Effect of Municipal Solid Waste Incineration Ash on Microstructure and Hydration Mechanism of Geopolymer Composites

The geopolymerization process is an appropriate way of disposing of municipal solid waste incineration fly ash (MSWIFA), and possesses the advantages of immobilizing the heavy metals and making full use of its pozzolanic properties in manufacturing green, cementitious materials. In this study, coal fly ash (FA) and metakaolin (MK) were used to prepare a geopolymer composite, with MK partially replaced by different proportions of MSWIFA through the alkali-activation method. The microstructure and hydration mechanism of the geopolymer composites containing MSWIFA were investigated through mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and Fourier transform-infrared spectroscopy (FT-IR) tests; and the immobilization effect of the geopolymer paste on heavy metal ions was explored through inductively coupled plasma-atomic emission spectrometry (ICP-AES). The MIP analysis showed that the addition of MFARR had an overall degrading effect on the pore structure of the matrix. When the content of MSWIFA reached the maximum of 35%, the porosity and average pore diameter increased by 25% and 16%, respectively, corresponding to the case without MSWIFA. However, the pore size distribution exhibited an improving trend when the MFARR was increased from 15% to 25%. The SEM images revealed that the integrity of the micromorphology of the geopolymer mortar became weaker after adding MSWIFA. When the MSWIFA content was increased to 35%, the microstructural compactness decreased and more pores and microcracks appeared in the matrix. The FT-IR pattern study suggested that all the geopolymer composites had a similar internal structure, consisting of O-H, C-O, Si-O-Si, and Si-O-Al. The main component of the geopolymer paste hydrated at 28 d remained dominated by calcium silica-aluminate (C-A-S-H), when the MSWIFA ranged from 0% to 35%. Finally, the ICP-AES results showed that the leaching concentrations of the geopolymer paste of J-40 at 28 d for Cd, Cr, Cu, Pb, and Zn met the requirements of Chinese standards.

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