In this paper, a high temperature treatment procedure was designed to evaluate the effect of holding temperature on thermal evolution process of Li + substituted Cs-based geopolymer (Cs 0.7 Li 0.3 GP), including the thermal analysis, phase composition and microstructure evolution. With rising of holding temperature, amorphous unheated Cs 0.7 Li 0.3 GP gradually transformed into a multiphase system during the high temperature treatment process, which consisted of pollucite (CsAlSi 2 O 6 ), spodumene (LiAlSi 2 O 6 ) and amorphous glass phase. In the multiphase system, Cs + ions were in the form of pollucite grains, while Li + ions were in the form of spodumene nanocrystallines distributed in amorphous matrix. The pollucite grains gradually coarsened with rise in holding temperature, and the densification of the resulting products were also improved synchronously, which were related to the presence of amorphous glass phases. The amorphous glass phase would be in a molten state when holding temperature over 800 °C. And the presence of molten amorphous phase would make the mass transfer process easier, which could contribute to the growth of the crystal grains and the elimination of the pores.
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°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 (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 (1− x ) Na x GPs ceramics increased from 4.80×10 -6 K −1 ( x =0) to 7.26×10 −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.
Wei Chen,Alicia C. Garofalo,Haining Geng,Yan Liu,Dongwen Wang,Qiu Li
Cesium-based geopolymer is more refractory than K + and Na + based geopolymers after the transformation to ceramic material, whose thermal resistance feature is highly appreciated by some industries, especially in the engineering field where the materials in some cases are exposed to environments with high temperatures. In this study, cesium-activated geopolymer composites (CsGPc), with 5–35 wt% cordierite reinforcements, were prepared and evaluated on the high temperature performance. When 35 wt% cordierite was added, the compressive strength increased by 72% compared to the reference plain CsGP. Residual compressive strength (after heating at 1400 °C) of CsGPc with 35 wt% cordierite increased by 106% compared to the specimen before the thermal exposure . After thermal exposure up to 1400 °C, the geopolymer matrix was transformed into a pollucite-based ceramic composite. Amorphous phases in CsGPc gradually crystallized to form pollucite as the main phase with cordierite as secondary phase. The microstructure of CsGPc and the obtained ceramics showed a clear sintering process and crystallization of pollucite. Shrinkage was reduced and the coefficient of thermal expansion (CTE) varied from 4.50 to 1.89 × 10 −6 K −1 with the cordierite inclusion. The performance of obtained ceramic composites was superior compared to the initial CsGPc, which would be excellent for heat resistance applications.
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