Synergistic biochar–fly ash blended cementitious systems: Experimental, microstructural, and predictive insights into strength development of sustainable mortar and concrete

The decarbonization of cementitious materials has accelerated interest in hybrid binders that reduce clinker content while maintaining mechanical performance and durability. This study examines the combined use of Class F fly ash (FA) and Arundo donax–derived biochar (BC) as partial cement replacements in mortar and concrete, integrating experimental testing with predictive modeling of strength development. Nine mortar mixtures containing 0–25 % FA and 0–10 % BC were evaluated for workability, setting time, and compressive strength up to 60 days. Based on statistical performance rankings, selected mixtures with 15 % FA and varying BC contents were scaled up for concrete testing, including compressive strength, surface electrical resistivity, and microstructural analysis using scanning electron microscopy (SEM). Results identify 15 % FA combined with approximately 5 % BC as the most effective composition, providing a favorable balance between mechanical performance, durability indicators, cost efficiency, and carbon reduction. This mixture achieved compressive strength comparable to the control at later ages while limiting the early-age strength penalties associated with higher biochar dosages. The observed synergy arises from FA-driven pozzolanic densification and BC-induced internal curing that supports sustained hydration. Predictive modeling confirmed this composition as an optimal region within the FA–BC design space, yielding up to 11 % material cost savings and 160–180 kg CO₂/m³ reduction without compromising structural performance. Overall, moderate biochar incorporation in conjunction with optimized fly ash replacement offers a practical and scalable pathway for low-carbon cementitious materials. • Investigated the combined use of fly ash and Arundo donax biochar as partial cement replacements in mortar and concrete. • Biochar at 5–10 % reduced workability and accelerated setting, while enhancing later-age hydration via internal curing. • A 15 % FA replacement optimized compressive strength, achieving 42 MPa at 60 days, comparable to the control mixture. • Concrete with 15 % FA and 5 % BC reduced material cost by up to 11 % and CO₂ by ∼160 kg/m³ while maintaining performance. • Microstructural analysis showed dense C–S–H near FA particles and localized hydration in biochar pores, indicating synergy.