Eco-Mechanical Optimization of Hybrid Fly Ash-Marble Dust Concrete: Microstructural Densification and Decarbonization Potential

The rapid expansion of global infrastructure demands sustainable alternatives to highly carbon-intensive Ordinary Portland Cement (OPC) and depleting natural aggregates. Concurrently, the accumulation of industrial by-products, such as coal fly ash (FA) and marble dust (MD), presents severe environmental and disposal challenges. This study investigates the synergistic effects of a hybrid concrete matrix utilizing FA as a partial cement replacement (20%, 30%, and 40%) and MD as a constant fine aggregate replacement (25%), specifically optimized for rigid pavement applications. Comprehensive experimental evaluations including compressive strength, splitting tensile strength, and water absorption (sorptivity were conducted at 7, 28, and 56-day curing intervals. The findings establish a precise microstructural optimum at the 30% FA and 25% MD substitution ratio. While early-age strength exhibited a predictable deficit due to the dilution of OPC, the optimal hybrid mix achieved a 56-day compressive strength of 4,226 psi, surpassing the 4,000-psi rigid pavement design requirement and exceeding the control mix by 7%. This late-age surge is attributed to a dual-action densification mechanism: the highly crystalline MD acts as an early-age physical micro-filler, while the amorphous FA drives late-age pozzolanic secondary hydration. Furthermore, this synergistic packing heavily restricted the capillary pore network, drastically reducing water absorption and enhancing macro-level durability against moisture ingress. Beyond structural and durability enhancements, the optimized matrix reduces raw binder costs by approximately 30% and significantly lowers the clinker factor. By mitigating greenhouse gas emissions and generating monetizable carbon credits on the voluntary market, this hybrid concrete presents a highly scalable, economically incentivized decarbonization pathway for the construction sector.