This paper presents the results of a study held to compare the properties of geopolymer mortar prepared with the use of class C fly ash. Utilization of cement kiln dust (by-pass) with its high alkali content in the activation of geopolymer mortar specimens to create nonconventional cementitious binders was investigated. Many different geopolymer mortar mixtures were tested, all prepared by using fly ash-to-sand ratio of 1:3. Mixtures evaluation was based upon replacing the fly ash with cement kiln dust (by-pass) in the selected mixtures. The cement kiln dust (by-pass) content percentages were 5%, 10%, 15% and 20% by weight of fly ash as a partial replacement. Alkaline liquid to fly ash ratio of 0.50 was used. At the used alkaline liquid to fly ash ratio, the strength developments have been observed at 3, 7, and 28 days. Geopolymer mortar specimens were prepared with different concentrations of NaOH solution of M10, M14 and M16 and were cured at 60 ºC for 24 hours inside an oven. Thus, it is necessary to study the effects of the geopolymer binder on the behavior of compressive strength of geopolymer mortar. In this study, the Local Alkaline Activator in Egypt and Natural River sand as a fine aggregate in fly ash based-geopolymer mortar was investigated. This paper illustrates the development of mechanical properties. Hence it has been found that the compressive strength for geopolymer mortar at 28 days was in the range of 29.9MPa – 41.6MPa