As a composite material, concrete's performance can be expected to depend on the properties of the interfaces between its two major components, aggregate and cement paste. Bentur  has reviewed the dependence of mechanical properties on ITZ microstructure. Durability will also be strongly influenced by ITZ microstructure and its effects on transport properties. Indeed, increases in electrical conductivity and permeability due to ITZ's have been observed by Ping et al.  and Costa et al. , both using simplified specimen geometries.
Over the years, the general microstructural features of the ITZ in ordinary portland cement concrete have been explored by a number of researchers including Zimbelman , Uchikawa , and Scrivener and Gartner . The general consensus reached is that the ITZ is a region, up to 50 micrometers wide around each aggregate, containing more porosity and larger pores than the bulk cement paste, and often containing large calcium hydroxide crystals. Due to these features, the ITZ is often considered to be the weak link in concrete with respect to mechanical properties and durability.
In this paper, a digital-image-based cement microstructurd model is applied to simulate ITZ microstructural development. Reasons for the differences between ITZ and bulk paste microstructure will be developed and methods for improving the structure of this weak link will be explored.