Next: Quantitative Analysis I
Figure 1 provides a composite of real specimens containing a) 0, b) 10, and c) 20% silica fume and hydrated for 28 days. In the micrographs, cement particles are bright white, CH and pozzolanic material (silica fume and C-S-H) are light grey, primary C-S-H is dark grey, and pores are black. The scale is such that the full image height corresponds to about 160 micrometers. The effects of silica fume on microstructural development in both the bulk paste and interfacial zone are readily apparent. As the silica fume content increases, the amount of CH decreases, becoming nearly nonexistent in the system containing 20% silica fume. The overall microstructure is markedly denser in the specimens containing silica fume as the pozzolanic reaction fills volume more efficiently than does the crystallization of CH. In terms of interfacial zone microstructure, the inhomogeneity present in the neat paste specimens is greatly reduced in the ones containing silica fume. The microstructure of the 20% silica fume specimens is particularly homogeneous, with little difference in appearance between interfacial zone and bulk microstructure.
Figure 1: SEM backscattered electron images of interfacial zone microstructure for systems hydrated 28 days containing a) 0%, b) 10%, and c) 20% by weight silica fume. Flat dark surface at bottom of each image is aggregate.
Figure 2 provides a composite similar to Fig. 1 but for model systems before hydration and after hydration to an extent equivalent to that achieved in the comparable real specimens. Phases are the same greylevels as in Fig. 1 except for the pozzolanic material which is now dark grey. The degree of hydration values (based on a set of 6 SEM images for each silica fume content) were determined to be 0.81, 0.85, and 0.86 for the 0, 10, and 20% silica fume specimens respectively. The features first observed in the real specimens are consistently reproduced in the model systems. The ability of the small silica fume particles to fill in the increased porosity near the aggregate surface is clear in the systems before hydration. As in the real specimens, the model specimen containing 20% silica fume exhibits a much denser, more homogeneous microstructure than the neat paste system.
Figure 2: Images of model interfacial zone microstructures before and after hydration for 0, 10, and 20% silica fume additions (left to right 0-10-20%). Images are taken just above aggregate surface.