Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 27

involve penetrations by other steelwork at each floor level. In these circum-
stances any bleeding settlement could be disastrous in causing cracking at
vital locations.
Tremie concrete, and particularly any concrete that has to resist free
falling through water, also benefits from the incorporation of silica fume,
although other thickening agents such as viscosity modifying admixtures
are also used.
2.5.4 Shotcrete
Silica fume concrete can transform the economics of shotcreting and greatly
improve repair performance by its ability to reduce rebound and improve
adherence to the substrate in both the fresh and hardened state.
2.5.5 Surface finish
The inhibition of water movement through the mix is very beneficial for sur-
face appearance. Effects such as hydration staining, sand streaks, bleeding
voids on re-entrant surfaces, and settlement cracking can be avoided. A
possible problem is that the properties of the particular silica fume can
cause a substantial effect on colour. This is due to any carbon content and
is apparently more influenced by the size of the carbon particles than by
their percentage by weight.
2.6 RICE HUSK ASH (RHA)
Rice husk ash (RHA) is produced by burning rice husks (i.e., hulls or shells)
that contain a large proportion of silica. It has similarities with silica fume.
Chemically it is like silica fume in being almost pure silica. Its similarity to
slag is that the conditions of production are very important. As slag must be
cooled very rapidly to achieve a glassy or amorphous state (glassy is amor-
phous as opposed to crystalline; they are not alternatives) so RHA must be
burnt at between 550°C and 800°C to achieve that state. Burning at too
high a temperature gives a crystalline silica that is not reactive and a health
risk. However, it is important that the burning should be complete or the
ash will have a high carbon and variable content, which is anathema to the
uniform and effective performance of admixtures. However there has been
a report of ash with up to 23% of carbon being used successfully (Dalhuisen
et al., 1996). This was in tropical conditions where air entrainment was not
required. Unfortunately much of the so-called RHA marketed is not true
amorphous material. Most regions with rice husk resources use rice husk
for energy but few have utilised the benefit of producing true RHA through
controlled burning. A dangerous problem created by uncontrolled burning