Multi-scale modelling for diffusivity based on practical estimation of interfacial properties in cementitious materials

Abstract

The interfacial transition zone (ITZ) between matrix and aggregate is a weak region because of its physical nature of relatively high porosity and low rigidity. ITZ microstructural configurations such as its thickness and volume fraction play an important role in transport and mechanical behaviours of cementitious composites. However, such ITZ characteristics are usually overestimated since random sampling planes from conventional sectional plane analysis technologies rarely pass through the normal to the surface of anisotropic aggregates. In this study, we first propose a normal line sampling (NLS) algorithm to evaluate the overestimation degrees of these microstructures for complex geometrical shaped aggregates like Platonic particles. The NLS algorithm applies the sampling lines along the normal of aggregate boundary at the given section plane that is significantly different from the previous methods in the literature. Then, the ITZ volume fraction can be determined by the ITZ thickness according to a generalized formula. We also extend these practical ITZ characteristics to estimate the effective diffusivity of cementitious composites by the classical effective medium approximation. The derived results show that the ratio of the apparent ITZ thickness to the actual ITZ thickness is dependent on the sphericity which is a shape descriptor of particles. Moreover, the overestimation degree of ITZ volume fraction and diffusivity of three-phase composites are governed by aggregate volume fraction, particle size distribution and particle shape. Finally, the results reflect that the aggregate shape has a significant effect on the apparent ITZ thickness, and the sampling process of ITZ thickness by using NLS algorithm is similar to the sampling techniques in experiment.