Structural transformation of zinc in metakaolin-based potassium-geopolymer as revealed by x-ray absorption spectroscopy

The uncertainty and limited reports on zinc (Zn) immobilization mechanisms in geopolymer, both in the initial state and after alteration, hinder the prediction of the long-term stability of solidified wastes. This study aims to elucidate the mechanisms of immobilization by determining the structural configuration of Zn in metakaolin-based potassium geopolymer using X-ray absorption spectroscopy (XAS). Zn-geopolymers were prepared by mixing metakaolin with a ZnCl 2 solution and an activator composed of potassium silicate and potassium hydroxide. The geopolymers were cured at 40 °C and 25 °C for 48 h, then subjected to leaching in deionized water for up to 90 days. Leachate analysis showed that Zn leaching was limited to 0.14%. Aluminum (Al) leaching was lower than that of silicon (Si), while the chloride (Cl) released remained limited. Solid-phase characterization indicated that the primary geopolymer framework was preserved after leaching. XAS analysis revealed that Zn initially exhibited tetrahedral coordination with oxygen (O) and was corner-shared with tetrahedral Si or Al. During leaching, Zn underwent a structural transformation, adopting octahedral coordination and forming a brucite-like co-precipitate with octahedrally coordinated Al. This study shows that Zn remains retained within the geopolymer system under altered conditions, changing its structure.