Improvements in cement production and prediction of cements' performance properties require 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 in order to more completely characterize them, and so provide an improved basis for investigating relationships among cement properties and performance properties.
RMs 8486, 8487, and 8488 are 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 (Figure 1) is intermediate in crystal size and exhibits heterogeneous phase distribution relative to the other clinkers. Alite occurs as subhedral to anhedral crystals approximately 25µm in size. Belite occurs in large clusters with an approximate crystal size of 15µm. 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.
Clinker 8487 (Figure 2) exhibits a fine crystal size and relatively homogeneous phase distribution, with the exception of belite and free lime, which both occur as clusters within the microstructure. Alite occurrs as anhedral grains approximately 12µm and comprises the bulk of the clinker. The matrix contains subhedral aluminate and fine-grained ferrite. Some of the ferrite is intimately intermixed with the aluminate. Some of the pores contain alkali sulfates. This clinker may be difficult to examine microscopically because of the fine texture of the aluminate and ferrite and the uneven distribution of the belite and free lime.
Clinker 8488 (Figure 3) is the most coarsely crystalline of the three clinkers and exhibits the most homogeneous distribution of the constituent phases. Subhedral to anhedral alite crystals up to 110µm form the bulk of the clinker. Belite occurs as rounded, evenly distributed crystals of about 35µm diameter. Ferrite occurs with both lath-like and dendritic habit while aluminate crystals may be found between ferrite crystals.
Figure 1: RM 8486 polished section prepared using a 30 s HF vapor etch with a field width of 260 µm (upper) and 100 µm (lower image). Alite appears as brown, euhedral to subhedral crystals exhibiting hexagonal outlines, belite as blue-to-brown with the internal lameller structure, ferrite appears white, aluminate as gray crystals within the ferrite network, and periclase as gray, equant crystals in the matrix.
Figure 2:RM 8487 polished section prepared using a 30 s HF vapor etch with a field width of 260 µm (upper) and 100 µm (lower image). Alite appears as blue to brown, subhedral to anhedral crystals, belite as blue, circular crystals, the interstitial phases are not clearly distinguished here though ferrite appears white and aluminate as gray crystals.
Figure 3: RM 8488 polished section prepared using a 30 s HF vapor etch with a field width of 260 µm (upper) and 100 µm (lower image). Alite appears as brown, euhedral to subhedral crystals, belite as blue, circular crystals exhibiting internal lameller structure, the interstitial phases ferrite appear white and aluminate gray.