Pozzolanas: rice husk ash and pulverised fuel ash
Traditionally, rice husk has been considered a waste material and has generally been
disposed of by dumping or burning, although some has been used as a low-grade fuel.
Nevertheless, RHA has been successfully used as a pozzolana in commercial production in a
number of countries including Columbia, Thailand and India.
Research and pilot projects have been undertaken in most of the major rice-growing
countries of the world. However it has been estimated that the total world production of
cement based on RHA is only 30,000 tonnes per year, mainly undertaken in small-scale
village production units. It is clear therefore that considerable potential exists to expand
production on both a small and large scale.
Republic of Korea
Table 1. Production of principal rice growing countries (million tonnes)
Processing and production of RHA
To produce the best pozzolanas, the burning of the husk must be carefully controlled to
keep the temperature below 700°C and to ensure that the creation of carbon is kept to a
minimum by supplying an adequate quantity of air. At burning temperatures below 700°C
an ash rich in amorphous silica is formed which is highly reactive. Temperatures above
700°C produce a crystalline silica which is far less reactive.
The presence of large quantities of carbon in the ash will adversely affect the strength of any
concrete or mortar produced using RHA cements. Where possible, the carbon content of the
ash should be limited to a maximum of 10%, although some studies have suggested higher
percentages can be tolerated with only a relatively small decrease in strength.
Rice husks which have been burnt in large open heaps to dispose of waste husks, or burnt as
a fuel in an industrial furnace, are unlikely to produces ashes with the specification
described above. In particular, they are likely to be crystalline due to high combustion
temperatures. Although this does not rule out their use as a pozzolana, ashes composed of
crystalline silica will require a considerable amount of grinding (see below) to produce an
acceptable reactivity. Even then they are unlikely to match the quality of amorphous ash.
There are several designs of small simple incinerators, normally made of fired clay bricks,
which are capable of burning ash at temperatures below 700°C and without excessive
quantities of carbon. The temperature is monitored by a pyrometer (an industrial instrument
for measuring high temperatures) and rapid cooling is necessary if the temperature rises
above 650°C. This is normally achieved by removing the ash and spreading it on the ground.
Incinerators of this type are normally used in banks of three or four to produce approximately
one tonne of ash per day.