The structural evolution and crystallization of potassium‐based geopolymer (K2O·Al2O3·4SiO2·11H2O) on heating was studied by a variety of techniques. On heating from 850–1100°C, potassium‐geopolymer underwent significant shrinkage and surface area reduction due to viscous sintering. Small, 15–20 nm sized precipitates present in the unheated geopolymer coarsened substantially in samples heated between 900° and 1000°C. However, the microstructural surface texture was dependent on the calcination conditions. Leucite crystallized as the major phase after being heated to >1000°C, although a minor amount of kalsilite was also formed. Prolonged heating for 24 h at 1000°C led to the formation of ∼80 wt% of leucite, along with 20 wt% of remnant glassy phase. The surface of geopolymers heated to 1000°C attained a smooth, glassy texture, although closed porosity persisted until 1100°C. Thermal shrinkage was completed by 1100°C, and the material reached 99.7% of the theoretical density of tetragonal leucite.

The structural evolution and crystallization of a cesium-based geopolymer (Cs 2 O . Al 2 O 3 . 4SiO 2 . 11H 2 O) 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 (Cs 2 O . Al 2 O 3 . 4SiO 2 ) on heating above 9001C. Its low crystallization temperature is believed to be due to the presence of nuclei in the geopolymer precursor, which are formed after curing at 501C 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 12001C. The microstructure of unheated geopolymer had B20-30 nm-sized precipitates that coarsened on heating above 10001C. By 13501C, the geopolymer surface had a smooth, glassy texture, although large macropores and closed pores remained. After heating to 16001C, the closed pores were removed, and the geopolymer reached B98% 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.

The atomic pair distribution function (PDF) method was used to study the structure of cesium aluminosilicate geopolymer (CsThe geopolymer was prepared by reacting metakaolin with cesium silicate solution followed by curing at 50 °C for 24 h in a sealed container. Heating of Cs-geopolymer above 1000 °C resulted in formation of crystalline pollucite (CsAlSi 2 O 6 ). PDF refinement of the pollucite phase formed displayed an excellent fit over the 10-30 Å range when compared with a cubic pollucite model. A poorer fit was attained from 1-10 Å due to an additional amorphous phase present in the heated geopolymer. On the basis of PDF analysis, unheated Cs-geopolymer displayed structural ordering similar to pollucite up to a length scale of ∼9 Å, despite some differences. Our results suggest that hydrated Cs + ions were an integral part of the Cs-geopolymer structure and that most of the water present was not associated with Al-OH or Si-OH bonds.