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TECHNICAL PAPER COLLECTOR’S EDITION

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particles and the cement paste plays a very film of CH at the interface . In addition, no
significant role in the cement-aggregate bond. interfacial cracks were observed in the study and
Even though the effect of this bond on concrete amorphous C-S-H“gel” was seen surrounding
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performance is a subject of controversey , its the aggregates, thus greatly reducing porosity.
significance on the mechanical properties and In the presence of silica fume, rupture appeared
2
durability has been established . as transgranular because of stronger bond.
Regourd and co-workers have also reported an
In normal Portland cement concrete the transition improvement in cement paste-aggregate bond
zone is less dense than the bulk paste and also in blended cements containing only 5 percent by
lime-rich because it contains a large number of weight silica fume .
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plate-like crystals of CH. The presence of this lime-
rich zone is because of the role of sand particles in Thus, it is expected that the presence of silica fume
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acting as sinks for crystallized CH . in mortars and concretes improves the mechanical
properties because of the enhancement in
SEM studies by Regourd showed that the cement interfacial or bond strength. This strengthening
aggregate interface is better crystallized and mechanism is not only connected to the chemical
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more porous . It is, therefore, more easily subject formation of C-S-H (i.e., pozzolanic reaction) at the
to microcracking by tensile stresses caused by interface, but to the microstructural modification
applied environmental loads. As a result the (i.e., CH orientation, porosity, and transition zone
transition phase between aggregate particles and thickness) as well.
hydrated cement paste is the weakest link of the
concrete system and it has a great influence on the
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properties of the concrete . The pattern of rupture STRENGTH ENHANCEMENT BY NON-
POZZOLANIC ACTIVITIES
is described as intergranular .
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Not all investigators agree that pozzolanic reaction
activity is the source of strength enhancement.
In the presence of silica fume the interface
morphology becomes different and this becomes
the dominant factor for strength improvement. Chatterji and co-workers have examined the
hydration characteristics of Portland cement-silica
fume pastes and concretes by X-ray diffraction
Charles-Gilbergues and co-workers have 21
investigated the influence of silica fume on the (XRD) . The silica fume content varied from 16 to
thickness of the transition phase in mortars, and 30 percent by weight of cement and the water/
on the degree of orientation of the CH crystals cement ratio ranged from 0.4 to 1.0. The XRD
in it . They have reported that compared to the results showed that CH was present in the pastes
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reference mortar containing only Portland cement, for at least up to 4 months, and up to 2 years in
the thickness of the transition phase decreases the concrete mix. It was noticed that 30 percent
with the addition of silica fume. Also, the presence silica fume could not consume the CH liberated
of silica fume reduces the degree of orientation of by 70 percent Portland cement even though the
the CH crystals in the transition phase. curing temperature was 50°C. The absence of CH
from one of the pastes after 5 days of curing was
attributed by the investigators to the lack of water
Regourd, on the other hand, reported complete due to low water/cement ratio.
absence of both oriented CH and the continuous

The Indian Concrete Journal | November 2018 55