Formation of Ceramics from Metakaolin-Based Geopolymers: Part I-Cs-Based Geopolymer

The structural evolution and crystallization of a cesium‐based geopolymer (Cs2O·Al2O3·4SiO2·11H2O) on heating was studied by a variety of techniques including X‐ray diffraction, thermal analysis, dilatometry, pycnometry, specific surface area, and microstructural investigation. The Cs geopolymer gradually crystallized into pollucite (Cs2O·Al2O3·4SiO2) on heating above 900°C. Its low crystallization temperature is believed to be due to the presence of nuclei in the geopolymer precursor, which are formed after curing at 50°C for 24 h. The Cs‐based geopolymer was found to be more refractory compared with K‐ and Na‐based geopolymers. Significant shrinkage, due primarily to viscous sintering, did not occur until the samples were heated to above 1200°C. The microstructure of unheated geopolymer had ∼20–30 nm‐sized precipitates that coarsened on heating above 1000°C. By 1350°C, the geopolymer surface had a smooth, glassy texture, although large macropores and closed pores remained. After heating to 1600°C, the closed pores were removed, and the geopolymer reached ∼98% of the theoretical density of pollucite. Higher than expected levels of Cs were found near large voids, as seen by scanning electron microscopy and transmission electron microscopy analysis. The presence of this extra Cs was due to Cs left behind in pore water, which was not bound within the geopolymer structure.