Structures And Properties Of Pollucite-Corundum Composite Solidified Form

High content of¹³⁷Cs（reach to above 40%by weight）can be stably solidified by pollucite lattice.And therefore,pollucite has been proposed as an ideal candidiate for immobilizing radioactive¹³⁷Cs due to its high hydrothermal stability and low thermal expansion.However,much more strict and complex conditions might be required in preparation of pure pollucite.Hence,the composite technology is currently used to obtain the pollucite-type¹³⁷Cs solidified forms.Based on the similarity in compositon and crystal structure among geopolymer,pollucite（CsAlSi₂O₆）,leucite（KAlSi₂O₆）and analcite（NaAlSi₂O₆）,Cs-defined geopolymer gels with the theoretical chemical composition of（K/Na）_1-x-x Cs_xAlSi₂O₆?wH₂O was prepared by the geopolymer precursor method in this paper.Then,pollucite-type ceramic solidified forms were obtained by thermal treatment of Cs-defined geopolymer gels.In order to ensure the long-term thermal stability and mechanical stability of Cs-defined geopolymer gels and the pollucite-type ceramic solidified body,corundum fillers were introduced into the geopolymer preparation,corundum/Cs-defined geopolymer gels composite was hence prepared.Corundum/pollucite composite ceramic solidified forms were obtained by thermal treatment of corundum/Cs-defined geopolymer composites.Effects of Cs content on the composition,structure and properties of Cs-defined geopolymer gels,pollucite-type ceramic solidified forms,corundum/Cs-defined geopolymer composites and corundum/pollucite composites were investigated.The main results are as follows:（1）Firstly,K_1-xCs_xAlSi₂O₆?wH₂O（x=0,0.1,0.2,0.3,0.37）geopolymer gels were obtained.The crystal structure of K_1-xCs_xAlSi₂O₆ was transformed from tetragonal leucite to cubic leucite or pollucite（x≥0.2）after heat treatment at 1100°C,which proved that a certain amount of Cs can stabilize the leucite cubic phase to room temperature.The increase of lattice parameters indicated that the final product was doped leucite or pollucite.（2）Na_1-xCs_xAlSi₂O₆?wH₂O（x=0,0.1,0.2,0.3,0.37）geopolymer gels was also obtained by the partially replacement of Cs for Na.Na_1-xCs_xAlSi₂O₆?wH₂O gels were firstly crystallized into sodium-nepheline during thermal treatment,and then transformed to analcite with the increase of Cs content and temperature.However,Cs and Na are difficult to replace each other in analcite or pollucite structure,and the obtained product is a two-phase product containing analcime and pollucite.（3）The addition of corundum can greatly improve the mechanical properties and high temperature stability of the geopolymer gels,and corundum/pollucite composites with better integrity can be obtained after thermal treatment.The corundum/pollucite composites ceramic solidified forms with high mechanical properties,high temperature stability,low leaching rate and high thermal conductivity would be an ideal choice for solidified materials for Cs-containing radioactive wastes.