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For the experiments at the ESRF, Cement and Concrete Reference Laboratory (CCRL) cement 133, issued in June 1999, was used. This cement was selected because it has been well characterized in the CCRL proficiency sample testing program [13] and also via computer modelling [11]. Detailed information on the particle size distribution and mineralogical phase distribution for this cement can be found in the NIST Cement Images online database [14].
To prepare a sample cement paste for viewing in the microtomography unit at the ESRF, the appropriate masses of cement (typically 50 g) and water were added to a small plastic beaker. The pastes were first mixed by hand for 1 minute in the beaker, followed by 1 minute of mixing using a "drill" mixer (a mixing blade attached to a regular electric drill). The sides of the beaker were then scraped and one final minute of drill mixing employed. Small pats of the paste were then carefully "extruded" into the sample tube molds, shown in Figure 1. The molds were either capped or left open depending on the desired curing conditions (sealed, open to drying, or open with water periodically added on top to maintain saturation). In general, pastes with water-to-cement mass ratios (w/c) of 0.3 to 0.45 were prepared and viewed after various periods of hydration. The specific gravity of cement is typically about 3.2, so that a 50:50 volumetric proportion of cement and water would correspond to a w/c of about 0.31. In addition, samples of a packed dry cement powder were prepared and viewed using the microtomography unit. In some cases, after viewing the original dry packed powder, water was added to the sample tube mold and the hydrated "packed" cement paste was viewed after various hydration times. For most pastes, it was not possible to obtain a stable microtomography image prior to the setting of the cement paste (at about 4 h), due to local motion of cement particles within the pore solution. The only exception to this was the sample of cement paste that was first prepared as a dry packed powder in the tube mold, with the subsequent addition of water. For this specimen, we were able to obtain a reasonable image immediately after addition of the water, as the particles basically remained in their original packed configuration.
For the Plaster of Paris, a commercial locally-available product was utilized and the paste was prepared at a water-to-solids mass ratio (w/s) of 1.0. The low viscosity of these pastes allowed the tube molds to be easily filled. Plaster of Paris hydration was viewed after 4 h, 7 h, 15.5 h, and 5 d. In addition, a sample of the dry Plaster of Paris powder was placed in a separate tube mold and viewed using the microtomography setup.
The clinker brick was a hard-burned clay brick that had been examined previously using the ESRF microtomography unit [6] at a resolution of 6.67 micrometers per pixel. For this experiment, a small 2 mm x 2 mm x 2 mm cube of the brick was carefully sawn from a larger specimen and mounted for viewing with the microtomography unit.
All samples were imaged on the 3-D microtomography (µ CT) unit developed on beamline ID 19 at the ESRF. The system utilizes a large monochromatic parallel beam and a 2-D area detector. The specimen to be imaged is mounted on a translation/rotation stage allowing precise alignment in the beam. To compile a 3-D image set for a material, a series of over 1000 radiographic images are recorded at different angular positions from 0 to 180º. After conversion to light by a flourescent scintillator screen, the radiographic images are digitized using a Frelon camera [15], which consists of a 2-D charge coupled device (CCD) array with 1024 x 1024 elements, covering an area of 19.5 mm by 19.5 mm. The distance from the sample to the scintillator is 8 mm. The Frelon camera has a resolution of 2 µm full width-half maximum. A 3-D filtered backprojection algorithm is then used to reconstruct a 3-D image of the specimen from the series of 2-D projections [16]. For the operating conditions and optical setup employed in this study, it was possible to acquire a 1024 x 1024 x 1024 image with a voxel dimension of 0.95 µm in just over twenty minutes (1.2 seconds per radiographic image). Including sample preparation, adjustment, and equipment setup, each data set required about 1 h of total clock time.
The (greylevel) intensity at each voxel in the final 3-D image corresponds to the linear attenuation coefficient of the material contained in that voxel. Thus, cement particles will appear much brighter than hydration products, which will appear brighter than water-filled or air-filled porosity.