Synthesis of geopolymer composites from blends of CFBC fly and bottom ashes

Blends of circulating fluidized bed combustion (CFBC) fly and bottom ashes of the same coal origin were investigated as raw materials for geopolymer synthesis. Reactivity of the low-reactive CFBC fly ash (CFA) was enhanced by an alkali-fusion pretreatment, which was optimized by an L16 (44) orthogonal array. It was found that, at a relatively low sodium hydroxide to CFA mass ratio of 0.5, effective alkali fusion could be achieved at 350 °C for 0.5 h. The fused CFA was blended with ground CFBC bottom ash (CBA) at mass ratios of 2.00, 1.00, 0.55, 0.29, and 0.12, and activated by two sodium silicate solutions (21.6 wt% and 34.5 wt%). Geopolymer pastes were cured at 40 °C for 7 days, reaching a highest compressive strength of 34.0 MPa. Characterization of the raw materials and geopolymer products was also conducted by an alkaline dissolution test, thermogravimetric–differential thermal analysis (TG–DTA), X-ray diffractography (XRD), scanning electron microscopy (SEM), as well as Fourier transform infrared spectroscopy (FTIR). The results of this study suggest that, by a moderate alkali-fusion pretreatment at temperatures slightly higher than the melting point for sodium hydroxide (318 °C), low-reactive CFA can be recycled together with CBA for production of value-added geopolymer composites.