The structural evolution and crystallization of potassium-based geopolymer (K2O.Al2O3.4SiO(2).11H(2)O) on heating was studied by a variety of techniques. On heating from 850-1100 degrees 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 degrees and 1000 degrees C. However, the microstructural surface texture was dependent on the calcination conditions. Leucite crystallized as the major phase after being heated to > 1000 degrees C, although a minor amount of kalsilite was also formed. Prolonged heating for 24 h at 1000 degrees C led to the formation of similar to 80 wt% of leucite, along with 20 wt% of remnant glassy phase. The surface of geopolymers heated to 1000 degrees C attained a smooth, glassy texture, although closed porosity persisted until 1100 degrees C. Thermal shrinkage was completed by 1100 degrees C, and the material reached 99.7% of the theoretical density of tetragonal leucite.