Cleaner utilization of co-combustion bottom ash in concrete: Pilot-scale validation for sustainable construction

The co-combustion of biomass and solid recovered fuel generates substantial quantities of bottom ash (BA), and safe value-added utilization in construction remains insufficiently explored. This study evaluates the feasibility of using co-combustion bottom ash (CCBA) as a fine aggregate replacement in concrete through comprehensive mechanical, durability, and environmental assessments supported by pilot-scale validation. Concrete mixtures with 0%, 50%, and 100% CCBA were studied as replacement of natural sand. Incorporating CCBA improved mechanical performance, with a maximum 10.48% increase in 28-day compressive strength of concrete compared to the natural-sand reference. Durability assessment demonstrated excellent resistance to sulfuric acid (5% sulfuric acid solution immersion) with only a 9.7% strength loss at 100% BA replacement, and strength gains of up to 21.3% after sulfate-chloride exposure. Concrete samples also showed strong freeze-thaw resistance, reduced sorptivity with BA content, and improved microstructural densification supported by EIS analysis. Leaching tests confirmed effective immobilization of toxic metals within the cementitious matrix, meeting Finnish environmental regulations without ash pre-treatment. Pilot-scale production of 10 tons of concrete barrier blocks further validated the field performance, showing no visible damage after exposure to harsh Nordic climatic conditions. The results demonstrate that CCBA is a technically viable, environmentally safe, and sustainable alternative to natural sand for large-scale concrete production.