Immobilization mechanism of Pb in fly ash-based geopolymer

•The behavior of Pb in geopolymer depends on the chemical property of the Pb compound.•Pb participates in the formation of geopolymer network, forming leaded amorphous gel.•Dissolution of lead compounds is a vital step in the formation of leaded geopolymer.•Pb compounds, soluble in alkali, are chemically bonded into the geopolymer gel.•PbS, inert to alkali, is trapped in the geopolymer by physical encapsulation.

Geopolymer possesses good immobilization capacity for Pb. There are two rival interpretations regarding the immobilization mechanism of Pb. This research investigates the behavior of 3 Pb compounds in geopolymer and clarifies the immobilization mechanism. When Pb contamination is added in the form, soluble in sodium hydroxide solution, the Pb is converted to an amorphous form and participates in the formation of geopolymer network. Successful immobilization of these species relies on chemical bonding and physical encapsulation. On the contrary, the Pb compound inert to sodium hydroxide solution is segregated from the binder and trapped by physical encapsulation.