ABSTRACT
Cement mortar can be treated as a multiphase system at the meso-scale level. The constituent phases are the fine aggregate, bulk cement paste, and the interfacial transition zone (ITZ) surrounding the fine aggregate particles.
The purpose of this paper is to model the influence of the aggregate on the diffusivity of cement mortar by numerical and analytical methods. The ITZ is more porous than the bulk cement paste due to the wall effect. The ITZ diffusivity was modeled based on its porosity gradient from the aggregate surface. The ITZ volume fraction was calculated by a three-dimensional geometric model with randomly distributed aggregates. According to the actual aggregate gradation, spherical aggregate particles were generated and distributed randomly into a given cube with periodic boundary conditions in a descending order of sizes. Then a concentric shell of the same thickness was added to each particle to represent the ITZ.
Given the diffusivity and volume fraction of each phase, the mortar diffusivity was predicted by analytical and numerical methods. For the numerical method, the diffusivity was simulated based on the three-dimensional geometrical model. After discretizing the geometric by voxel cubes, the diffusivity was attributed to each voxel according to its phase type. Only the diffusion between two face-sharing voxel cubes was considered. Then the diffusion process in the mortar was simulated under the constant boundary conditions. When a steady-state condition is reached, the diffusivity of the mortar can be calculated.
To reach an analytical solution, a three-phase composite spherical element is introduced. Each element is composed of a spherical core and three concentric shells. The cement mortar is represented by many elements of different sizes. The effective diffusivity is then calculated based on the general self-consistent method. To consider the effect of the aggregate shape, the analytical solution for the cement mortar containing spheroidal aggregate was also presented. The cement mortar was considered as a two-phase medium: equivalent aggregate (consisting spheroidal aggregate and ITZ) and bulk cement paste. The effective diffusivity was then obtained by using an average scheme for a two-phase medium containing randomly oriented spheroidal inclusions. The predicted mortar diffusivity was in conformance with the measured diffusivity. One of the comparison is shown in Fig. 1. Comparison of measured diffusivity and predicted results for spheroidal aggregates of different aspect ratios λ. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig183_1.tif"/>
The modeling of the aggregate influence on the diffusivity can also be used for the prediction of chloride diffusion in the concrete.
