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

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 sodium-substituted cesium-based geopolymers, Cs((1-x))Na(x)GPs (where x=0, 0.1, 0.2, 0.3, and 0.4), were prepared and treated at 1300 degrees 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((1-x))Na(x)GPs was not completely amorphous after the substitution of Na+ and the crystallinity of Cs((1-x))Na(x)GPs gradually increased with the rise of sodium content. Meanwhile, the average particle sizes of Cs((1-x))Na(x)GPs also increased evidently with increases in sodium substitution. The final product after heat treatment mainly consisted of pollucite (CsAlSi2O6) 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((1-x))Na(x)GPs ceramics increased from 4.80x10(-6) K-1 (x=0) to 7.26x10(-6) K-1 (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.