Initial ITZ Microstructure

Continuum modelling has been used to study the effects of particle size distribution and water-to-cement (w/c) ratio on the initial porosity of the ITZ and bulk cement pastes [12,13]. The cement particles are represented as solid spheres, with a center and a radius, as described above. For simulations without hydration taking place, this is actually a more accurate way to place the particles around the aggregate surface. When hydration takes place, then the system must be sub-divided into pixels in order to allow the digital-image-based hydration code to work [8].

Figure 1 shows graphs of initial porosity vs. distance from an aggregate surface for a random particle packing of two different cements and three different w/c ratios. The computational volume consisted of a three-dimensional box 200 µm on a side. Because of the inefficient packing of the cement particles near the aggregate surface, the ITZ cement paste is observed to contain much less cement and much more water-filled capillary porosity than the cement paste far away from the aggregate surface. This inefficient packing of cement particles, the so-called "wall effect," is one of the major reasons for the formation of the ITZ in cement-based composites [1].

Figure 1: Porosity fraction near aggregate surface prior to hydration for two different cements (reprinted from Ref. 13). The median cement particle diameters of the two cements are: A1 - 28 µm, A7 - 11 µm.

The results of this modelling indicate that the extent of the ITZ corresponds closely to the median cement particle diameter on a mass basis (28 µm for Cement A1 and 11 µm for Cement A7, respectively). What is meant by the "extent" or "thickness" of the ITZ is that distance over which the porosity is significantly greater, by a factor of 10% or more, than the bulk porosity. The w/c ratio does not appear to affect the thickness of the ITZ region (as we are ignoring any effects of bleeding). The w/c ratio does however appear to affect the porosity gradient, which will alter both the ITZ and bulk cement paste w/c ratios, in the following manner.

The competition between ITZ and bulk cement paste plays a major role in determining the properties of a concrete, as will be shown later in this chapter. There is clearly less cement in the ITZ regions. The w/c ratio in the ITZ cement paste must then be higher than the overall w/c ratio. The w/c ratio in the bulk (non-ITZ) cement paste must then be lower than this overall value, because the average or overall w/c of the concrete has been fixed by choosing the initial masses of water and cement to be mixed with the aggregates. If these masses are conserved during the mixing and curing process, then the average w/c must be fixed. Therefore, a region of higher than average w/c ratio implies that there must exist a region of lower than average w/c ratio. This realization is critical in the interpretation of experimental measurements of transport properties of mortars and concretes, as will be discussed further below (see section 4).

For digital-image-based simulations of single ITZ microstructure, a simple geometry, such as a square or flat plate in 2-D or a cube or thin plate in 3-D, is generally used to represent the aggregate. The essential point is that the radius of curvature of the aggregate surface be much larger than the typical cement particle radius of curvature. This is because the typical aggregate diameter is much larger than the typical cement particle diameter (300-500 µm vs. 10-20 µm). Figure 2 illustrates a two-dimensional microstructure before and after hydration, using a flat plate aggregate (radius of curvature is infinite). Once an aggregate is placed in the microstructure, the cement particles are placed at random locations, such that they do not overlap any portion of the aggregate. The simple geometries used for the aggregates facilitate the subsequent quantification of ITZ microstructure, specifically the determination of the phase fractions present as a function of distance from the aggregate surface.

Figure 2: Original (left) and 65% hydrated (right) ITZ microstructures. The aggregate is gray, and all cementitious materials, hydrated and unhydrated, are white, with black representing water-filled porosity. The space between the thin gray vertical bars and the aggregate is the approximate extent of the ITZ region.

In addition to the wall effect contributing to the microstructure of the ITZ, digital-image-based simulations have identified a secondary mechanism active in ITZ formation, the so-called "one-sided growth" effect [14]. As hydration occurs in regions far away from an aggregate surface, the porosity is filled in with hydration products coming from all directions. However, near an aggregate surface, reactive growth occurs only from the cement paste side. Using computer simulations, this effect was isolated and quantified by allowing the initial cement particles to freely overlap the aggregate surface, thus eliminating the wall effect. While the wall effect typically scales with the median diameter of the cement particles, the one-sided growth effect appears to be active only over a distance of a few micrometers, at most. In addition to studying formation mechanisms, digital-image-based simulations have also been employed to study the effects of mixing and flocculation on the initial distribution of cement particles near an aggregate surface [15].