Transformation of hazardous residues from laboratory waste treatment plant into compressed concrete paving units

Abstract In this study, we investigated the transformation of hazardous residues, including fly ash, silica gel, bottom ash, and sludge, from a laboratory waste incineration system at the Sustainable Environment Research Center into compressed concrete paving units (CCPU). The raw fly ash was washed twice with water to remove Cl − . The secondary washed fly ash (SWFA) was then mixed with silica gel, bottom ash, and sludge using various formulations designed through response surface methodology (RSM) in Design Expert 8.0.6 software. The mixed materials were subsequently melted at 1,400 °C. The results of the RSM analysis indicated that the optimal conditions were achieved when the encapsulating phase (silica gel) comprised 60% of the total sample, while the washed fly ash constituted 10% of the encapsulated phase (SWFA, bottom ash, and sludge). This configuration minimized the amount of unmelted materials and maximized the percentage of mass reduction. The slag with optimal melting properties was combined with cullet, clay, cement, and water, then molded, cured, and transformed into CCPU. The physical properties of specimens met the standards for Grade-A CCPU, and the leaching availability results indicated that metals were effectively encapsulated within the CCPU. Overall, the CCPU provides adequate physical strength and will not contribute to environmental pollution when recycled.