The current work presents the effect of alkali concentration on fly ash based geopolymer material. In this work geopolymer material was prepared with a fixed Si/Al ratio (1.85: 1) and varied solid to water ratio and NaOH concentration. All samples were tested to establish the role of alkali on the strength. Samples prepared with 8M of NaOH showed the highest compressive strength. In this regards, Fourier transform infrared spectroscopy (FTIR) results support this observation where broad band becomes sharper and shifts towards lower wavenumbers. In XRD the broad hump slightly shifted towards higher values which indicate the formation of new product. These findings show that to obtain required processing time at desired mechanical strength for a specific application of geopolymer, the suitable concentration of alkali must be established. © 2014 Trans Tech Publications, Switzerland.