Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 30

Table 2.3 Indian classification of pozzolans
Activity Strength (MPa)
Very inactive <1.4
Inactive 1.4-2.8
Poor active 2.8-4.1
Intermediate 4.1-5.5
Active 5.5-6.9
Very active >6.9
Source : After Hammond, A. A., Appropriate
Building Materials for Low Cost Housing, Proc. of
Symp., Nairobi, Kenya, E. & F. N. Spon, New York,
1983, 73-83.
chemically. After processing, the benefits of this material have been found
to be similar to those obtained using a high-quality silica fume.
2.8 COLLOIDAL SILICA
A French development chemically produces silica in a colloidal form rather
than as a by-product from silicon ferrosilicon production. The material is
even finer than silica fume but, being in a liquid suspension, does not pres-
ent the same handling difficulties. It is more expensive but used at a lower
dose rate than silica fume. It is claimed to be particularly effective and
economical for shotcreting (Prat, 1996).
2 .9 METAK AOLIN
Metakaolin is a relatively new entrant to the pozzolan for concrete field. It
is produced by calcining kaolin, otherwise known as the china clay used
for ceramics. As with rice husk ash, it is important that it be fully calcined
but that the temperature not exceed approximately 800°C as this would
cause the formation of “dead burnt”, nonreactive mullite. The material is
an aluminosilicate that reacts with free lime in a similar manner to silica
fume and producing similar benefits when used in similar proportions of
5% to 15%.
Proponents point to the fact that metakaolin is a purpose-made controlled
product, whereas most pozzolans are by-products or waste materials.
Another important advantage is the relatively high aluminate component
that improves chloride binding. Being essentially a white pigment, it
produces concrete of a lighter shade. Since it also reduces efflorescence, it is
particularly suitable for coloured concrete.