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.