Linear Modelling and Performance Limits of Fly-Ash and GGBS Substitution for Low-Carbon RC 25/30 Concrete in the United Kingdom

Abstract

Embodied carbon from ordinary Portland cement dominates the cradle-to-gate footprint of structural concrete. This study evaluates the decarbonisation potential of fly ash (FA) and ground-granulated blast-furnace slag (GGBS) in a UK reference mix RC 25/30. Twenty-seven FA and twenty-three GGBS environmental-product-declaration (EPD) data points, which report environmental impacts of materials, were converted to kilograms of carbon dioxide equivalent per kilogram (kg CO₂ e kg⁻¹) and fitted with ordinary least-squares models (R² > 0.98). Within a 0–50% substitution window, embodied carbon declines almost linearly; 20% FA lowers emissions by 11.8% and 40% GGBS by 27.2%. Strength datasets confirm safe windows of 15–20% FA and 30–50% GGBS, whereas FA above 30% delays early-age strength. Sensitivity analyses show that the carbon benefit disappears at transport distances above 2,655 km (road) or 15,110 km (sea) and is further reduced when carbonation uptake and high-loss-on-ignition (a measure of unburnt material present in fly ash) processing are considered. A decision matrix links carbon targets, programme constraints, and supply limits to recommended substitution levels. Results indicate that moderate FA and GGBS replacements deliver reliable reductions without compromising performance, but only when addressing regional sourcing, material quality, and whole-life assessments. Future work should incorporate dynamic allocation factors and region-specific transport inventories.