This paper study the strength and durability of the fly ash based Geo- Polymer Concrete (GPC) prepared with sodium silicate and sodium hydroxide as activators. The concretes were prepared with varying fly ash content of 396, 408, 418 & 426 Kg/m3. Ratio of sodium silicate solution-to-sodium hydroxide solution by mass of 0.4 to 2.5 was used. This ratio was fixed at 2.5 for most of the mixtures. Molarity of sodium hydroxide (NaOH) solution used was in the range of 8M to12M. The ratio of activator solution-to-fly ash by mass, in the range of 0.3 and 0.45 was used. Compressive strength of the GPC in the range of 15-30 N/mm2 was obtained. The performance of these concretes in aggressive environments was also studied, using acid resistance and sulphate resistance tests. The test results show that GPC specimens are better in resisting acid attack and sulphate attack than normal concrete comparatively.