Structural Performance of Geopolymer Concrete Incorporating Nigerian Fly Ash and Kaolin Under Elevated Temperatures

Geopolymer concrete offers a low-carbon alternative to ordinary Portland cement concrete and is of particular relevance in Nigeria, where fly ash from coal-fired power generation and kaolin deposits provide locally available aluminosilicate precursors. This study investigates the structural performance of geopolymer concrete incorporating Nigerian fly ash from the Egbin Thermal Station in Lagos State and calcined kaolin from Kankara in Katsina State under ambient and elevated temperature conditions, addressing a research gap in geopolymer concrete behaviour using Nigerian source materials. Geopolymer mixes were formulated at fly ash to kaolin mass ratios of 100:0, 75:25, and 50:50, activated with sodium hydroxide and sodium silicate solutions at molar ratios of 8, 10, and 12. Mechanical properties at ambient temperature and after controlled exposure to 200, 400, 600, and 800 degrees Celsius were characterised through compressive strength, split tensile strength, elastic modulus, and microstructural analysis by scanning electron microscopy and X-ray diffraction. Results indicate that the 75:25 fly ash to kaolin blend at 10 molar NaOH activation achieved the highest ambient compressive strength of 41.8 N/mm2, significantly outperforming the reference ordinary Portland cement concrete of 38.2 N/mm2. After exposure to 400 degrees Celsius, the geopolymer mix retained 91 percent of ambient strength, compared to 67 percent for the Portland cement control, demonstrating superior fire resistance. Microstructural analysis confirmed geopolymer gel homogeneity and reduced micro-cracking in post-heating specimens. The study establishes performance benchmarks for geopolymer concrete using local Nigerian materials and recommends field trials in structural column applications in high-temperature industrial environments. Keywords: geopolymer concrete, fly ash, kaolin, elevated temperature, structural performance.