Geopolymer mortars were produced by replacement of high calcium fly ash with cement at the percentages of 5%, 10%, 15%, 20%, 25%, and 100%. Sodium hydroxide was used as the activator and sodium/binder ratios, by weight, were 10%, 12%, 14%, 16%, 18%, and 20% in mortar mixtures. Some physical properties such as unit weight, water absorption, apparent porosity and ultrasound pulse velocity tests were performed on specimens kept in ambient temperature. Flexural and compressive strength tests were made on specimens kept in ambient temperature, and exposed to temperatures of 200 degrees C, 400 degrees C, 600 degrees C, and 800 degrees C for residual performances of mortars. After exposing of specimens to high temperatures, ultrasound pulse velocity and loss in weights were also determined for deterioration levels. An experimental design is also achieved to find optimum solutions for Na/binder and cement/fly ash ratios by using D-optimal design method after establishing response surface models for compressive and flexural tensile strengths. The highest compressive strength of 35.65 MPa was obtained in mortar specimens containing 14% sodium at 20% cement replacement with fly ash. Similarly, residual strengths were observed higher than those of other mortar specimens at the same sodium and cement replacement ratios.