Structural characterizations of aluminosilicates in two types of fly ash samples from Shanxi Province, North China

Abstract

In order to determine the structural characterization of aluminosilicates in two types of fly ashes, two samples from Shanxi Province, China were selected for study. One was from a pulverized coal boiler (FA-1), and the other from a circulating fluidized bed boiler (FA-2). FA-1 had a much higher content of silicon dioxide (SiO2) (70.30%) than FA-2(42.19%), but aluminum oxide (Al2O3) was higher in FA-2 (25.41%) than in FA-1 (17.04%). The characterizations were investigated using various methods including X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), magic angle spinning nuclear magnetic resonance (MAS-NMR) spectrometry, and X-ray photoelectron spectroscopy (XPS). The XRD analysis showed that FA-1 contained aluminosilicate glass, quartz and mullite, while FA-2 contained significant amounts of amorphous aluminosilicate, quartz and gypsum. The FTIR results showed an increased substitution of Al3+ for Si4+ as the band of asymmetric stretching vibrations Si-O(Si) (1100 !!cm−1) moved to 1090 cm−1 for FA-2, much lower than for FA-1(1097 cm−1). Moreover, the sharpness of the bands in the 1250-1000 cm−1 region for FA-2 indicates that the silicate structure of FA-2 was more ordered than for FA-1. It can be understood from the 29Si MAS–NMR results that Q4(mAl) (Q4 are connected via 4 bridging O atoms to mAl) is the main structural type in FA-1 and FA-2, and that FA-2 contains more Al, which substitutes for Si in the Q4 structure. 27Al MAS–NMR demonstrated that a combination of tetrahedral, pentahedral, and octahedral Al existed in FA-1 and FA-2. The Si 2p XPS spectra suggested that there were three forms of Si, including bridging Si (Si–O2), non–bridging Si (Si–O), and SiO2 gel. The content of Si–O2 for FA-1 was 37.48% higher than Si–O (28.57%), while the content of Si–O2 was 30.21% lower than Si–O (40.15%) for FA-2. The Al 2p XPS spectra showed that octahedral Al was the dominant form for FA-1 with a content of 40.25%, while the main phase was tetrahedral Al for FA-2 with a proportion of 37.36%, which corresponds well with the 27Al MAS–NMR results.