The stability of U(VI) doped calcium silicate hydrate (CSH) phases was evaluated in claystone formation waters that possess high ionic strengths and carbonate contents. Such phases were synthesized by direct U(VI) incorporation and characterized with time-resolved laser-induced fluorescence spectroscopy (TRLFS), infrared (IR) spectroscopy, powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The time-dependent release of Ca, Si and U from U(VI) doped CSH phases into brines that contained 2.5 M NaCl, 2.5 M NaCl/0.02 M Na2SO4, 2.5 M NaCl/0.02 M NaHCO3 or 0.02 M NaHCO3 was monitored for three calcium-to-silicon (C/S) ratios over 32 days. Subsequently, changes of the U(VI) speciation and CSH phase mineralogy caused by leaching were investigated with TRLFS, IR spectroscopy and XRD. Results indicated that composition and pH of the leaching solution as well as presence and solubility of secondary phases such as portlandite and calcite determine the U(VI) retention by CSH phases under high saline and alkaline conditions. At high ionic strengths, the Ca release from CSH and secondary phases like calcite is increased. Under hyperalkaline conditions only small amounts of U(VI) were release during leaching. A decrease of the pH due to the additional presence of carbonate was linked with an increased U(VI) release from CSH phases caused by the occurrence of a calcium uranyl carbonate in the supernatant solution.