of their chemical formula, form in U-containing cements is also reviewed. (C) 1997 International Centre for Diffraction Data.