There are two basic mechanisms that have been found to underlie interfacial zone formation [5]. Bleeding may also contribute, but we are ignoring this gross effect in the model presented here. The two mechanisms are the wall effect and the one-sided growth effect.
The wall effect is a packing effect. An aggregate surface acts as a wall, against which the packing of cement particles is inefficient, and results in an originally high porosity, low cement region around each aggregate. The one-sided growth effect is different. Consider a small volume of space in the bulk cement paste. On the average, hydration products are growing into this region from all directions. Now consider the region close to an aggregate surface. Growth only occurs from the cement paste side, not the aggregate side, which will also contribute to a higher porosity in this region. The wall effect is the more important effect, but the one-sided growth effect also contributes. To simulate an interfacial zone in concrete, a single aggregate is first placed in the hydration volume (area) and then the cement particles are randomly placed such that no particles overlap one another or the aggregate. The hydration algorithm is then executed as described previously.