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SEM and X-ray Imaging and Phase Separation

The experimental and image processing techniques which have been developed for segmenting two-dimensional images of cements into their five major phases (C3S, C2S, C3A, C4AF, and gypsum) have been described in detail in [7]. The cement of interest is dispersed in a low viscosity epoxy which is subsequently cured. A polished surface is prepared and viewed in the scanning electron microscope. Based on the signal intensity of the backscattered electron image and the intensities of the X-ray images for Ca, Si, Al, Fe, and S, each pixel in the two-dimensional image can be identified as either porosity or one of the five phases of portland cement. For example, strong signals for iron, aluminum, and calcium indicate the presence of the C4AF phase. Once an initial segmentation is performed, a type of median filtering is applied to remove some of the random noise present after the segmentation process. Here, each non-porosity pixel is replaced by the majority solid phase present in a limited neighborhood (e.g., 3 x 3) centered at the pixel. The final images are then analyzed to determine the area fraction of each phase and the perimeter fraction of each phase in contact with porosity. Images of the two CCRL cements analyzed in this project using these procedures are provided in Figures 2 and 3. Tables 1 and 2 provide the measured area and perimeter phase fractions along with the phase fractions calculated from the reported oxide compositions [5] using the conventional Bogue calculation [8].


  
Figure 2: Final segmented two-dimensional image of CCRL Cement 115. Phases from brightest to darkest are: C3A, gypsum, C4AF, C3S, C2S, and porosity. Image is approximately 250 x 200 µm.
\begin{figure} \special{psfile=cem115.ps hoffset=0 vscale=100 hscale=100 angle=0} \vspace{11.6 cm}\vspace{0.12in}\end{figure}

Table 1: Phase Fractions for CCRL Cement 115
Phase Perimeter fraction Area fraction Bogue volume fraction
C3S 0.504 0.605 0.547
C2S 0.239 0.221 0.214
C3A 0.045 0.032 0.081
C4AF 0.076 0.097 0.093
Gypsum 0.136 0.046 0.065


  
Figure 3: Final segmented two-dimensional image of CCRL Cement 116. Greylevel assignments are the same as in Figure 2.
\begin{figure} \special{psfile=cem116.ps hoffset=0 vscale=100 hscale=100 angle=0} \vspace{11.6 cm}\vspace{0.12in}\end{figure}