Investigation on compressive strength and microstructural characteristics of slag-fly ash-based sustainable concrete under elevated temperature

Purpose Concrete is widely utilized in the construction industry. The concrete-making process requires the hydration of cement, resulting in carbon footprints. Additionally, a significant quantity of natural aggregates are mined illegally, causing ecological imbalance and major environmental problems. granulated blast furnace slag (GBFS) and fly ash (FA), the commonly available industrial by-products, are utilized as fine aggregate and binding material, respectively, for improving the heat-resistance performance of sustainable concrete. Design/methodology/approach M40-grade concrete has been cast with GBFS and FA as alternative natural fine aggregate and binder material, respectively. The uniaxial compression behaviour of the sustainable concrete after exposure to 200°C, 400°C, 600°C and 800°C was examined, including mass loss, failure modes, initial elastic modulus, peak strain, stress–strain behaviour and microstructural analysis. Findings Deterioration of compressive strength and elastic modulus has been observed, but deterioration was 12–23% and 5–14% higher for elastic modulus at 600°C and 800°C, respectively. FA-based blends exhibited higher residual compressive strength and energy absorption capacity up to 600°C. Substitution by GBFS only reduces peak strain and compression toughness, while the combination of GBFS and FA enhances the same. The microstructure and mineralogical phase have improved owing to the combination of FA and GBFS at elevated temperatures due to more peaks of CSH and insignificant micro-cracks. Originality/value This article demonstrates the strength and potential of concrete incorporating GBFS and FA for application in adverse environmental conditions with higher thermal gradients. In the authors’ opinion, test results are in favour of the design and practical implementations of such industrial waste-based sustainable concrete mixes under elevated temperatures.