A Performance-Weighted Environmental Assessment of Ultra-High-Volume Fly Ash Substitution in Portland Cement Concrete

Fly ash substitution for cement in Portland cement concrete (PCC) has been regarded as a sustainable solution, but its widespread application remains constrained by concerns over mechanical performance and durability of PCC, especially at higher replacement rates. This study evaluates PCC mixes incorporating fly ash Type C (FA-C) or Type F (FA-F) across cement replacement rates from 10% to 90%, tracking fresh-state workability, compressive strength, and surface electrical resistivity at 7, 14, and 28 curing days. A process-based life cycle assessment (LCA) with the TRACI 2.1 method quantified global warming potential (GWP, kg CO2/m3) under a raw-material-plus-batching-electricity boundary for each mix. A Performance Index (PI) normalizes GWP against both compressive strength and electrical resistivity, producing a performance-weighted environmental efficiency metric (GWP/PI). A sensitivity analysis across five weighting scenarios tested the robustness of mix rankings under varying priorities for structural versus ironic transport resistance performance, and a structural threshold analysis identified mixes meeting strength requirements. FA-C at 50% cement replacement exceeded the OPC control in 28-day compressive strength (42.9 vs. 36.2 MPa) and electrical resistivity (9.88 vs. 8.50 kΩ·cm), while reducing GWP by 48.3% relative to the OPC control (40.24 kg CO2/m3). FA-F at 30–50% replacement exhibited a distinct strength–resistivity decoupling, demonstrating that strength only evaluation underrepresents the environmental efficiency of durability-critical applications. The GWP/PI metric revealed that raw GWP reduction alone misrepresents environmental efficiency. FA-C at 50% achieved a GWP/PI of 17.73, which is a 56% improvement over the OPC control. These findings question the conventional <30% substitution ceiling at 28 days under standard moisture curing and demonstrate that performance-weighted LCA metrics provide a more informed basis for sustainable concrete mix design.