Sustainable concrete with olive oil sludge: mechanical performance, weathering resistance, and life cycle metrics

This study explores the use of olive oil mill sludge (OOMS) in wet, thermally dried, and biochar forms as sustainable partial replacements for cement in concrete. A two-level factorial experimental design was used to investigate the effects of sludge type, treatment temperature, and replacement level (5-15%) on mechanical performance, durability, microstructure, and life cycle metrics. Microstructural analyses revealed enhanced matrix densification with thermally dried sludge, while biochar increased porosity at higher contents. Compressive strength reached 32 MPa with 15% thermally dried sludge and 35 MPa with 5% biochar. Sludge-modified concretes maintained structural integrity under wetting-drying and freeze-thaw cycles, though microbial biofilm formation was highest with thermally dried sludge. Life cycle assessment showed that cement production contributes more than 80% of total impact. Replacing cement with thermally dried sludge reduced global warming potential by 10%, while biochar reduced human health and resource impacts by 5-7% and provided additional energy savings through pyrolysis. Life cycle costing revealed that the lowest production cost was achieved with the thermally dried sludge mixture. Multi-criteria decision analysis ranked 15% thermally dried sludge as the most sustainable option, followed by 5% biochar.