Previous: Quantitative Analysis I
Since silica fume is typically greater than 90% SiO2, it can be expected to modify the overall Ca/Si molar ratio in the cement paste, since a predominantly calcium silicate cement is being partially replaced by an almost pure silica compound. Previous experimental results [10,22] have shown that in concrete, this Ca/Si molar ratio increases dramatically in the interfacial zone as the aggregate surface is approached. Since initially (before hydration) this Ca/Si ratio would be constant (assuming the cement is relatively homogeneous), hydration is resulting in a preferential increase of Ca relative to Si in the interfacial region. This is in agreement with the frequently observed large crystals of CH in the interfacial zone. Using the model results, one can examine the effects of silica fume on the Ca/Si ratio in the interfacial zone.
Figure 5 shows the variation in the Ca/Si ratio as a function of distance from the aggregate for the three model systems before any hydration. For the plain portland cement system, the curve is perfectly flat as the cement is homogeneous so that the Ca/Si ratio is independent of the cement volume present at a given distance. However, when silica fume is added, the overall Ca/Si ratio is reduced since the fume is assumed to be pure silica. Additionally, since the interfacial zone has a higher porosity, more silica fume per unit volume is present near the aggregate than in the bulk paste. Thus, for systems with silica fume, the Ca/Si ratio is not constant but decreases as the aggregate is approached. As Ca preferentially moves into the interfacial zone during hydration, an excess of silica is already present there to undergo the pozzolanic reaction.
Figure 5: Ca/Si molar ratio vs. distance from aggregate surface for three silica fume contents of model systems before hydration.
The effects of hydration on the Ca/Si ratio curves are shown in Fig. 6. As seen experimentally [10,22], for the neat paste after hydration, the Ca/Si ratio increases dramatically as the interface is approached. For the systems with silica fume, however, hydration produces a fairly constant Ca/Si ratio throughout the composite microstructure. Thus, once again, the microstructural development is seen to be much more uniform when silica fume is present. The excess Si initially present in the interfacial zone offsets the deficiency in cement in this region and reacts with incoming Ca to produce C-S-H and somewhat eliminate the weak area normally present at the aggregate-cement paste interface.
Figure 6: Ca/Si molar ratio vs. distance from aggregate surface for three silica fume contents of model systems after 77% hydration.