Role of sewage sludge fly ash in controlling microstructure, mechanical performance and environmental safety of fired clay ceramics

The increasing production of sewage sludge and the limited disposal options have intensified interest in the valorisation of sewage sludge fly ash (SSFA) as a secondary raw material in construction materials. This study investigates the influence of high-phosphate SSFA on the microstructure, mechanical performance and environmental safety of fired clay ceramics. Clay-ash composites containing 30% SSFA from two wastewater treatment plants were produced and fired at 1050 °C. The incorporation of SSFA reduced water absorption from 1.8% in the reference sample to 1.1% and 1.5% in ash-modified ceramics. Compressive strength decreased from 98.0 MPa to 80.1-84.7 MPa, while flexural strength increased to 19.0 MPa in samples containing ash from Łódź. SEM observations revealed that porous ash particles modified pore morphology and acted as crack-deflecting inclusions, reducing the brittleness of the ceramic matrix. Radiological analysis showed that the activity concentration index remained below the regulatory limit (I = 0.73-0.92), while heavy-metal leaching values remained within the limits specified for inert waste. The results demonstrate that phosphate-rich sewage sludge fly ash can function both as a sustainable secondary raw material and as a microstructural modifier influencing pore evolution, fracture behaviour and environmental stability of fired clay ceramics.