The world is facing the challenges of climate changes due to the increase in CO2 emissions. Cement production is one of the biggest contributors to CO2 emissions due to combustion processes that require high temperatures. The new development in building construction showed that fly ash based Geopolymer concrete can be as structure materials to reduce or even eliminate ordinary Portland cement concrete. This paper presented the research results of fly ash based geopolymer concrete mechanical properties, like the compressive strength, flexural strength, and Elastic Modulus. The data were collected from the test results of geopolymer concrete specimens made from 2 sources of Fly Ash with varying concentration of sodium hydroxide solution from 4 M up to 12 M. The range of achieved compressive strength was from normal to high strength concrete. The correlation between the compressive strength and flexural strength and the Elastic Modulus of geopolymer concrete will be the highlight of this paper. The mathematical formulation of compressive strength and its flexural strength and Elastic modulus will be determined and compared with normal Portland Cement Concrete.