Background/Objectives: To address the environmental constraints due to cement production, strength properties of high-calcium fly ash based Geo Polymer Concrete (GPC) has been explored in this paper. Methods/Statistical Analysis: High-calcium (class C) fly ash from Neyveli Thermal Power Plant has been used for the production of GPC. Combination of sodium silicate and sodium hydroxide solution was used as alkaline solution. Alkaline solution to fly ash ratio was varied as 0.35 and 0.45. Concentration of sodium hydroxide solution was maintained as 8M and 12M. Three types of curing such as ambient curing, oven curing and External exposure curing were adopted. Findings: The compressive strength of Geo Polymer Concrete was tested at 7, 14, 28 and 60 days. Flexural strength was determined at 28 days. From the test results it was found that 1) Alkaline solution to fly ash ratio of 0.35 and 12M concentration of NaOH solution produced high strength in Geo Polymer Concrete, 2) Compressive strength of external exposure concrete was more than that of oven and ambient cured concrete. Application/Improvements: Class C fly ash can be used for the production of Geo Polymer Concrete with acceptable strength.