Effects of Cr3+, Cu2+, and Pb2+ on Fly Ash based Geopolymer

The effects of Cr3+, Cu2+, and Pb2+ on compressive strength, reaction products, and pore structures of fly ash based geopolymer were studied. In addition, the immobilization and bonding interaction between heavy metal and fly ash based geopolymers were investigated by X-ray photoelectron spectroscopic (XPS) and environmental scanning electron microscope (ESEM) techniques. The experimental results showed that the incorporation of Cr3+, Cu2+, and Pb2+ had a great effect on the later compressive strength and resulted in producing reinhardbraunsite in the solidified body. Moreover, the Pb2+ reduced the total pore volume of the solidified body, while Cr3+ and Cu2+ increased it. The XPS results indicated that O(1s), Si(2p), and Al(2p) bind energy increased due to Cr3+ and Cu2+ addition, but it did not change significantly due to Pb2+ addition. The microstructure of calcium silicate hydrate (C-S-H) gel and sodium silicoaluminate hydrate (N-A-S-H) gel changed in different degree according to the ESEM results. The immobilization of Cr3+, Cu2+, and Pb2+ using fly ash based geopolymer is attributed not only to the physical encapsulation, but also to the chemical bonding interaction.