Strength and Microstructural Assessment of Sustainable Green Concrete

Based on the experimental investigation into the strength and microstructural behaviour of ternary blend geopolymer concrete, the following conclusions are drawn:   • The optimum binder proportion for maximum compressive strength under ambient curing is FA 60%: GGBS 25%: MK 15% (F60G25M15 — Mix 3), with 1.0% steel fiber by volume. • The maximum 28-day compressive strength achieved was 55.19 N/mm² for Mix 3 with 1.0% steel fiber, representing a 33.8% gain over the OPC reference mix (41.26 N/mm²). • Increasing metakaolin content from 5% to 15% (with proportional reduction in GGBS) progressively enhanced compressive strength, attributable to the higher reactive surface area and amorphous aluminosilicate content of MK. • Steel fiber addition at 1.0% provided an incremental strength improvement of 7.0–15% over plain geopolymer mixes, with greater enhancement in higher-strength mixes. • SEM micrographs confirmed that FA morphology is spherical, MK is angular and platy, and GGBS is angular and irregular. The hardened concrete matrix exhibited a dense, compact reaction product consistent with dual C-A-S-H and N-A-S-H gel formation. • XRD analysis of the hardened concrete identified Quartz, Calcite, Gismondine, and prominent C-S-H gel peaks (2θ = 37–43°), confirming that C-A-S-H gel formation is the primary mechanism of strength development. • EDAX elemental analysis showed increasing Si and Ca content from Mix 1 to Mix 3, corroborating the microstructural strength development trend and the role of calcium in C-A-S-H bridging. • The ternary blend FA–GGBS–MK geopolymer concrete presents a technically viable and environmentally sustainable alternative to OPC concrete for structural applications, offering significant CO₂ reduction through utilisation of industrial by-products.

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