| The municipal solid waste incineration (MSWI) fly ash is classified as a kind of hazardous waste due to the high quantities of leachable toxic heavy metals. Therefore, the MSWI fly ash must be detoxified or stabilized before the landfill or reuse. Geopolymerization has been considered as an effective alternative for the disposal of MSWI fly ash. Geopolymers are inorganic polymeric materials with a chemical composition similar to zeolites but possessing an amorphous structure. Immobilization of MSWI fly ash with geopolymer has the advantage of using large volumes of waste material and also reduce the energy consumption and the emission of greenhouse gases associated with the production of ordinary cement.On the basis of the physico-chemical properties of MSWI fly ash, this work focuses on the mechanical properties and stability of geopolymer including acid-base resistance, heat resistance and freeze-thaw stability, which is prepared from metakaolin, alkali activator and MSWI fly ash. The properties are evaluated by compressive strength and leaching concentration of heavy metals.The MSWI fly ash was characterized by Brunauer-Emmett-Teller (BET) measurement, X-Ray Fluorescence (XRF), laser particle analyzer. It was found that the major elements in fly ash were O, Ca, Cl, Si, Al, Na and K, and primary heavy metals in fly ash included Pb, Zn,Cu, Cd, Cr and Hg. Of them, the heavy metals Pb and Cu, accounting for 76.50% and 83.16%, respectively, were relative unstable in the MSWI fly ashes. The average particle diameter was 15.08?m and pore volume was 0.0095 cm3·g-1.When 40% fly ashes are added, molar ratio of SiO2/Al2O3 (A) was 5.1, molar ratio of Na2O/SiO2 (B) was 0.30, with the optimal curing humidity of 60%,7 days compressive strength of the fly ash-based geopolymer could reach 41.25MPa,91% of the steady compressive strength, and the leaching concentrations of Cr, Cu, Zn, Cd, Hg and Pb were 0.292 mg·L-1,0.108 mg·L-1,0.006 mg·L-1,0.003 mg·L-1,0.003 mg·L-1 amd 0.003 mg·L-1, respectively, far less than their original contents (Cr 0.839 mg·L-1, Cu 0.634 mg·L-1, Zn 3.070 mg·L-1, Cd 0.002 mg·L-1, Hg 0.020 mg·L-1 and Pb 57.700 mg·L-1). By 15 days the compressive strength had reached a plateau.The stability tests of fly ash-based geopolymer indicated that:(1) leached by simulated acid rain (pH 3.0,4.0 and 5.0) and immersed in acid solution (pH 3.0 and 4.0) and aqueous alkali (pH 9.0,11.0 and 13.0) had slight influence on the compressive strength and leaching concentration of heavy metals. These resluts confirmed the prepared geopolymer was acid-base resistant; (2) the compressive strength was maintained effectively after successsive thermal process at the temperatures in the range of 200?to 1000?and freeze-thawing recycles (up to 55), and the leaching concentrations of heavy metals were far below national standard.Systemic physico-chemical charaterization by methods that include X-ray diffraction (XRD), X-Ray Fluorescence(XRF), scan electron microscopy (SEM), fourier transform infrared (FT-IR) spectroscopy, thermogravimetry-differential thermal analysis (TG-DTA), was used to elucidate the relationgship between the structure and stability of geopolymers. Besides, a mechanism accounting for immobilization of the heavy metals from the MSWI fly ash with geopolymer has been presented. It is speculated that heavy metals participate in constituting the structure of geopolymer, take the palce of Na+ and K+, meanwhile are adsorbed by the aluminium ions on the geopolymeric skeleton. The geopolymer is crystalloid structure consisting of toroidal chains and the ring molecules combining with each other forms sealed cavities. Consequently the heavy metals can be encapsulated in cavities or held in geopolymer by adsorption. |
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