Fly ash from the incineration of domestic waste contains heavy metals, which is harmful to the environment. To reduce and prevent their contamination, heavy metal ions need to be sequestered. In this study, the geopolymer prepared by fly ash, a kind of power plant waste, is used to cure the heavy metal Pb²⁺, and to investigate the effect of different concentrations of Pb²⁺ on the compressive strength of the solidified body at different ages; the curing effect is judged by the toxic leaching concentration of heavy metals; the resistance of the solidified body to immersion is evaluated by comparing the change in strength before and after leaching; the fly ash-based geopolymer solidified body is compared with the cement solidified body in terms of curing effectiveness; the properties of the geopolymer and its mechanism of curing heavy metals are explored by microscopic tests. The results show that the fly ash-based geopolymer solidified body has good resistance to immersion; the optimum curing concentration of Pb²⁺ in fly ash-based geopolymers is 2.0%; compared to pure geopolymers, the strength of the solidified body at 28 d decreases by only 13.0%, and the leaching concentration of Pb²⁺ is 4.73 mg·L⁻¹, which meets the specification requirements; the curing effect of the fly ash-based geopolymer is better than the cement solidified body; the microscopic test results indicate that the curing of Pb²⁺ by the fly ash-based geopolymer is a combination of both chemical bonding and physical fixation.

Spontaneous combustion coal gangue (SCCG) is considered to be an aluminosilicate-based solid waste containing various toxic ions. The alkali-activation method for this material can not only fully use its potential hydration activity but also solidify the hazardous components to some extent. Through introducing additional Pb²⁺, the solidification behavior of heavy metal Pb²⁺ for an SCCG-based geopolymer was studied in the present paper. The solidification efficiencies were evaluated by Pb²⁺ leaching rates under neutral and acidic conditions, while its mechanism was explained by the methods of XRD, TG, FT-IR, SEM, and MIP. The results show that the Pb²⁺ solidification efficiency increases along with the curing age, and acidic rather than neutral conditions lead to a more intensive solidification capacity. Judging by the permissive maximum value of 5 mg/L, the Pb²⁺ original concentrations under neutral and acidic circumstances should be lower at 2.0 wt.% and 3.0 wt.%, respectively. The Pb²⁺ absorption is dominated by the physical process, due to the formation of no new hydration products. However, the Pb²⁺ addition would interrupt the reconstruction of the Si-Al network structure, slowing the accumulation of N-A-S-H gel and the densifying of the matrix. When the Pb²⁺ concentration grows, the sizes of hydration productions shrink continuously, more defects appear in the microstructure of the geopolymer, and the pore structure deteriorates rapidly, all of which accelerate the diffusion of toxic ions to the external condition.