Factors Affecting Consolidation in Geopolymers for Stabilization of Galvanic Sludge

This study explores the potential of metakaolin-based geopolymers, activated using sodium hydroxide and sodium silicate, for the solidification and stabilization of heavy metals present in galvanic sludge—a hazardous industrial waste rich in chromium (Cr), nickel (Ni), and iron (Fe). The research investigates factors affecting the cold consolidation of the pastes, such as NaOH molarity (8 or 10 M) and time of preparation of activating solutions (24 h in advance or soon before the fresh paste preparation), the sequence of experimental steps (the sludge added to the fresh paste or to the powder of metakaolin) and amount of waste (10 or 20 per cent by weight over metakaolin). The final products were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and environmental scanning electron microscopy (ESEM). Mechanical performance and durability assessments, including compressive strength and water stability tests, were conducted to evaluate the suitability of the geopolymer for construction applications. Leaching tests according to EU regulation demonstrated promising heavy metal immobilization, highlighting the effectiveness of the geopolymerization process in reducing metal leachability. It was found that the factors affecting immobilization are more evident for Cr than for Ni, whose immobilization percentages are very high. In particular, it was observed that preparing the mixture by adding sludge after metakaolin activation increased Cr immobilization from 83% to 89%. Similarly, preparing the activating solution 24 h before mixing the sludge and geopolymer increased the percentage from 89 to 95.