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To quantitatively compare the model to experiment, phase fractions were measured as a function
of distance from the aggregate surface for both sets of specimens. Previous research has
suggested that one key effect of silica fume in enhancing interfacial zone microstructure is the
increased homogeneity of the total C-S-H and cement phase fractions [8]. The total (C-S-H
+ unreacted cement + unreacted silica fume) phase fractions as a function of distance from the
aggregate surface are plotted in Fig. 3.
Distance is in pixels for the model specimens with the
(weight) average cement particle being 16 pixels in diameter. For the real specimens, distance is
in micrometers, with the average cement particle estimated to be about 40 micrometers from
viewing the cement powder under the SEM. Once again, considerable agreement is exhibited
between the real and model specimens.
Silica fume is found to increase the total (C-S-H + cement + silica fume) phase
fraction in
the bulk paste and reduce the dramatic decrease in this phase fraction occurring as the aggregate
surface is approached. If one considers the vicinity near the aggregate where this total phase
fraction exhibits its minimum value as the weak link in the strength of the composite system, silica
fume would increase the strength of concrete by increasing this phase fraction's minimum value.
With 20% silica fume, this phase fraction is flat nearly all the way to the aggregate surface,
suggesting a superior paste-aggregate bond and true stress transfer between paste and aggregate.
While strength is influenced by the amount and homogeneity of the load-bearing phases in
the paste in concrete, durability of concrete will depend greatly on transport properties
(diffusivity, permeability) which have been shown to be strong functions of system porosity
[13,19]. Figure
4 provides plots of the capillary porosity fraction as a function of distance
from the aggregate surface for the model and real specimens. Here again, reasonable agreement is
exhibited between model and experimental results, as the presence of silica fume decreases the
steep porosity gradient present as the aggregate surface is approached, as well as decreasing the
bulk capillary porosity. When analyzing Fig. 4, it
should be noted that the systems containing
silica fume are hydrated further than the neat paste specimens. Thus, a small porosity reduction
(on the order of 3% for an increase in 
of 0.05) is achieved
due to the increased hydration, while the major portion of the porosity reduction is due to the
ability of the pozzolanic reaction to fill a greater volume with C-S-H product than that occupied
by the CH and silica fume reactants. From these experimental and model results, it is expected
that silica fume would improve the durability of concrete as well as increase its strength. Indeed,
direct experimental evidence of improved transport and durability properties of systems
containing silica fume has been presented [20,21].
Figure 3a: Total C-S-H + unreacted cement + unreacted silica fume phase fractions vs. distance
from aggregate surface for three silica fume contents: real system.
Figure 3b: Total C-S-H + unreacted cement + unreacted silica fume phase fractions vs. distance
from aggregate surface for three silica fume contents: model system.
Figure 4a: Porosity vs. distance from aggregate surface for three silica fume contents for real
systems.
Figure 4b: Porosity vs. distance from aggregate surface for three silica fume contents for model
systems.
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