Mechanical Properties, Durability, and Microstructural Characterisation of Steel Fibre-Reinforced Fly Ash Geopolymer Concrete

India generates approximately 220 million tonnes of fly ash (FA) annually from coal-fired thermal power plants — NTPC, TANGEDCO, and APGENCO collectively contributing over 60 million tonnes — of which only 67% is currently utilised in cement manufacture, embankment filling, and mine reclamation. The 33% unutilised fraction represents both an environmental liability and an unexploited pozzolanic resource of considerable value for producing geopolymer concrete (GPC), a cement-free binder system in which alkaline activation of aluminosilicate-rich fly ash by sodium hydroxide and sodium silicate produces an amorphous three-dimensional N-A-S-H (sodium aluminosilicate hydrate) gel network with compressive strength comparable to OPC concrete but substantially lower CO₂ footprint.This investigation characterises the combined influence of fly ash replacement level (0-100% by binder weight in six increments), alkaline activator ratio (NaOH:Na₂SiO₃ mass ratios of 1:1.5, 1:2.0, and 1:2.5), and hooked-end steel fibre volume fraction (0%, 0.5%, 1.0%, 1.5%) on compressive strength at 7, 28, and 90 days, split tensile strength, flexural strength with post-crack ductility, rapid chloride penetration test (RCPT) permeability, and drying shrinkage. A total of 132 mix combinations were prepared and tested at the Advanced Structural Materials Laboratory, Mepco Schlenk Engineering College. Workability was assessed by flow table and T500 slump flow. Microstructural characterisation using SEM, EDS, and XRD identifies the N-A-S-H gel formation, unreacted FA particle morphology, and steel fibre-matrix interface transition zone (ITZ).The optimum mix — 70% FA replacement, NaOH:Na₂SiO₃ = 1:2.0, 1.0% fibre volume fraction — achieves 28-day compressive strength of 54.7 MPa, flexural strength of 7.2 MPa, split tensile 4.6 MPa, RCPT 1,124 Coulombs (Low permeability class), and 37.3% lower CO₂ emission than an equivalent OPC M40 concrete. Steel fibre addition at 1.0% volume fraction increases flexural strength by 43.1% over unreinforced GPC and nearly eliminates post-crack brittleness, measured by toughness indices I₅=3.9 and I₁₀=7.4. Beyond 1.5% fibre volume, workability drops below T500 = 3.5 seconds, restricting practical application for pump-placed construction.