Experimental Investigation and Microstructural Characterisation of Alkali-Activated Geopolymer Mortar from Nigerian Industrial By-Products for Structural Repair Applications

Structural repair mortars for deteriorated reinforced concrete must exhibit compatibility with existing concrete substrates in terms of modulus of elasticity, thermal expansion coefficient, and bond strength, while demonstrating long-term durability in tropical climatic exposure. Alkali-activated geopolymer mortars offer potential superiority over cement-based repair systems in both sustainability and durability, but their formulation using Nigerian industrial by-products and performance characterisation for structural repair applications represent a largely unexplored research area. This study presents an experimental investigation and microstructural characterisation of alkali-activated geopolymer mortars formulated from calcined clay from Kogi State, ground granulated blast furnace slag from Ajaokuta Steel Company in the North Central geopolitical zone, and palm oil fuel ash from Ogun State in the South West geopolitical zone. Mortar batches were prepared at precursor blend ratios of calcined clay to slag to palm oil fuel ash of 60:40:0, 50:30:20, and 40:40:20, activated with sodium hydroxide and sodium silicate at silica moduli of 1.0, 1.5, and 2.0. A comprehensive mechanical and durability characterisation programme was conducted encompassing fresh state workability, setting time, compressive and flexural strength development to 90 days, bond strength to existing concrete substrate by slant shear and pull-off tests, chloride permeability, freeze-thaw resistance, and sulphate attack resistance. Microstructural analysis employed scanning electron microscopy with energy dispersive spectroscopy, X-ray diffraction, and thermogravimetric analysis to characterise reaction product evolution. Results identify the 50:30:20 precursor blend at silica modulus 1.5 as the optimal formulation, achieving 28-day compressive strength of 38.6 N/mm2, bond strength 41 percent higher than an equivalent Portland cement repair mortar, and chloride permeability classified as very low by ASTM C1202. Keywords: geopolymer mortar, alkali activation, structural repair, Nigerian industrial by-products, microstructure.