Mechanical Performance and Carbonation Behaviour of GGBS-Biochar Concrete: A Life Cycle Assessment

The incorporation of supplementary cementitious materials (SCMs) and carbon-rich additives has emerged as an effective approach to improve the performance and durability of concrete while reducing clinker consumption. While fly ash–biochar systems have been investigated for low-carbon concrete, their application is often constrained by slow early-age reactivity and increased porosity. This study investigates the synergistic effects of ground granulated blast-furnace slag (GGBS) and wood-derived biochar as partial cement replacements on the mechanical properties, carbonation behaviour, and microstructural characteristics of concrete. Concrete mixtures containing 20%, 40%, and 60% GGBS and 2.5%, 5%, and 7.5% biochar were evaluated individually and in combination. Mechanical performance was assessed via compressive, tensile, and flexural strengths tests, while water absorption and porosity were determined as durability-related properties. Accelerated carbonation curing (ACC) was employed to enhance CO₂ uptake, which was quantified using thermogravimetric analysis (TGA). The evolution of carbonation products and microstructure was examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results demonstrated a synergistic effect at 20% GGBS and 5% biochar, resulting in a 56.15% increase in CO₂ uptake compared to the control mix, while maintaining compressive strength comparable to conventional concrete. The improved performance was attributed to pore refinement and enhanced matrix densification, whereas higher replacement levels led to increased porosity and reduced mechanical performance. Life cycle assessment (LCA) further indicated reduced environmental impact for optimized mixtures. The findings demonstrate that moderate incorporation of GGBS and biochar can enhance carbonation behaviour while maintaining structural performance, supporting their application in low-clinker concrete systems.