Development of the Rietveld method for quantitative x-ray diffraction analysis of complex mixtures including coal combustion by-products.

摘要

Quantitative X-ray diffraction (QXRD) analysis of coal combustion by-products (CCBs) and their hydration products is not straightforward. The number of crystalline phases in CCBs range from four to more than ten. X-ray powder patterns of CCBs typically have overlapping peaks, preferred orientation, broad peaks from varying levels of crystalline order, and shifted peaks due to solid solution compositions for some phases. The familiar internal standard methods for QXRD, such as the Reference Intensity Ratio (RIR) method, have two problems: (1) finding representative standards for the determination of RIR values and (2) finding suitable analytical peaks (i.e., peaks having minimal overlap). The Rietveld method, which has been applied to QXRD of multiphase materials only in the last decade, does not have these limitations.; The Rietveld method was evaluated for its application to QXRD of CCBs. A cost-effective protocol was developed for the application of the Rietveld method to complex phase assemblages, such as CCBs, for use in a laboratory staffed primarily by users with general familiarity with XRD or the Rietveld method. Items addressed were the precision and accuracy of the method on complex phase assemblages, the internal standard, the scan parameters, microabsorption, and the choice of structure for each phase to be quantitated. An internal round-robin was conducted on a test mixture to assess the reproducibility of the method.; The Rietveld method was then applied to a number of systems, including the National Institute of Standards and Technologies (NIST) Fly ash Standard Reference Materials (SRMs), CCB materials from disposal sites across the U.S., and a set of magnetically enriched fractions of a Class “F” fly ash obtained from the United States Geological Survey (USGS).; In order to investigate the NIST Fly ash SRMs, a set of synthetic ashes was produced to examine the accuracy of the method on these systems. One ash was also designed to test the accuracy of the method when the limits of the Rietveld program General Structure Analysis System (GSAS) were exceeded, as in the case of one of the NIST Fly ash SRMs. A novel averaging procedure was developed, tested, and then applied.