Impact of elevated temperature on immobilization of SeO42- and CrO42- in sulfoaluminate cement matrix

Abstract

Safe disposal of nuclear waste is an inevitable requirement for the rapid development of nuclear power. Sulfoaluminate cement (SAC) exhibits excellent performance and potential application in nuclear waste solidification. In this study, a temperature-variable leaching test (90 °C and 25 °C) and an ettringite crystal synthesis experiment were designed to analyze the influence of elevated temperature on SeO₄^2- and CrO₄^2- in SAC matrices. The results showed that, on the one hand, high temperature promoted the oxyanions leaching; on the other hand, it accelerated the early hydration of SAC and subsequently hindered further leaching. The decomposition of ettringite at high temperatures weakened both the physical and chemical solidification mechanisms. Changing from 90 °C condition to 25 °C condition could cause a small amount of ettringite to reform within pores, which significantly inhibited oxyanions leaching. No obvious competitive or induced effects between SeO₄^2- and CrO₄^2- were observed. In SAC matrices, the distributions of Se and S were integrated, whereas Cr tended to be enriched alone. SeO₄^2- tended to mix with SO₄^2- and participate in the formation of ettringite together, and CrO₄^2- tended to precipitate alone to form CrO₄-ettringite. This paper presents new insights into the solidification and migration behavior of oxyanions when the solidification matrix undergoes phase transformations.