Compressive strength and environmental assessment of UHPC matrices incorporating electric arc furnace slag and off specification fly ash

Ultra-High-Performance Concrete (UHPC) offers exceptional mechanical performance but typically relies on large quantities of Portland cement and highly processed powders, increasing its environmental burden. This study investigated the feasibility of producing high-strength UHPC with fine Electric Arc Furnace Slag (EAFS) and high-loss-on-ignition Off-Specification Fly Ash (OSFA). The raw materials were characterized by X-ray fluorescence, X-ray diffraction, scanning electron microscopy, and particle-size analysis. An orthogonal central composite design was combined with the modified Andreasen and Andersen packing model to formulate 18 mixtures. Portland cement content, water-to-binder ratio, and polycarboxylate-based high-range water-reducer dosage were varied, while EAFS, OSFA, and quartz micro-sand were adjusted through particle packing and volumetric closure. Fresh performance was assessed through spread flow, and compressive strength was measured at 1, 7, and 28 days under conventional curing. Spread flow ranged from 199.2 to 273.0 mm, while 28-day compressive strength varied between 130.4 and 150.1 MPa. Water-to-binder ratio predominantly controlled flowability, whereas the coupled cement-rich composition and admixture dosage governed strength development. The best-performing experimental mixture, containing 650 kg/m³ PC, 262.8 kg/m³ EAFS, and 61.5 kg/m³ OSFA at a w/b ratio of 0.170, achieved the highest 28-day compressive strength of 150.1 MPa while maintaining a spread flow of 273 mm. The study focuses on the unreinforced UHPC matrix; therefore, the hardened-state assessment is restricted to compressive strength and should not be interpreted as a comprehensive evaluation of the tensile, flexural, post-cracking, or structural performance of fiber-reinforced UHPC. Furthermore, under the adopted cradle-to-gate system boundary, cut-off allocation approach, and regional transportation assumptions, the life-cycle assessment indicated around 5.4–19.3% lower environmental burden across the ten evaluated impact categories relative to conventional UHPC, including a 19.3% reduction in global warming potential. Overall, the results demonstrate that EAFS and high-LOI OSFA can be incorporated into dense, high-compressive-strength UHPC matrices through integrated statistical and packing-based design, although tensile, flexural, durability, and microstructural characterization is required before their broader structural performance can be established.