Resistance of metakaolin-MSWI fly ash based geopolymer to acid and alkaline environments

•The immobilization of MSWI fly ash with metakaolin-based geopolymers is explored.•The resistance of this geopolymer to acid and alkali is investigated.•This geopolymer presented excellent stability in the acid and alkaline environments.•This immobilization is an effective method for reducing the amount of fly ash.

The goal of this work is to study the acid and alkali resistance of metakaolin-municipal solid waste incineration (MSWI) fly ash based geopolymer as heavy metals' stabilization/solidification (S/S) binder. The S/S effectiveness and mechanism of metakaolin-MSWI fly ash based geopolymers were investigated by the compressive strength test, leaching concentration of heavy metals, X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The results indicated that the lowest compressive strength had 36.1 MPa after acid rain leaching for 2000 mL at pH = 3.0, and the compressive strength could still reach 40.0 MPa after aqueous alkali immersing for 30 d. The leaching concentration of Pb, Zn, Cu, Cr, Cd and Hg from MSWI fly ash significantly decreased after geopolymerization, values 0.003, 0.006, 0.108, 0.292, 0.003 and 0.003 mg·L− 1 respectively. XRD analyses showed that no new crystalline phase appeared in the mineral compositions when MSWI fly ash was present. SEM results demonstrated that the metakaolin-MSWI fly ash based geopolymer presented a more closely layered structure, and thus, heavy metals were prevented from leaching out. FTIR images indicated that the MSWI fly ash was involved in the geopolymerization. This work is beneficial to the understanding and future application of metakaolin-MSWI fly ash-based geopolymer as S/S material.