Valorisation of Inorganic Fractions of Waste Generated by Hydrothermal Treatment of Sewage Sludge in Alkaline Cement

Hydrothermal processing technology provides an innovative and promising solution to achieve significant reductions in the volume of sewage sludge and the recovery of raw materials. In this work, we evaluated the possibility of using inorganic mineral residue (IMR) from hydrothermal sewage treatment in a geopolymer binder. Initially, the waste was characterised, and thermal treatment was carried out at 800 °C to eliminate the organic matter. Calcined clay (3 h at 800 °C) was used to manufacture the geopolymer. Calcined clay/residue mixtures at ratios of 100/0, 90/10, and 80/20 were prepared and activated using a 6 M NaOH solution. The pastes were cured for 20 h at 85 °C, their compressive strengths were evaluated, and the reaction products were characterised using XRD and microscopy. The results show low reactivity in the residue. However, it was observed that some of the phosphorus present in the residue could be incorporated into the products of alkaline activation. A leaching study was also carried out, in which all the toxic metals remained confined except for chromium.