Evaluation of Fly Ash-Based Geopolymer Mortar for Repair of Deteriorated Reinforced Concrete Structures in North East Nigeria

This study evaluates the performance of fly ash-based geopolymer mortar as a repair material for deteriorated reinforced concrete structures in the North East geopolitical zone of Nigeria. Much of North East Nigeria's built infrastructure suffered damage during the Boko Haram insurgency and subsequent reconstruction is ongoing, while conventional cementitious repair mortars have shown limited durability in the hot, semi-arid exposure conditions of the region. Fly ash from a power station source is activated with sodium hydroxide and sodium silicate solutions at different molar ratios to produce geopolymer mortars. Mix variables include activator modulus (1.0, 1.5, and 2.0) and fly ash-to-activator ratio. Compressive strength, bond strength to substrate concrete (pull-off test), drying shrinkage, and resistance to sulphate and chloride attack are evaluated following relevant ASTM and EN test methods. Performance is compared with a proprietary polymer-modified Portland cement repair mortar as the reference. Findings show that optimised geopolymer mortar achieves compressive strength above 45 MPa at 28 days, bond strength exceeding 1.5 MPa, and superior resistance to sulphate attack relative to the reference mortar. Drying shrinkage is marginally higher than the polymer-modified reference. The study recommends geopolymer mortar as a technically superior and locally producible repair material for structural rehabilitation in North East Nigeria. Keywords: geopolymer mortar, fly ash, concrete repair, North East Nigeria, durability.