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Previous: Saturated and sealed
The model can be further validated against experiment by comparing SEM micrographs of real sample microstructures to two-dimensional images from the 3-D model. Figures 2 and 3 provide a comparison at similar scales for Cement 115. The two most distinguishing features between the sealed and saturated specimens are the amount and size of the unhydrated cement grains and the amount and size of the observable capillary pores. One can clearly observe that there are more and larger unhydrated cement grains in the sealed specimens (particularly in terms of the alite grains which are surrounded by a dense layer of calcium silcate hydrate gel), in agreement with their measured lower degrees of hydration. Based on quantitative SEM measurements of the unhydrated cement area fractions, after 94 days, the saturated sample has achieved a degree of hydration of 0.79 ± 0.04, while the sealed sample has only achieved a value of 0.66 ± 0.04, based on the averages from eleven fields of view (1024 pixels x 800 pixels, 0.25 µm/pixel) for each sample, and expressed with an uncertainty of one standard deviation. By contrast, the corresponding degrees of hydration for the two model systems shown are 0.80 and 0.69, respectively. In addition, due to the emptying of the capillary pores during chemical shrinkage, many relatively large pores remain in the sealed specimens. Once again, the effects observed in the real specimens are quite accurately duplicated by the model, as the real and model images exhibit many similar features.
Figure 2: Comparison of real SEM images for saturated (left) and sealed
(right) hydration for Cement 115 with w/c=0.3.
Unhydrated cement is white, porosity is black, calcium hydroxide is
light grey, and calcium silicate hydrate gel and
other hydration products are dark grey. Areas imaged are approximately
128 µm x 190 µm.
Figure 3: Comparison of model images for saturated (left) and sealed (right) hydration for Cement 115 with w/c=0.3. Greyscale assignments are the same as those given above for the real images. Areas represented by the images are 100 µm x 100 µm.