Structures And Properties Of Ceramic Waste Form Based On Pollucite

Pollucite (chemical structural formula:CsAlSi2O6) has been considered to be a potential alternative material for immobilizing radioactive 137Cs due to its large capacity of loading Cs, structural stability and high temperature resistance and other superior properties. However, synthesis condition of the pure pollucite is relatively harsh. Based on the similar chemical compositions and crystal structures between pollucite and leucite or analcite, and the accessible raw materials and preparation conditions of leucite or analcite, the paper offered to research the structures and performances of pollucite based ceramics (K1-xCsxAlSi2O6 or Na1-xCsxAlSi2O6) by using Cs to substitute for K or Na.In this work, crystal structure models of K1-xCsxAlSi2O6 and Na1-xCsxAlSi2O6 have been built firstly, and crystal structures of K1-xCsxAlSi2O6 and Na1-xCsxAlSi2O6 ceramics have been simulated through the first-principles method. The computational simulated results indicated that the transformation of main crystal phase from tetragonal to cubic phase in K1-xCsxAlSi2O6 and Na1-xCsxAlSi2O6 crystal structures occurred with increasing of Cs content, and the cubic structure of K1-xCsxAlSi2O6 was easier to form than Na1-xCsxAlSi2O6. However, the imperfect cubic phase was formed since for the conditions x=0.4 and x=1 the characteristic peak of the cubic structures was non-splited in the simulated diffraction patterns of K1-xCsxAlSi2O6. Based on the computational simulation, K1-xCsx-geopolymers with chemical compositions of about K1-xCsxAlSi2O6·nH20 have been used as precursors to prepare K1-xCsxAlSi2O6 ceramics. And the experimental results verified that the increasing of Cs content would result in the formation of cubic structures with lower crystallization temperatures which was consistent with the computational simulation. Although, the characteristic peak split in XRD parttens for K1-xCsxAlSi2O6 ceramics through K1-xCsx-geopolymers heated upon 1100? or more also indicated the imperfect cubic phase of K1-xCsxAlSi2O6 ceramics except for the case x=0.3, 0.4,0.5 and 1, consistent with the simulations results. Phase transformation from cubic to tetragonal when cooling down or the formation of intermediate tetragonal phase might be the main reason of characteristic peak split.Additionally, the product consistency test leaching results showed that the leaching rates of Cs+ in K1-xCsxAlSi2O6 ceramics were excessively low and K1-xCsxAlSi2O6 ceramics possessed superior chemical durability, suggesting that the K1-xCsxAlSi2O6 ceramics had great potential in immobilizing radioactive 137Cs.Ba, ? decay product of Cs, had a certain influence on the crystal structures of CsAlSi2O6 ceramics. When heated upon at 850?, the crystallinity of Cs1-xBax-geopolymer showed a downward trend with Ba content increasing, indicating that some Ba2+ ions could not exist in crystal lattice. When the sintering temperature reached 1100?, the shift of characteristic peaks in the XRD patterns showed some Ba2+ might occupy the crystal lattice position of Cs+ in CsAlSi2O6 structure, although, Ba content should be controlled in the range of x<15%.