The influence of calcium and its content on the structure formation, hardening, and performance of fly ash based geopolymeric binder was the objective of our investigation. Calcium hydroxide was added to fly ash in different amounts. Since it is known that the formed structure determines certain properties of the material, the coherence between different types and various ratios of the reaction products on thermal properties such as strength after thermal treatment up to 1,100 degrees C, thermal resistance under load, creep in compression, and axial dilation were investigated. The results were compared to the type and composition of the reaction product, which was detected, for example, by Si-29 NMR spectroscopy and X-ray diffraction. Along with calcium containing zeolitic phases, the calcium built C-S-H-phases using the silicon from the fly ash, both of which crystallize or convert into new phases at elevated temperatures.