Study On Immobilization Of Sr²⁺ By Na-based Geopolymer

A new sodium ion-excited geopolymer（Na-based geopolymer,Na GP）on Sr²⁺immobilization is prepareded to solute the problem of traditional cement and glass-ceramic immobilization technology with low immobilizing fraction and complicated preparation process.The effects of Sr²⁺content,heat-treatment temperature and B₂O₃content on the microstructure and mechanical properties of NaGP were systematically investigated with using non-radioactive 87Sr²⁺to simulate radioactive 90Sr²⁺.And the immobilization capability and immobilization mechanism of Na GP and its ceramization products on Sr²⁺immobilization under different storage circumstances were systematically discussed.The results show that Na GP exhibited a more loose microstructure,and its mechanical properties were significantly decreased in case of introducing Sr（NO₃）₂into NaGP to form irregularly shaped SrCO₃ particles.The maximum immobilization capacity of NaGP to Sr（NO₃）₂ was 14.22 wt%measured by the mechanical properties.Compared with cement,the immobilization performance of Na GP on Sr²⁺is superior,especially when the content of Sr²⁺is lower than 5.89 wt%.This is due to the main immobilization mechanism of NaGP is chemical adsorption and formation of insoluble Sr CO₃ precipitates,but the main immobilization mechanism of cement is chemical adsorption.Ceramization transformation of NaGP can be realized when it was treated at high temperature.NaGP gradually transformed from amorphous phase to crystalline phase with the treatment temperature increase,and its microstructure gradually changed from an amorphous gel-like structure to a densely-grained microstructure.The apparent porosity first increased and then decreased,which is caused by decomposition of Na NO₃ resulting in the generation of pores with the temperature increase and then subsequent sintering.The leaching resistance propertity of Na GP in different storage conditions varied greatly due to different heat-treatment temperatures.Among it,Na GP exhibited best leaching resistance performance after treated at 1200℃,and leaching resistance performance of Na GP treated at 800℃and 1000℃is poor,which is mainly determined by the state of the microstructure of NaGP and the presence of Sr²⁺.The immobilization mechanism of heat-treatment NaGP on Sr²⁺is mainly the formation of crystalline phase Sr Si O₃,Sr amorphization and solid solution.The ceramization transition temperature deceased by introduction of B₂O₃ into NaGP,and the compressive strength of NaGP gradually increased with the increase of B₂O₃ content at room temperature.When the B₂O₃ content reached 4.0 wt%,the compressive strength of NaGP was 27 MPa,which was 16 MPa higher than that of NaGPwithout B₂O₃.The main reason is that B₂O₃ accelerated the formation of Na GP and make its microstructure more dense.In addition,the ceramization transformation temperature of NaGP with 4.0 wt%B₂O₃ content is 685℃,which is 162℃lower in comparation with Na GP without B₂O₃.The ceramization transition temperature of NaGP decreased for the reason that the liquid phase B₂O₃ significantly increased the diffusion and migration rate of the solid phase particles.The leaching resistance performance of Na GP slightly improved owing to incorporation of B₂O₃ during heat-treatment process,which was mainly related to B₂O₃ promoting sintering,but irrelevant with B₂O₃ percentage whthin 5 wt%and 10 wt%.The law of leaching behavior for NaGP on Sr²⁺under different storage circumstances not affected by the introduction of B₂O₃.