Effects of Na⁺ substitution Cs⁺ on the microstructure and thermal expansion behavior of ceramic derived from geopolymer

Abstract

As part of a series of studies, effects of Na + substitution on the thermal evolution of cesium-based geopolymers on heating were studied. A series of sodiumsubstituted cesium-based geopolymers, Cs (1Àx) Na x GPs (where x=0, 0.1, 0.2, 0.3, and 0.4), were prepared and treated at 1300°C for 2 hours to obtain the corresponding ceramic products. The thermal evolution process was disclosed by virtue of a variety of technical, including TG-DTA, thermal shrinkage, XRD analysis, SEM, and TEM investigation. The results indicated that unheated Cs (1Àx) Na x GPs was not completely amorphous after the substitution of Na + and the crystallinity of Cs (1Àx) Na x GPs gradually increased with the rise of sodium content. Meanwhile, the average particle sizes of Cs (1Àx) Na x GPs also increased evidently with increases in sodium substitution. The final product after heat treatment mainly consisted of pollucite (CsAlSi 2 O 6 ) and amorphous glass phase. The particle size of pollucite grain gradually decreased as more Cs + were replaced maybe owing to the role of Na + in the nucleation process of pollucite. Two forms of Na + present in the final products: A small portion was present in the pollucite grains due to Na + partial occupied the crystallographic sites of Cs + ; and the rest were present in the amorphous glass phase among the pollucite grains. The average coefficient of thermal expansion (CTE) of resulting Cs (1Àx) Na x GPs ceramics increased from 4.80910 -6 K À1 (x=0) to 7.26910 À6 K À1 (x=0.4) with increases in sodium substitution, which could be due to the amorphous glass phase had a relatively higher CTE than that of pollucite. K E Y W O R D Sgeopolymer, high-temperature treatment, microstructure, pollucite, thermal expansion 1 | INTRODUCTION Radioisotopes of cesium ( 137 Cs and 134 Cs) were very soluble in water and the resulting radiation would spread to a wide area and continue to radiate to the surrounding for a long time, even for centuries, which had become the largest problems after nuclear accidents in Chernobyl and Fukushima. 1-3 Pollucite, with stoichiometric formula of CsAlSi 2 O 6 , had a cubic Ia-3d symmetry with a zeolite-like structure. The structural channels in pollucite were constructed by six-oxygen rings and had a diameter of only 2.8 A, which was smaller than the diameter of the Cs + of 3.34 A. 4-7 Radioactive Cs + will be encapsulated after the formation of pollucite and the trapped Cs + would not be