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Figure 2 provides a complete 2-D (slice) image for the data set for the dry cement powder with no addition of water. A dense packing of bright cement particles surrounded by darker air space is clearly visible. In the two-dimensional image, particle sizes are observed to range between several micrometers and about 100 µm, as is typical of most portland cements [9]. There is some indication of variable levels of brightness within individual cement particles, corresponding to the different cement minerals (silicates and aluminates). Additionally, gypsum particles present in the cement powder appear darker and more uniform than the cement particles. There is some evidence of slight experimental/imaging artifacts, as evidenced by the bright ring surrounding the exterior of the sample. For this reason, a 300 voxel by 300 voxel by 300 voxel subvolume was selected from the exact center of the data set in the xy plane. Based on analysis of the greylevel histogram obtained for this subvolume and a subsequent segmentation of the subvolume into particles and air (porosity), the initial packing of cement particles would correspond to a w/c of about 0.28 if the air in the tube mold were replaced by water. Individual cement particles could be extracted from this data set and characterized with respect to size and shape, as has been performed previously for aggregate particles [7].
Figure 3 shows a 2-D slice image for the w/c=0.35 cement paste hydrated for 16 h. Many unhydrated cement particles are still visible in this image. Different types of hydration products are seen to fill the initial water-filled space between the particles. Because the sample was cured under sealed conditions, some of the original water-filled pores have been converted to air (water vapor)-filled pores due to the chemical shrinkage and self-desiccation that occur during the hydration reactions [19].
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As shown in Figure 4, it is informative to view the greylevel histograms for the subvolumes as a function of hydration time. In Figure 4, the disappearance of cement (high greylevels) and water (low greylevels) over time to create hydration products (middle greylevels) can be clearly distinguished. Based on these histograms, a segmentation of the subvolumes into water/air, hydration products, and unhydrated cement particles was attempted. Water and air-filled pores were selected as all voxels with a greylevel below 46. Hydration products were selected as having a greylevel between 46 and the local minimum in the range of (99,120) shown for each curve in Figure 4. Unhydrated cement was identified as all voxels having a greylevel greater than this local minima. Based on this segmentation, the starting w/c and degree of hydration of each data set could be estimated. The following estimates of w/c were thus obtained for 300 voxel by 300 voxel by 300 voxel portions of the top and bottom halves of each data set, respectively: 0.28 and 0.285 for the 4 h data set, 0.29 and 0.27 for the 12 h, and 0.335 and 0.317 for the 40 h. All of these are lower than the nominal w/c=0.35 used to prepare the cement paste, suggesting that extrusion into the very small tube molds may have lowered the paste w/c by expelling water and densifying the cement pastes. It is interesting that the lowest obtained average value of 0.28 is very close to the value obtained for the data set based on a packing of dry cement particles directly into the tube. The determined degree of hydrations (volume fraction of the cement which has reacted) for the subvolumes from the top and bottom halves of each data set are then: 0.19 and 0.21 at 4 h, 0.36 and 0.36 at 12 h, and 0.47 and 0.46 at 40 h. These values can be contrasted against previously measured values (based on loss on ignition measurements) for a w/c=0.3 cement paste of: 0.19 at 8 h, 0.40 at 24 h, and 0.49 at 72 h [11]. The comparison is reasonable, with some suggestion that the x-rays may be accelerating the hydration at early times. To assess the possibility of an acceleration due to local heating of the specimen within the x-ray beam, a thermocouple was inserted into one of the specimens during an image acquisition. A slight temperature rise on the order of 2 ºC was measured during the image acquisition, suggesting minimal acceleration due to thermal effects.
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Figure 5 shows a 2-D slice for hydrated cement paste prepared at a w/c of 0.45 and hydrated for 137 h while Figure 6 shows a 3-D image of a small subvolume of the same paste with only the unhydrated cement particle "cores" shown. Due to the higher w/c and the longer hydration time, there are fewer unhydrated cement particles in this image than in that for the w/c=0.35 paste shown in Figure 3. Upon careful examination of the image, individual needles and crystals of hydration products can be clearly observed. Shells of hydration product are visible around each of the larger unhydrated cement particles. Analyzing the greylevel histogram in a manner similar to that described above for the w/c=0.35 pastes, estimates of w/c and the degree of hydration of 0.47 and 0.62, respectively, are obtained. This degree of hydration value compares favorably to the values previously determined by loss on ignition for a w/c=0.45 paste [11]: 0.60 after 72 h and 0.70 after 168 h. Thus, the microtomography-determined values for w/c and degree of hydration for this data set both appear reasonable. For the 3003 subvolume, the segmented capillary pores (31.7 % of the overall volume) were evaluated using the program percolate.c. Using a burning algorithm, it was determined that about 98 % of the pores are part of a percolated (connected) pathway across the microstructure, indicating a highly connected "pore network." In future microtomography experiments, it would be of interest to view well-hydrated (e.g., several months) specimens to investigate the de-percolation of the capillary pores that is expected to occur around 20 % porosity [20,21].
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