Effects of phase composition on the cesium leachability of cement-based waste forms

Although portland cement has long been used as an encapsulant/host phase for a variety of radioactive waste streams, the phase chemistry/stability relations which exist are still poorly defined. It was the objective to investigate low temperature phase relations, predominantly in the system CaO-Al/sub 2/O/sub 3/-SiO/sub 2/-H/sub 2/O, in order to better understand the underlying physical/crystal chemical principles which ultimately govern the leachability of a given ion. The present paper, dealing with cesium leachability, is the first in a series of such papers. A series of mixtures in the vicinity of Stratling's compound (2CaO . Al/sub 2/O/sub 3/ . SiO/sub 2/ . 8H O) were prepared from Type I portland cement, calcium aluminate cement and condensed silica fume. Samples were formulated using either deionized water or deionized water containing added cesium hydroxide. After curing at 380C in sealed glass jars, the samples were characterized via compressive strength, x-ray diffraction and scanning electron microscopy. Cesium leachability of 28-day cured samples was determined using a modified MCC-1 leach test. Solutions were analyzed for Ca, Al, Si, Na, K and Cs. All samples were found to be as strong if not stronger than their non-cesium containing counterparts. Leaching results indicated that the enrichment of the bulk composition of a mixture in silica and/or alumina enhanced cesium retention. Phase and composition data were used to construct a highly schematic, phase equilibrium diagram for the lime-rich section of the system CaO-Al/sub 2/O/sub 3/-SiO/sub 2/-H/sub 2/O. Leach data are evaluated in terms of proposed phase assemblages. Phases responsible for cesium retention may include Stratling's compound, Al-substituted calcium-silicate-hydrate, Si-substituted hydrogarnet and a yet unidentified cesium-containing hydrate