PRIpossibility of producing more dense concrete with a very low porosity, the pores being small
and discontinuous, and therefore, with a high strength and a low permeability.
2 2g 2
! X The specific gravity of silica fume is 2.20. The silica
fume is available as—produced in untensified from with bulk density of 200-300 kg/m^3 ; densified
from with bulk density of 500-600 kg/m^3 ; in the form of micro pellets with bulk density of
600-800 kg/m^3 ; or in a slurry form with the desired concentration (generally with density of
1400 kg/m^3 ). The other basic characteristics are: specific surface area of 1500-2000 cm^2 /g ,
standard grade slurry pH value of 4.7, specific gravity of 1.3-1.4 and dry content of micro silica
of 48-52 per cent.
X With the addition of silica fume the slump loss with time is directly proportional
to the increase of silica fume content. This is simply due to the introduction of large surface area
in the concrete mix by its addition. Although the slump decreases but the mix is highly
cohesive. However, with the adjustment in the aggregate grading and by the use of super
plasticizer the demand for additional water can be minimized.
2 2f X Silica fume reduces bleeding significantly. This effect is caused
mainly by the high surface area of the silica fume to be wetted, thereby reducing the free water
left in the mixture for bleeding. Moreover, the silica fume reduces bleeding by physically
blocking the pores in the fresh concrete. In the absence of bleeding and due to slow movement
of water from interior to the surface, it is essential to finish the concrete as soon as possible after
it has been placed and compacted to protect the surface from drying out. It therefore, requires
early curing; membrane curing is most effective.
2 X 2 The addition of silica fume in small amounts to ordinary concrete mixtures
(250-300 kg/m^3 ) has no significant effect on setting times. However, set retarding occurs with
the increased silica fume content. It can also be used in combination with fly ash or blast-
furnace-slag to develop strength at early ages.
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X The attributes of silica fume have found their use in shotcrete applications,
pumped concrete, mining and chemical industries. The high strength concrete made with silica
fume provides high abrasion/erosion resistance. Silica fume influences the rheological properties
of the fresh concrete, the strength, porosity and durability of hardened mass. Silica fume
concrete with low water content is highly resistant to penetration by chloride ions.
WIH sgi2r u2er
The combustion of agricultural residues volatises the organic matters and a silica-rich ash is
produced. Of all the agricultural wastes, rice husk yields the largest quantity of ash with about
93 per cent silica which gives it puzzolanic properties.
When burned in ordinary way rice husks produce a crystalline silica ash. However, if
burned under suitable conditions, a highly reactive black non-crystalline silica residue having
puzzolanic properties is produced. Temperature and duration of combustion are of utmost
importance for good quality rice husk ash. The right temperature is 700°C for 2-3 hours. Thus,
a controlled combustion of rice husk in electricity generation plants produces amorphous or
non-crystalline silica with about 85-90 per cent cellular particles. These particles are highly
micro porous and possess a very high specific surface (5 × 10^5 to 10 × 10^5 cm^2 /g).