Structural Characterizations of Aluminosilicates in Two Types of Fly Ash Samples from Shanxi Province, North China

• Main Authors: Yunxia Liu, Fangui Zeng, Beilei Sun, Peng Jia, Ian T. Graham
• Format: Article
• Language: English
• Published: MDPI AG 2019-06-01
• Series: Minerals
• Subjects: fly ash; structural characterizations; aluminosilicate; spectroscopic analysis; FTIR
• Online Access: https://www.mdpi.com/2075-163X/9/6/358
• ISSN: 2075-163X

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.