Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 24

400

350

300

250

200

OPC

150

30%

100

50%

65%
80%

50

0

0 10 20 30 40 50 60 70 80 90 100

Age (days)

Figure 2.4 Effect of replacement percentages of GGBS on 91-day autogenous shrinkage
strain. (From Alfred, J. M., and Lim, S. N., Factors Affecting the Autogenous
Shrinkage of Ground Granulated Blast-Furnace Slag Concrete, 8th CANMET/
ACI International Conference on Fly Ash, Silica Fume, Slag and Natural
Pozzolans in Concrete, ACI SP 221, 2004, pp. 783-769.)

Silica fume (also known by one proprietary name “Microsilica”) is a
by-product of the manufacture of silicon, ferrosilicon, or the like, from
quartz and carbon in electric arc furnaces. It is usually more than 90%
pure silicon dioxide, and is a superfine material with a particle size of the
order of 0.1 micron and a surface area of over 15,000 m 2 /kg (i.e., a hundred
times greater than cement or fly ash). Its relative density is similar to that
of fly ash at about 2.3 but, owing to its extreme fineness, it has a very low
bulk density of only 200 to 250 kg/m 3 in its loose form. For this reason it is
usually handled either in a condensed form or as a 50/50 slurry with either
water or a superplasticising admixture. In the condensed form, particles are
agglomerated by aeration and the bulk density increases to 500 to 700 kg/
m 3 . The problems with maintaining a stable slurry have meant that slurried
silica fume is rarely used.
There is disagreement as to whether use of silica fume increases water con-
tent. This depends on how it is used. At very low water to cement ratio, the
silica fume particles displace water reducing the requirement for water. To be
fully effective it must be dispersed so that it occupies spaces between cement
grains and must not remain in clumps of silica fume particles. Undispersed
lumps of silica fume can act as sites for alkali silica reaction (Maas et al.,
2007). The authors' experience and research (i.e., Lagerblad and Utkin, 1993)
have shown problems with satisfactory dispersion of the silica fume. The
mixing and grinding action in many mixers does not seem to be adequate
to ensure dispersion. One method to overcome this tendency is to premix