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Phase composition estimates by X-ray powder diffraction and Rietveld analysis are becoming more widely used in the cement industry. The ASTM C01.23 Compositional Analysis subcommittee developed test method C 1365 Standard Test for ‘Determination of the Proportion of Phases in Portland Cement and Portland-Cement Clinker Using X-Ray Powder Diffraction Analysis.’ A round-robin analysis involving 11 laboratories was initiated to assess the precision and bias of this approach and to develop guidelines for Rietveld analysis of hydraulic cements. Four cements were prepared using NIST SRM clinkers spiked with known amounts of one or more of the following minerals: gypsum, bassanite, anhydrite, and calcite. Specimens were provided with instructions that laboratories analyze the whole cement, collect data in triplicate, and repack the specimen for each run. Results of the round robin were used to estimate inter- and intra-laboratory precision and bias of phase abundance determinations. Results show an improvement over previous cement round-robin studies utilizing traditional internal-standard based, peak area measurement methods.