This chapter has shown how the microstructure around a single aggregate, as influenced by aggregate properties, cement size, and the presence of mineral admixtures, can be quantitatively modelled using mainly micrometer-scale digital-image-based models, in order to be able to handle the random microsructural changes that occur during hydration. The geometry and topolopy of the many ITZ regions that occur in a real material have been studied using millimeter-scale continuum models. These models can handle some of the effect of aggregate shape, and can quantitatively explore the interconnectedness of the ITZ regions. The effect on the ionic diffusivity or electrical conductivity of a mortar or concrete of ITZ geometry and properties can be studied in a multi-scale approach, with reasonable agreement with available experimental evidence.
Work to be done includes obtaining more experimental measurements of concrete diffusivity and conductivity, along with degrees of hydration, and aggregate and cement particle size distributions, to better compare with the multi-scale model predictions. The multi-scale model probably needs to be improved in its use of effective medium theory, the formula used for cement paste diffusivity as a function of porosity, and the distribution of hydration products between bulk and interfacial transition zone cement paste. Also, the effect of mineral admixtures on ITZ and bulk microstructure and transport properties must be quantified and added to the multi-scale model. Further work in these areas is on-going.