X-Ray Powder Diffraction Study of NBS Fly Ash Standard Reference Materials

Abstract

AbstractThe fly ash Standard Reference Materials (SRMs) issued by the U.S. National Bureau of Standards have been studied by X-ray powder diffraction (XRD). Based on observations of large diffuse scattering maxima in their X-ray diffractograms, it was evident that all of the ashes had a high glass content. SRM 1633a and 2689, derived from the combustion of bituminous coal, contained different amounts of quartz, mullite, hematite and ferrite spinel (magnetite). SRM 2891, derived from subbituminous coal had quite a different chemical composition and a more complex crystalline phase assemblage, that included these four phases plus anhydrite, tricalcium aluminate, lime, periclase and minor phases. SRM 2690, also derived from subbituminous coal, had only quartz, mullite and ferrite spinel as detectable phases in its diffractogram. Analytical CaO is an important factor in determining the phase assemblage; SRM 2691 had 25.8 wt%, SRM-2690 had 8.0%, and the ash derived from bituminous coals had only 1.6-3.0%. The changing composition of the glass phases in the SRMs is detected in a shift in the position and shape of the diffuse scattering maximum in the diffractograms. Use of an internal intensity standard permitted quantitative comparisons of the relative amounts of crystalline phases among the four fly ash SRMs.