On the Glass in Coal Fly Ashes: Recent Advances

Abstract

Recent advances in the characterization of the glassy aluminosilicate phases in coal fly ashes are reviewed and discussed in terms of the development of new models describing their mechanism of formation, composition, structural relationships and chemical reactivity. Characterization techniques such as electron microscopy, x-ray diffraction, infrared and Raman spectroscopy, nuclear magnetic resonance and Mössbauer spectroscopies, thermal analysis, acid dissolution, silanation, and density fractionation have been used as experimental probes, and reveal a range of information relating to the glass phases from the macro, through the micro, to the nano-structural levels. Complex inter- and intra-particle heterogeneity in ash is observable not only in bulk chemical and mineralogical analyses but also at the molecular level in the constituent glasses. Remarkable relationships between ash particle size, density and glass composition have been observed and lead to a new proposed mechanism for ash morphogenesis based on temperature-viscosity effects in the mineral precursor melts in the boiler flame. A glass-ceramic model for fly ash particle structure is proposed and discussed in terms of phase relations in the CaO–SiO2–Al2O3 and FeO–Fe2O3–A12O3–SiO2 systems, modification of the aluminosilicate glasses by alkali and alkaline earth metal cations, and microheterogeneity involving glass devitrification and phase separation.

Identifying Glass Compositions in Fly Ash

“…The amount of calcium-modified glasses identified in each fly ash tracked with the amount of bulk CaO content reported for each fly ash in Table 1; the highest CaO content and amount of calcium-modified glass was measured in the CC fly ash, followed by the LEGS fly ash, the ML fly ash, and the FO fly ash. These data suggest good reactivity in alkaline solutions for these fly ashes, since the two calcium-containing phases are highly modified glasses, differing greatly from the ideal silicate glass structure as described earlier and by Hemmings and Berry (1987). The mixed glass phase is modified to a greater extent than the CAS phase due to its iron content, which means that it has significant amounts of three network modifiers (calcium, aluminum, and iron) to introduce disorder to the ideal silicate glass structure as discussed by Hemmings and Berry (1987).…”

“…The compositions of the crystalline phases are not listed in the tables, as they correspond to those for crystalline quartz (SiO2), lime (CaO), or iron oxides, such as hematite, magnetite, or maghemite (Fe2O3 or Fe3O4). Other than quartz, it is unlikely that most crystalline phases measured using X-ray diffraction could be identified visually, since they often form as finely disseminated grains within a glassy matrix as micro-or nano-crystalline materials (Hemmings and Berry, 1987). Each fly ash's compositional analysis for a single field of view is presented next.…”

“…First, the crystalline phase may not have been present in the specific point location where data were collected. Alternately, the iron may have been substituted into a crystalline or glassy phase in sufficient quantity to result in a strong intensity in the X-ray map; substitution into crystalline phases, such as mullite and network-former substitution, in glasses frequently occurs in fly ash with elemental iron (Hemmings and Berry, 1987;McCarthy, 1987).…”

“…In fly ash, calcium contributes to the disordered nature of the glassy phases, which can increase the reactivity of the raw material (Hemmings and Berry, 1987). For use as a partial Portland cement replacement, ASTM C618-12 (2012) classifies fly ash as either Class C or Class F, with the former specified to have a lower sum of SiO2, Al2O3, and Fe2O3 and, thus, a higher CaO content.…”

“…Coupled with the point-counting method to define the glassy phase composition for each glass, a more specific understanding of the fly ash composition is achieved using MSIA. Other methods that have been used to characterize glasses in fly ash include the location of the fly ash's vitreous halo in X-ray diffraction (Hemmings and Berry, 1987;Kilgour and Diamond, 1987;Duxson and Provis, 2008), the fly ash's vitreous alumina content (Fernández-Jiménez and Palomo, 2003), and the morphology and size of the fly ash particles.…”