SEM and X-ray analysis provide a convenient and powerful means for examining the microstructure of fly ash particles [2,4]. Here, the SEM/imaging techniques developed previously [5] for imaging Portland cements have been modified for the analysis of fly ash samples. In addition to the backscattered electron (BSE) image, X-ray images are collected for Al, Ca, Cl, S, and Si. Based on the intensities of these signals, a separation into the following phases is performed: silica (SiO2), an aluminosilicate (SiAl2O5), calcium chloride (CaCl2), anhydrite (CaSO4), a calcium aluminosilicate (CaSi2Al2O8), tricalcium aluminate (Ca3Al2O6), and inert (other) material [2,3]. The initial segmented image is typically processed with a median filter [1] to improve the image quality by removing much of the noise within individual phase regions. A series of final processed images for a variety of fly ashes is provided in Appendix A of this document. These final images can be analyzed to determine phase volume fractions, phase surface area fractions, and correlation functions to be used in reconstructing representative three-dimensional images [1].
In reality, of course, the phase composition of the fly ash is more complex than being simply composed of the compounds listed above. However, in approximating the fly ash to be composed of these compounds, we are operating in much the same spirit as that which utilizes the Bogue compounds (C3S1, C2S, C3A, and C4AF) as a representation of a Portland cement.