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A digital-image-based model has been applied to investigating a variety of concrete ITZ microstructures. In the absence of bleeding, the model suggests that ITZ microstructural features are due to both the wall and the one-sided growth effect, and that the ITZ thickness will largely be controlled by the median particle size of the cement. Denser, more uniform ITZ microstructures can be achieved in a variety of ways. Fine pozzolanic mineral admixtures improve ITZ microstructure by partially offsetting the wall (packing) effect and producing extra CSH due to their pozzolanic reaction with CH. Both size and pozzolanic reactivity are paramount in modifying ITZ microstructure with small (1 micrometer) highly reactive admixtures such as silica fume providing the most improvement. Lightweight absorptive aggregates aalso partially offset the wall effect by rearranging the distribution of cement particles near the aggregate. Some decrease in the density of the paste is still observed in the immediate vicinity of the aggregate due to inefficient packing and the one-sided growth effect. The use of porous cement clinker as aggregate reduces both the one-sided growth and wall effects.
The microstructure model has been verified by comparing the microstructure of model and real concretes containing various amounts of silica fume. The good agreement obtained suggests that the model accurately represents microstructural development in the ITZ's of concrete. As the demand for high performance concrete increases, the capability to engineer ITZ microstructure will become more important. Simulation of microstructural development appears to be a powerful tool in achieving this goal.