Sintering of metakaolin‐based Na/Ca‐geopolymers and their immobilization of Cs

Abstract

Metakaolin‐based Na/Ca‐geopolymers with a designed composition close to feldspar were used as precursors of Cs immobilization form materials. The sintering behaviors of geopolymers and their sintered materials' immobilization of Cs ions were investigated; the results indicated that the major phases of sintered geopolymers are plagioclase feldspars and feldspathoid. With the addition of Cs to the geopolymers, Cs‐leucite phase and caesium silicates were formed at 1150°C and 1170°C, respectively. It was found that Cs addition can slightly decrease the sintering temperature of geopolymers and make the grains finer. In addition, in order to reduce the volatilization of Cs ions during sintering, sintering temperatures of Cs‐geopolymer were further decreased to 850°C by introducing B2O3. After the geopolymer and Cs‐geopolymer were sintered using a low‐temperature liquid‐phase process, they remained structurally plagioclase feldspars and feldspathoid. Leaching on Product Consistency Test indicates that the leaching concentrations of Cs‐geopolymer samples sintered at higher temperatures are lower than those of samples sintered at lower temperatures. Thus, the increased volatilization of Cs ions at the higher temperatures and the formation of Cs‐leucite phase and caesium silicates can lead to the decrease of leaching concentrations in leachates. Therefore, low sintering temperatures and the fabrication of Cs crystalline ceramics are the key factors toward improving the immobilization of Cs ions.

B2O3‐modified geopolymer and its liquid‐phase sintered products for immobilization of strontium

“…The phase‐transition temperature was 795°C for 0.5 wt% B 2 O 3 in the precursor, while 690°C for 4.0 wt% B 2 O 3 in the same precursor. This result could be attributed to increased liquidity induced by fluxing agent B 2 O 3 enabling to transform solid‐phase diffusion into liquid‐phase diffusion during the sintering process of atomic rearrangement, which lowered the sintering temperature 37 . The relative intensities of endothermic peaks also reflected the distinct degree of crystallization of different samples.…”

“…The further mass loss was interpreted as complex chemical reaction like decomposition of SrCO 3 as well as effects of ceramicization for (Na, Sr)-GP when temperature was ascended by nearly 200 enabling to transform solid-phase diffusion into liquidphase diffusion during the sintering process of atomic rearrangement, which lowered the sintering temperature. 37 The relative intensities of endothermic peaks also reflected the distinct degree of crystallization of different samples. After 800 • C, for TG curves, the product of adding 0.5 wt% B 2 O 3 underwent a slight mass increase, implying the interference of the tested atmosphere, N 2 flow.…”

Vitrification of wastes: from unwanted to controlled crystallization, a review

“…Another study used a geopolymer process to produce alkali activated solution (CsOH, NaOH) with colloidal silica, which was then mixed with metakaolin (Al, Si source) and B 2 O 3 then heat treated >700 °C to crystallize pollucite [He et al, 2020]. If higher conversion temperatures are used for the geopolymer, Cs is volatilized and crystalline phases tend toward Cs-substituted leucite (feldspar, nominally KAlSi 2 O 6 ) and CsAl 2 Si 5 O 12 [Chen et al, 2019b].…”

Geopolímeros e sua potencial aplicação na imobilização de metais pesados: Uma década de revisão

“…Atualmente, as cinzas volantes Onutai, et al, 2020) e o metacaulim (MK) (Chen, et al, 2019;Ofer-Rozovsky, et al, 2019), que são ricos em componentes reativos de Si/Al, são as matérias-primas mais comumente utilizadas para a síntese de geopolímeros para a imobilização dos metais pesados. Nos últimos anos, muitos outros resíduos sólidos foram desenvolvidos para substituir ou substituir parcialmente as cinzas volantes e MK para a preparação.…”