Comparison between geopolymer reaction and cement hydration in solidification of fly ash generated in municipal solid waste incineration

This paper compares two techniques that solidify the fly ash generated in municipal solid waste incineration (MSWI), namely, the geopolymer reaction and the cement hydration. The comparison focuses on the difference between solidified bodies in strength and heavy metal phases. Taking compressive strength as the indicator, the author investigated how the dosages of alkali activator and sodium silicate affect the solidified body of the geopolymer reaction, and contrasted the strengths of solidified products from geopolymer reaction and cement hydration of MSWI fly ash under the same cost of agents. In addition, the Netherlands leaching test program NVN 7431 was adopted to explore the impacts of geopolymer reaction and cement hydration on the occurrence phases of heavy metals in the MSWI fly ash. The results show that the optimal parameters of geopolymer reaction solidification are Na/FA = 2.8mol/kg and 2Na(2)SiO(3)/FA = 0.2mol/kg. Under these parameters, the 7d strength of the solidified body reached 18.2MPa at the agent cost of RMB 250 yuan per each ton of fly ash. By contrast, the strength of the solidified body after cement hydration at the same agent cost was only 12MPa. It costs RMB 750 yuan/ton for cement hydration to realize the same strength. Moreover, the leaching tests reveal that the water-soluble phase and the acid-soluble phase heavy metals were converted into aluminosilicate phase in both geopolymer reaction and cement hydration, indicating that the phase distribution of heavy metals are similar between geopolymer system and cement system. The research findings provide valuable reference to the technical selection for MSWI fly ash treatment under different scenarios and objectives.