Integrated mechanical, durability, and sustainability assessment of low-clinker hybrid-blended HSC under sulfate and elevated temperatures exposure conditions

Hybrid-blended waste powders can produce high-strength concrete (HSC) with enhanced durability and improved environmental sustainability through reduced embodied carbon. In this study, nine blended HSC mixtures incorporating silica fume (SF), dealuminated metakaolin (DK), and limestone powder (LS) in binary, ternary, and quaternary binder systems were produced at a constant water-to-binder ratio of 0.30 and cement replacement levels up to 35%. At ambient conditions, results showed that SF-DK systems exhibited the greatest mechanical properties enhancement, with the 15SF5DK mixture achieving the highest 28-day compressive strength, approximately 39.07% higher than the control mixture, with notable improvements in splitting tensile and flexural strengths. Additionally, scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM–EDX) analyses suggested that this performance was driven by microstructural densification and a reduced Ca/Si ratio of 2.21 within the binding matrix. Transport-related properties were significantly improved in the blended systems, with water absorption and water penetration depth reduced by up to 40.9% and 29%, respectively, indicating a denser microstructure and reduced capillary connectivity. Furthermore, 8 months of exposure to a 10% sodium sulfate solution revealed a transition from an early transport-controlled regime to a later chemistry-governed deterioration phase. While SF-rich and 15SF10LS systems showed superior stability, standalone DK mixtures experienced severe late-stage degradation—with 10DK recording the highest 8-month mass loss of 5.80% and lowest strength retention (56.2%) due to reactive alumina phases. However, the quaternary blends successfully mitigated this chemical instability, restricting 8-month mass loss while preserving high residual strength. Following exposure to 500 °C, SEM–EDX microstructural observations revealed more silica-rich binding phases and lower Ca/Si ratios in the high-performing blended systems, consistent with improved residual compressive strength and reduced microstructural deterioration. From a sustainability perspective, the quaternary blend 15SF10LS10DK achieved the greatest environmental benefit, reducing embodied carbon by approximately 31.4% while maintaining high-strength performance. Collectively, the findings of the current study demonstrate that the strategic incorporation of SF, DK, and LS can effectively enhance the mechanical performance, durability, and environmental impact of low-clinker HSC.