Geoscience Reference
In-Depth Information
Table 7.3
Physical properties of silica fume (
after
Toutanji
et al.,
1999).
Properties
Values
Loss on ignition (%)
1.44
Specific gravity
2.25
Particle size (average) (micron)
0.1
ferrosilicon and metallic silicon in high-temperature electric arc furnaces. Silica fume
is extremely fine and dusty, with a typical particle size equal to 1.016
10
−
4
mm and
surface area of around 19,000m
2
kg
−
1
. Its pozzolanic activity with cement is estimated
at 120-200%. Cement and silica fume mix show a higher strength. Silica fume is said
to be a very effective pozzolanic as well as siliceous material, which in itself possesses
little or no cementitious property, but which in finely divided form and in the presence
of moisture will react chemically with calcium hydroxide to form compounds possess-
ing cementitious properties (Agarwal, 2006). Silica fume as a pozzolanic additive gives
cement increased strength, density and durability (Fleri and Whetstone, 2007).
There is limited available literature on cement, silica fume and peat, although sil-
ica fume is used as an additive to produce high-performance concrete. This is more
homogeneous than normal strength concrete, and if made with small aggregates it can
be compared to a strong rock. One of the benefits of using silica fume in Portland
cement-based composites is its performance as filler in capillary pores and the cement
paste-aggregate interface (Toutanji
et al
., 1999). Detwiler and Metha (1989) observed
that cement reacts with water to form calcium silicate hydrate and calcium hydroxide.
Silica fume reacts with the calcium hydroxide in the presence of water to form calcium
silicate hydrate. The increased calcium silicate hydrate gel and reduction in capillary
pores in the cement paste are the main factors in its increased strength and imperme-
ability. Silica fume and fly ash undergoes the same type of pozzolanic reaction. The
strength gain is more rapid than with fly ash but slower than that of cement. The
pozzolanic reaction is highly temperature dependent, so that the short-term strength
is lower at low temperatures. However, long-term strength is increased by replacing
part of the cement with silica fume (Janz and Johansson, 2002).
Some of the physical properties of silica fume are shown in Table 7.3.
The usual dosage of silica fume used along with cement in concrete mixtures is
5-10% by weight of cement (Bunke, 1988). Hooton (1993) has observed that addition
of 15 and 20% of silica fume decreases the tensile strength of 91 day-old concrete by
15 and 20% respectively. Toutanji
et al
. (1999) showed that the partial replacement
of cement by 8% of silica fume resulted in an increase in tensile strength of cement
mortar (cement and sand). The replacement of cement by a higher dosage of silica
fume (25%) resulted in a decrease in the tensile strength of mortar.
Kalantari
et al.
(2010) have attempted to stabilize or improve peat by using cement
as binders and silica fume (Figure 7.16) as additive. This study was carried out by
adding 5-50% of cement to peat and silica and measuring the compressive strength
(UCS) and California bearing ratio (CBR). Silica fume is a proven pozzolanic material
and its pozzolanic activity is estimated at 120-200% that of cement. The cement and
silica fume mix shows a much higher strength compared with cement when used alone.
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