Rietveld quantitative X-ray diffraction analysis of NIST fly ash standard reference materials

Rietveld quantitative X-ray diffraction analysis of the fly ash Standard Re ference Materials (SRMs) issued by the National Institute of Standards and Technologies was performed. A rutile (TiO2) internal standard was used to e nable quantitation of the glass content, which ranged from 65% to 78% by we ight. The GSAS Rietveld code was employed. Precision was obtained by perfor ming six replicates of an analysis, and accuracy was estimated using mixtur es of fly ash crystalline phases and an amorphous phase. The three low-calc ium (ASTM Class F) fly ashes (SRM 1633b, 2689 and 2690) contained four crys talline phases: quartz, mullite, hematite, and magnetite. SRM 1633b also co ntained a detectable level of gypsum, which is not common for this type of Ay ash. The high-calcium (ASTM Class C) fly ash, SRM 2691, had eleven cryst alline phases and presented a challenge for the version of GSAS employed, w hich permits refinement of only nine crystalline phases. A method of analyz ing different groups of nine phases and averaging the results was developed , and tested satisfactorily with an eleven-phase simulated fly ash. The res ults were compared to reference intensity ratio method semiquantitative ana lyses reported for most of these SRMs a decade ago. (C) 2000 International Centre for Diffraction Data. [S0885-7156(00)00403-6].