Immobilisation mechanism for nuclear waste containing aluminium by supersulfated cement containing phosphogypsum

Abstract

Supersulfated cement (SSC), which features a relatively low pH value, is increasingly considered as an effective alternative to Portland cement-based material for solidifying nuclear waste. Phosphogypsum (PG), an industrial waste, can be used to prepare SSC, but the impurities may affect the efficiency of SSC for nuclear waste immobilisation. Therefore, PG was pretreated by calcination and lime neutralization. The mechanism of supersulfated cement containing phosphogypsum for immobilizing nuclear waste was investigated in this study. Results show that the corrosion of aluminium rods by the PG-based SSC is much lower than that of Portland cement, of which calcined PG-based SSC has the least corrosion on aluminium rods. At the w/c ratio of 0.4, the pH value of PG-based SSC is lower than that of Portland cement. For calcined PG-based SSC with different w/c ratios, a high w/c ratio reduced the corrosion of aluminium rods slightly. The immobilisation mechanism of the SSCs on nuclear waste is as follows: when the aluminum-containing nuclear waste comes to contact with the cement paste, the protective layer of alumina is slowly dissolved, and aluminum reacts with OH− to form hydrogen and Al(OH)3. The formation of hydrogen bubbles may force cement away from the aluminum surface and allow ettringite and Al(OH)3 to form in water-filled pores around the metal, which relatively slows the rate of the corrosion reaction. Corrosion will end until the surface of the aluminum rod is covered with corrosion products or the free water in the cement is exhausted.