Sustainable UHPC Incorporating Water-Quenched Slag and Incineration Fly Ash for Infrastructure Covers

With the rapid increase in municipal solid waste and the associated production of incineration fly ash (IFA) in Taiwan, sustainable utilization of industrial by-products has become a pressing concern. This study evaluates the mechanical, environmental, and structural performance of ultra-high-performance concrete (UHPC) incorporating water-quenched slag (WQS) and IFA as partial replacements for cement or quartz powder. Laboratory-scale specimens were tested for compressive and flexural strength, followed by full-scale load-bearing tests on trench covers (60 × 35 × 4 cm) and manhole covers (120 × 60 × 5 cm) with varying steel fiber contents and welded steel mesh reinforcement. Mechanical behavior, heavy-metal leaching (TCLP), carbon emissions, and life cycle impact assessment (LCIA) were examined. The results show that WQS maintained or enhanced strength, while IFA caused strength loss and surface corrosion due to gas release during hydration. Trench covers with 15% WQS achieved the highest peak load (14,733 kg), exceeding heavy-traffic requirements, whereas IFA-based covers met the 10-ton standard but showed corrosion. Manhole covers did not reach the 75-ton design load, indicating applicability only for light or non-traffic areas. All UHPC mixes immobilized heavy metals within regulatory limits, and partial cement replacement reduced the carbon footprint by 60–120 kg CO2e/m3. LCIA further indicated that 20% IFA replacement provided the greatest overall environmental benefit. In conclusion, WQS-incorporated UHPC offers reliable structural and environmental performance, while IFA requires pretreatment or modification to ensure long-term durability.