Recently, Zhang and Gjorv [8] have characterized the microstructure of the interfacial zone in lightweight concrete as being more dense and homogeneous than that found in normal weight concrete. One possible reason for this observation is the rearrangement of the cement particles in the vicinity of an aggregate due to water absorption by the aggregate. To simulate a lightweight absorptive aggregate using the model, the original distribution of cement particles around the aggregate may be modified by moving all particles toward the aggregate surface to simulate water absorption by the (dry) aggregate. This results in the aggregate acting somewhat as a filter, drawing in water and pulling cement particles towards its surface [9].