Next: Conclusions Up: Main Previous: Percolation and chemical shrinkage
In addition to affecting the overall percolation of the capillary porosity and the amount of empty porosity, the cement PSD also has a significant effect on the local microstructure of the ITZ. It has been previously illustrated using computer modelling that the thickness of the ITZ region will be on the order of the median diameter of the cement PSD . Thus, the use of a coarser cement will increase the thickness of the ITZ region and also result in the presence of larger "pores" in this region. Since the largest pores empty first during self-desiccation, a large fraction of empty porosity may be created within the ITZ region . This is illustrated in Fig. 2 which shows 2-D plane sections from the initial and final microstructures (from the 3-D model) for the two different cement PSDs. One can clearly observe the larger pore sizes present with the coarser cement, particularly in the ITZ region. The final images show that often these larger pores empty during hydration, creating an ITZ microstructure that is significantly different from that of the bulk cement paste. While these large pores are likely detrimental from a strength viewpoint, they may actually reduce the autogeneous shrinkage in these systems, as for the same amount of chemical shrinkage, the emptying of larger pores will result in less of a decrease in internal RH and thus less measurable autogeneous shrinkage .
A quantitative analysis of this phenomena is provided in Fig. 3, which provides plots of the water-filled and empty capillary porosity as a function of distance from the aggregate surface for the initial and final microstructures. The appearance of a local maxima at a distance of about 5 pixels in the final empty porosity curves, particularly prominent for the systems cured under totally sealed conditions, corresponds to the large empty pores observed in the final microstructural images provided in Fig. 2. The peak is seen to be much higher for the coarser cement, due to the inefficient packing of the cement particles in the vicinity of the aggregate surface and the larger average pore size. For the water-filled porosity, the profile increases basically in a monotonic fashion as the aggregate is approached, due both to the initial cement particle packing and the one-sided growth effect . For the empty porosity, however, the value actually decreases very near to the aggregate surface, as the pores a finite distance from the aggregate are first emptied during self-desiccation and a "thin (several pixel) water film" remains on the aggregate surface to be filled in by subsequent hydration.