Advanced electron microscopic characterization of Portland cements

Cement and concrete materials are generally produced from anthropogenic waste materials and naturally occurring raw materials like clay, sand and limestone. Improving the efficiency of the material thus requires a thorough understanding of its variability, microstructure and composition. The optimization of cement production in the modern rotary kilns is beneficial in improving the product quality with low output variability, minimizing waste and reduction of the related CO₂ emissions and energy consumption. Cement clinkers play a vital role in optimizing the properties of cement and concrete materials. It consists of seven major phases, including alite, belite, periclase and alkali sulphates.

For most cement producers, the contemporary challenge is the increased demand for further reduction of CO₂ emissions. According to manufacturers, this is almost impossible, considering that the optimization of raw materials composition, kilns and fuels has reached a limit that cannot permit further significant emission reduction. Besides production optimization, alternative approaches have been employed to achieve further CO₂ emissions reduction. They include using alternative cement compositions like calcium aluminate, targeted addition of minor and trace elements during production and increasing cement reactivity. Cement being a multi-phase material, all these approaches demand the characterization of the modified materials and their constituent phases.

Although there are numerous techniques for characterizing cements, analysis of individual phases without interference is often not possible by bulk analysis. Electron backscatter diffraction (EBSD) in scanning electron microscopy (SEM) is a relatively new analytical method for the microstructural characterizing of cement and concrete. Further, a combination of EBSD and energy dispersive X-ray spectrometry (EDS) in one SEM can accurately provide the chemical composition and crystallographic identification of the cement clinker phases. Though EBSD analysis under low-vacuum conditions can be used for quantifying the crystal sizes of cement phases, analysis under high vacuum conditions is also possible. A protocol for obtaining optimal EBSD data for cement clinkers is provided.

Herein, Dr. Christiane Rößler and Professor Horst-Michael Ludwig from Bauhaus-University Weimar in collaboration with Dr. Dominik Zimmer and Dr. Patrick Trimby from Oxford Instruments GmbH described the protocol for preparing and characterizing cement clinkers using EBSD-EDS analysis in the SEM. Full characterization of the cement clinkers, including chemical – crystallographic, was carried out under both low- and high-vacuum conditions. The preparation of the sample as well as the EBSD, EDS and SEM setup, enabled the collection of sufficient data required for differentiating all the clinker phases at high resolution. Their research work is currently published in the journal, Cement and Concrete Research.

Sample surfaces prepared with argon broad ion beam polishing generated surfaces with high contrast electron backscatter diffraction patterns (EBSPs). The state-of-the-art EBSD camera and the improved EBSP quality under high-vacuum conditions played a crucial role in analyzing the crystal distortion in belite phase, which was found to be high. An approach to differentiate other cement clinker phases, including periclase, C₄AF, orthorhombic and cubic C₃A was also presented. Furthermore, Mg incorporation resulted in amorphization, and the presence of amorphous C₃A phases in white Portland cement was also confirmed.

In summary, the study is the first to characterize the composition and microstructure of all major clinker phases simultaneously in a single experiment. The variation of the band contrast, which was attributed to the different orientations of the individual crystal domains, was a good indicator of crystallinity. In a statement to Advances in Engineering, Dr. Christiane Rößler noted that correlating the EBSD-EDS results with clinker production parameters and the performance of the produced cement could facilitate the development of reaction-optimized clinker for further CO₂ emission reduction.