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The three-dimensional x-ray attenuation map of the cement mortar was generated at the National Synchrotron Light Source (NSLS) located at Brookhaven National Laboratory using Exxon's microtomography scanner at beamline X-2B. This instrument is an optical microscope that images scintillation events in a phosphor onto the surface of a 512x512 cooled CCD. X-rays illuminate a cylindrical specimen and radiographic images are acquired at a large number of discrete view angles as the specimen is rotated about its axis. This rotation axis is perpendicular to the x-ray beam and parallel to the CCD pixel columns. At the conclusion of a scan the image data from the CCD are sorted such that the specimen projections for each pixel row are collected together. These collected data are reconstructed to form a single 2-D tomographic slice. Since the synchrotron beam is highly collimated, a fast Direct Fourier inversion algorithm is used to rapidly reconstruct the 512 slices corresponding to each CCD row. Three dimensional volumes are built up by stacking the slices. Beam hardening artifacts are eliminated by using a Silicon monochromator to select a well defined energy.
Under ideal conditions the ultimate spatial resolution of this instrument is comparable to a light microscope and is about 1 micrometer. For this study the spatial resolution is limited by the specimen size which is about 3.5mm. With a field of view for the mortar specimen of slightly greater than 3.5mm, each pixel represents about 7 micrometers.
The mortar specimen in this study was scanned at 17 keV and radiographic images were acquired in 0.36 degree increments between 0 and 180 degrees of specimen rotation. Data acquisition and reconstruction were completed in 1.5 hours. This scanning protocol resulted in a reconstructed volume that is more noisy than what is usually achieveable. The protocol for future scans will be modified to reduce noise.