A detailed study of geopolymeric reaction products and mechanical properties of geopolymer pastes prepared with natural zeolite has been investigated by means of compressive strength, XRD and SEM/EDX analysis. Sodium silicate and sodium hydroxide solutions are used as an alkaline activator. The results of the investigation show that the activator ratio plays an important role on the mechanical development of geopolymer pastes. The geopolymeric gel and CSH phase with a low Ca/Si ratio are found as the main reaction product. The increased intensity of CSH phase has a significant effect on the improvement of the compressive strength. Decreased intensity of clinoptilolite and totally consumed clay mineral upon geopolymerization proves the involvement of the aluminosilicate phases in the geopolymeric reaction. The existence of sodium and increment of Si/Al ratio with respect to original zeolitic tuff were detected. The results also show that the investigated natural zeolite, which is emerged as an environmentally friendly, low-cost material, is suitable for the production of geopolymer cement.