Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 28

either in the fields or boilers is the disposal of the carcinogenic crystalline
ash produced. As most of the rice-producing countries are in the develop-
ing world, the care in disposing of this dangerous waste may not always be
up to the required standard. Foo and Hameed (2009) state that “the price
of the ash disposal cost (either in landfills or ash ponds) hitting as high as
$5/tonne in developing countries and $50/tonne in developed countries”.
The particles are “fluffy”. They are much larger than silica fume particles
and yet have a higher surface area due to their vesicular nature. Depending
on the production process, it may be necessary, and relatively easy, to grind
such particles to avoid excessive water demand and resistance to compac-
tion. With such a material, it is clearly important to evaluate product from
a particular source for performance and uniformity since it can range from
being as valuable as (and similar to) silica fume to being as deleterious as
silt when incorporated in concrete.
There are substantial quantities (millions of tons) of rice husks available
annually in many parts of the world. They constitute a potentially valu-
able resource if suitably prepared, rather than being a large-scale nuisance
and health hazard after burning indiscriminately to reduce volume or gain
energy from combustion.
2.7 NATURAL POZZOLANS
The concrete industry started with the Romans and was based on the natu-
ral pozzolans. The term pozzolan comes from an Italian word pozzolana ,
which means “earth of Pozzuoli”, which is a city near Naples. The mag-
nificent dome on the Parthenon in Rome is an inspiring example of the use
of concrete, particularly when you consider it was built 1900 years ago. In
spite of their impressive history, natural pozzolans have not been widely
used in the modern concrete industry. Industrial by-products such as fly
ash, GGBS, and silica fume have dominated as supplementary cementitious
materials. However, the tide may be turning. Although there is still a great
deal of fly ash available, modifications to coal-fired power generation have
made fly ash more variable and efforts to reduce dependence on energy
from coal will tend to increase this trend. GGBS and silica fume supplies
are quite limited. Another important advantage of natural pozzolans as a
substitute for Portland cement and other SCMs is when they are locally
available and do not require importation into less developed countries.
Natural pozzolans can be classified based on their chemical or mineral-
ogical composition or based on strength properties when it is reacted with
either Portland cement or lime. For more in-depth discussion of natural
pozzolans, refer to definitive guides such as ACI 232.1R-00.
Mehta (1987) classified natural pozzolans in four groups based on the
principal lime-reactive constituent present: (1) unaltered volcanic glass,