Environmental safety of incinerated sewage sludge ash reuse in cement-based composites under natural exposure

The growing demand for sustainable construction materials has increased interest in utilizing incinerated sewage sludge ash (ISSA) as a secondary resource in cement-based applications. The environmental safety of ISSA, in terms of heavy metal leaching under natural conditions, remains a key barrier to regulatory acceptance. This study describes the environmental performance of ISSA-cement composites exposed to outdoor conditions over a 62-day period. The leaching behavior of Zn, Cd, Pb, Cr, Ni, and Cu was assessed in rainwater-exposed samples containing 0-35% ISSA by mass. The results showed that Zn and Cd concentrations in the collected leachates were substantially lower than in reference rainwater, while the leachate pH remained slightly alkaline, mostly within the range of 8-9. These observations are consistent with pH-buffered geochemical immobilization mechanisms, including possible sorption and precipitation reactions within the cementitious matrix. Obtained results provide field-based evidence supporting the safe reuse of ISSA within circular economy and waste management frameworks.