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An experimental investigation was conducted to study the performance of fly ash based geopolymer mortar specimens inMagnesium Sulphate solution. Specimens were manufactured from low calcium fly ash by activation with a mixture of SodiumHydroxide and Sodium Silicate solution and cured thermally. 10% by weight Magnesium Sulphate solution was usedto soak the specimen up to 24 weeks. Performance of the specimens was evaluated in terms of visual appearance, variationof pH of solution, change in weight, and change in compressive strength over the exposure period. White deposits occurredon the surface of specimen which was initially soft but later converted to hard crystals. pH of solution increased noticeablyduring the initial weeks which indicate migration of alkalis from mortar specimens. At the end of 24 weeks samples experiencedvery little weight gain and recorded a loss of compressive strength by up to 56%.