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TECHNICAL PAPER COLLECTOR’S EDITION

2C S + 6H C S H + 3CH . . . (1) added to 500m1 of aqueous CH solution. Samples
3
3 2 3
were taken from each solution and examined with
3CH + 2S C S H . . . (2) a scanning electron microscope (SEM).
3 2 3

From the experimental observations, the following
By TGA, the amount of CH, expressed as a percentage
model was proposed for the reactions of silica
of the weight of the original C S, was determined at fume with CH in aqueous solutions : as soon as
3
different ages. A 14 percent reduction in CH content
silica fume comes in contact with water, part of
was found at 3 days with the 25 percent addition of the silica fume undergoes rapid dissolution. After
amorphous silica, while for 75 percent addition the
5 to 15 minutes of mixing (in general less than 1h)
reduction was 73 percent. Even greater reductions the silica in solution tends to form an amorphous
in the content of CH were found with the Portland silica-rich, Ca-poor “gel” which forms coatings on
cement pastes. For example, with a 25 percent the surfaces of the silica fume particles.
addition of amorphous silica a 65 percent reduction
in CH content with respect to the control paste was
found at one day of curing. At the same time the silica fume particles
agglomerate forming clusters of particles.
However, with the passage of time (after Ih) the
Based on the above experimental results, Nelson
silica-rich, Ca-poor “gel” coatings begin to dissolve
and Young have concluded that amorphous
and the silica fume clusters start reacting with CH
silicas of colloidal dimensions act as very reactive
to form C-S-H. The initial reaction of silica fume
pozzolans and the pozzolanic reaction may be to form the silica-rich, Ca-poor “gel” is a rapid
evident as early as the first day of curing. surface-dependent reaction. In contrast, the

formation of the C-S-H via the pozzolanic reaction
In a recent paper by Cheng-yi and Feldman, it was is temperature-dependent. This is evident from
shown that hydration reaction in Portland cement the fact that at 20°C the C-S-H formation becomes
is accelerated in the presence of silica fume abundant at 24h, while at 38°C the C-S-H formation
2+
particles because of their reaction with Ca ions becomes common at 8h.
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and their ability to act as nucleation sites for CH .
This reaction occurs within minutes after contact Grutzeck and co-workers also suggested that
with water and the reduced Ca ions affects the their proposed model for the pozzolanic reaction
2+
nature of the hydration products. Pozzolanic of silica fume by which the C-S-H is formed via a
reaction in this study occurred as early as 8 hours. silica-rich, Ca-poor “gel” intermediate, can explain

certain characteristics of Portland cement-silica
Grutzeck and co-workers have investigated the fume paste, mortar, or concrete mixes. The high
mechanism of the pozzolanic reaction of silica water demand and the premature hardening of
fume with CH in aqueous solutions . The silica silica fume-cement pastes could be explained by
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fume used in that work had 94 percent content of the taking up of the water for the formation of
2
SiO and surface area of 20.2m /g. The particles the silica-rich, Ca-poor “gel”. The formation of
2
were about 0.1μm in size. Two solutions were this “gel” thickens the cement paste and results
examined : one at 20°C with 0.33g of silica fume in the early setting of the paste unless retarders
added to 500m1 of aqueous CH solution, and the or superplasticizers are used. Furthermore, after
other at 38°C in which 0.28g of silica fume was some time, the silica-rich, Ca-poor “gel” will

The Indian Concrete Journal | November 2018 51