The changes in solution chemistry and mineralogy of spent oil shale were examined as a function of time to determine the extent of the weathering reactions. The computer-based solution chemistry model GEOCHEM was used to predict solution- and solid-phase chemistry, and x-ray diffraction analyses were used to determine the mineralogy of the materials. The solution chemistry evaluation provides evidence that diopside has the potential to precipitate after 16 hours of weathering. However, the system appeared to be in a state of nonequilibrium because significant changes occurred from the first day to the fifty-sixth day of hydration. The X-ray diffraction analyses did not provide conclusive evidence that specifies the mineral phases that make up the comentitious materials. However, calcite was a major component of the system and, undoubtedly, is a major ingredient in the formation of cements. Also, gypsum may contribute to the cementation process.