Electron microscope study of Synroc before and after exposure to aqueous solutions

A well-characterized Synroc was subjected to durability tests in de-ionized water at 70 °C and 150 °C, and silicate and bicarbonate solutions at 70 °C, to study the effect of temperature and solution composition on the mechanisms of aqueous alteration. SEM and TEM were used before and after durability experiments to characterize the primary and secondary phases in and on the Synroc samples, and to describe changes in morphology and chemistry. Leachant compositions after durability testing were analyzed using ICP/OES and ICP/MS and have been reported elsewhere.⁸ After durability testing, titaniferous surface layers were observed primarily on perovskite, the most soluble Synroc phase. Electron microscopy demonstrates that the surface layer formed at 70 °C is an amorphous Ti–O film and is fine grained anatase at 150 °C. Congruent dissolution is the major mechanism of perovskite alteration. However, at 150 °C, selective loss of Ca and Sr may occur locally. A number of additional secondary phases were identified, including Al–oxide/hydroxide, Fe–oxide, REE-bearing Ti–oxides, and several silicate phases. The abundance and composition of the secondary phases are related to solution composition and temperature.