Microsilica is predominantly silicon dioxide. Its prime characteristic is particle size
which would be as low as 0.2 micron, which is about 100 times smaller than Portland
cement grains. The extremely small grain size of microsilica is responsible for its high
reactivity with free lime in the concrete to form a strong and non-permeable paste. The
other important properties which established microsilica as a formidable building
material are its imperviousness to water, low permeability to chloride ion and resistant
to sulfate attack [Svenkerud 1991]. Table-1 shows typical composition of Elkam
Microsilica. Because of high surface area and high contents of amorphous silicon in
silica fumes, the latter acts as a highly active Pozzolan and reacts more quickly than
ordinary Pozzolans. The Pozzolanic reaction may begin as early as 2 days after cement
hydration [Mehta 1983, Sellervold et al.,1983] and the main Pozzolanic effect of silica
fume in concrete takes place between the ages of 3 and 28 days for curing at 20o
C. The
presence of silica fumes provides increased internal cohesion of fresh concrete. It is
possible to design fluid silica fume concrete with essentially no bleeding or segregation.
As a result, local areas of weakness such as bleed water channels and voids under coarse
aggregate particles can be eliminated. The transition zone between cement paste and
coarse aggregate particles is an especially critical region in most concrete. It is
frequently the weakest part because of bleed-water voids, yet it is under the greatest
stress because of the elastic mismatch between the cement paste and the relatively stiff
aggregate material. The presence of microsilica brings reduction of bleeding in fresh
concrete and in consequences, significant improvements in the density of the transition
zone and in the mechanical behavior of hardened concrete. The strength of the
transition zone can be further enhanced by a Pozzolanic reaction. Cheng and Zhang
[1986] cite the Pozzolanic reaction as being responsible for the increased strength of
transition zone in silica fume concrete. Silica fume particles may also act as nucleation
sites for the products of cement-hydration reaction. Mehta [1987] pointed out that this
effect would cause grain refinement of the hydration, products. One might expect
"grain refinement" to lead improved mechanical behavior as it does in metal and ice.