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Characteristic features of the interfacial transition zone, that region of the cement paste close to an aggregate surface, are higher capillary porosity and larger pores than in the bulk cement paste matrix [43,44]. These features are typically seen in the cement paste that is within 50 micrometers of an aggregate surface [45]. Using the cement paste microstructure model mentioned above, we have analyzed two major causes of these features in the interfacial transition zone microstructure: 1) the particle packing effect, and 2) the one-sided growth effect [46].
The particle packing effect arises from particles not packing together as well near a flat surface as in free space. Since the typical aggregate is many times larger than the typical cement particle, even for the fine aggregate, locally the aggregate surface appears flat to the surrounding cement particles. This inefficient packing causes less cement and higher porosity to be present initially near the aggregate surface, and so even after hydration this condition persists. The width of the interfacial transition zone will then scale with the median cement particle size [47]. This is the main contribution to the interfacial transition zone microstructure, but not the only one.
The one-sided growth effect arises in the following way. Consider a small region of capillary pore space located out in the bulk paste part of a mortar or concrete. On the average, there is reactive growth coming into this small region from all directions, since the cement particles are originally located randomly and isotropically. Now consider a similar small region of capillary pore space, but located very near an aggregate surface. Reactive growth is coming into this region from the cement side, but not from the aggregate side [46]. This will give rise to a higher porosity, as the net growth of solids will then be smaller in the interfacial transition zone region.