The application of XRD to quantitative phase abundance analysis dates back to the mid-1920s with the Portland Association Fellowship at the National Bureau of Standards (now National Institute of Standards and Technology, NIST) . The late 1950s through the 1980s saw increased use of XRD in the analysis of cements and the first international round robins on quantitative powder diffraction analysis [5, 6, 7] The results of these round robins were pessimistic on the suitability for XRD in the laboratory or cement plant due to the poor reproducibility and repeatability. In 1980, ASTM Committee C01.23 on Compositional Analysis established a task group to explore XRD methods for phase analysis of portland cements. Initial work on measurement of the interstitial phases of clinker - periclase, aluminates and ferrite , resulted in adoption of a standard test method for XRD analysis of portland cement and clinker . A qualification approach was adopted from ASTM C 114 on chemical analysis where the analyst may choose any method for analysis as long as he is able to demonstrate, or qualify, his approach through analysis of SRM materials. For the XRD method, the NIST SRM clinkers are used because they have certified phase composition  and the qualification limits are developed through round robin testing. The round robin testing was initially based on a standardization using pure clinker phases and either peak area or whole-pattern measurement techniques. The difficulty in standardization and measurement limited the number of laboratories willing to participate. It was then decided to analyze clinker data by the Rietveld method. This also allowed inclusion of some data sets in the literature that included NIST SRM clinker data. Round robin participants were selected that have experience in XRD analysis of portland cement clinker. Three additional data sets were taken from published studies that included the NIST clinkers [10, 11, 12]. Some guidance was provided on specimen preparation, and scan conditions (except for data sets from literature). Participants were requested to grind the clinker to −10 µm median particle size and collect duplicate data sets (with sample re-packing) from a two-theta range of at least 20º − 65º (Cu Kα). Results with a single estimate are shown here but were not included in the statistical analyses.