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TECHNICAL PAPER COLLECTOR’S EDITION

As a result, Chatterji and co-workers have concluded may he evident as early as 8 hours after curing
that the strength improvement achieved by adding starts.
silica fume to mortars or concretes must be due
to the physical nature of the product and not to its The high water demand and the premature
pozzolanic activity. hardening observed in silica fume-Portland cement
pastes is caused by the formation of a silica-
Buil and co-workers investigated the hypothesis rich, Ca-poor “gel” which forms coatings on the
according to which the principal granular effect of surfaces on the silica fume particles, and causes
silica fume is the filling of the spaces between the agglomeration of the particles. With time this “gel”
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cement grains . The filling of the spaces between starts dissolving allowing the silica fume particles
the cement grains would reduce the water/cement to react with CH to form C-S-H. However, this model
ratio and, thus, increase the compressive strength. of pozzolanic reaction may not be applicable to
However, the investigators could not verify the silica fume mortars or concretes. More research
hypothesis, since when silica fume was added to data is needed.
the mortar mixes, a higher amount of water had
to be provided to maintain a constant workability The pozzolanic reaction of silica fume depends
requirement. strongly on temperature at early ages, but not
at later ages. The consumption of CH and the
CONCLUSIONS formation of C-S-H is greatly accelerated with
increasing reaction temperature.
In Portland cement-silica fume composite the
pozzolanic reaction process is the major source In pastes, mortars, or concretes containing
accountable for strength enhancement. There are silica fume the volume of large pores (> 0.1µm),
three mechanisms associated with this process not the total porosity, is inversely related to
and they are as follows :
strength. The pozzolanic reaction of silica fume
pore-size refinement and matrix densification with CH is accompanied by pore refinement, i.e.,
transformation of large pores into finer pores. This
reduction in content of CH
process is instrumental in causing the strength
cement-aggregate interfacial refinement. increase observed in pastes, mortars, or concretes

Non-pozzolanic reaction processes have also containing silica fume.
been attributed to strength enhancements. Other
conclusions drawn from the review are listed The percentage of CH consumed by silica fume may
below: not be an accurate index of its pozzolanic activity.
A better index can be obtained by monitoring the
process of pore refinement in a hydrating Portland-
Silica fume, because of its very high content (85
to 98 percent) in amorphous SiO and its extreme silica fume cement.
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fineness (17 to 30m /g) is a very reactive pozzolanic
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material. It reacts with CH to form cementitious The incorporation of silica fume into mortars or
C-S-H, and the results of the pozzolanic reaction concretes leads to the formation of an amorphous
C-S-H, which fills up the empty spaces in the paste

56 The Indian Concrete Journal | November 2018