Improvements in cement production and prediction of cements' performance properties requires the application of material science which, in turn, requires the ability to determine and describe their micro- and macrostructures. Improved methods for determining the phase composition of cements using X-ray powder diffraction will facilitate this understanding. This project, part of the Partnership for High-Performance Concrete Technology at the National Institute of Standards and Technology (NIST), involves the development and testing of analytical methods necessary for characterization of cements. Rietveld refinements to model the complex X-ray powder diffraction (XRD) patterns of cementitious materials will provide phase, chemical, and structural information to more completely characterize them, and so provide an improved basis from which to investigate relationships between cement properties and performance properties.
RM 8486 is one of three NIST reference clinkers used for developing and testing methods of quantitative phase analysis [1]. These clinkers were selected as representative of the range of North American clinker production with respect to phase abundance, crystal size, and crystal distribution. The reference values are currently based upon an optical microscope examination of polished, etched sections. The XRD study is intended to provide both an additional estimate using an independent method of analysis, and data to examine inter- and intra- sample heterogeneity. The combined XRD and optical datasets are intended to establish certified values.
Clinker 8486 is intermediate in crystal size and exhibits heterogeneous phase distribution relative to the other clinkers (see Fig. 1). Alite occurs as subhedral to anhedral crystals approximately 25 µm in size. Belite occurs in large clusters with an approximate crystal size of 15 µm. Equant periclase crystals up to 15 µm are common throughout the microstructure. A medium- to fine-grained lath-like ferrite, with aluminate filling the inter-lath voids, forms the interstitial constituents.
Figure 1. RM 8486 polished section prepared using a 30 s HF vapor etch, field width: 250 µm.