Next: Image segmentation and Up: Two-dimensional Imaging of Previous: Two-dimensional Imaging of

Scanning electron microscopy image acquisition

Because different laboratories will have different scanning electron microscopy (SEM) equipment, only generalities can be presented in this portion of the documentation. Beginning with material preparation, the cement powder of interest is sampled and approximately 25 grams are mixed with a low viscosity epoxy resin [4] to form an almost dry paste. The epoxy is subsequently cured at 60^oC for 24 hours and the specimen is then cut, polished, and coated with a thin film of carbon in preparation for SEM viewing. Careful sample preparation is critical to the successful imaging of the fine cement particles [5,6].

The sample is then viewed in the SEM and analyzed based on a combination of the backscattered electrons and X-rays emitted due to the specimen-primary electron beam interactions. For viewing cement powders, typical settings are an accelerating voltage of 12 kV and probe currents of 2 nA and 10 nA for the backscattered electron and X-ray imaging, respectively [6]. Typically, in addition to the backscattered electron image, X-ray images are collected for Ca, Si, S, Al, and Fe. These images are then processed and combined to determine the distribution of phases amongst the cement particles comprising the image.

The major phases present in a typical portland cement are: tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A), tetracalcium aluminoferrite (C₄AF), and gypsum () footnote 1. Because brightness in the backscattered electron image is proportional to the average atomic number of a phase, the C₄AF phase shows up as the brightest phase, followed by C₃S, C₃A, C₂S, , and resin-filled voids. Unfortunately, the intensities for the C₃A and C₂S are similar enough that a complete and accurate separation of phases based simply on the backscattered electron image is not possible. Fortunately, the X-ray images can provide valuable supplemental information concerning the phases comprising the individual cement particles in the image.