Upcycling of furnace bottom ash as sustainable materials for low carbon concrete

Furnace Bottom Ash (FBA) has the potential for use as a supplementary cementitious material due to its aluminosilicate composition; however, its low reactivity and variable quality continue to limit practical application. Although several activation approaches have been proposed, many remain unsustainable or impractical for large-scale deployment. This study investigates the co-grinding of FBA with ground granulated blast furnace slag (GGBFS) as a practical activation route for upcycling ground FBA (GFBA) in low-carbon ternary concrete. Three GGBFS: GFBA co-blends with ratios of 2.5:1, 2.5:2, and 2.5:3 were prepared and compared with their counterparts from separately ground blends. A comprehensive experimental program was conducted to evaluate fresh properties, compressive strength (CS), flexural strength (FS), split tensile strength (STS), modulus of elasticity (MOE), water absorption, and abrasion resistance. Response surface methodology was employed to quantify the individual and interaction effects of the grinding method and GFBA replacement level, followed by multi-criteria optimisation using desirability analysis, Grey Relational Analysis, TOPSIS, and the Weighted Sum Model to identify the optimal mixture. Microstructural mechanisms governing performance were examined using SEM-EDS, XRD, and TGA, while environmental benefits were assessed through life cycle assessment. Results demonstrate the clear superiority of co-grinding, with co-ground mixtures exhibiting significantly enhanced mechanical performance. On average, co-ground samples achieved improvements of approximately 10.2% in CS, 6.9% in MOE, and 16.7% in FS compared with their separate ground counterparts. Abrasion resistance was also remarkably enhanced, with abrasion losses reduced by 16.2% and 32.3% at 28 and 56 days, respectively, relative to the mean values of the separate-grinding groups. Microstructural analysis confirmed that co-grinding promotes a denser, more homogeneous cementitious matrix. Life cycle assessment further revealed substantial environmental benefits, including a reduction of up to 30% in greenhouse gas emissions and 28% in ozone formation potential. These findings validate co-grinding as an effective enabling strategy for upcycling FBA into a high-performance, low-carbon binder system, aligning with cleaner-material design principles through clinker reduction and industrial by-product valorisation while maintaining service-relevant performance.