rice husk Ash


Rice husks are the hard protective coverings of rice grains which are separated from the grains during milling process. Rice husk is an abundantly available waste material in all rice producing countries, and it contains about 30%–50% of organic carbon. In the course of a typical milling process, the husks are removed from the raw grain to reveal whole brown rice which upon further milling to remove the bran layer will yield white rice.

Current rice production in the world is estimated to be 700 million tons. Rice husk constitutes about 20% of the weight of rice and its composition is as follows: cellulose (50%), lignin (25%–30%), silica (15%–20%), and moisture (10%–15%). Bulk density of rice husk is low and lies in the range 90–150 kg/m3.

Most of the evaporable components of rice husk are slowly lost during burning and the primary residues are the silicates. The characteristics of the rice husk ash (RHA) are dependent on (1) composition of the rice husks, (2) burning temperature, and (3) burning time. The chemical composition of RHA is significantly dependent on combustion conditions, and the burning temperature must be controlled to keep silica in an amorphous state to give rice husk ash its pozzolanic reactivity.

Application and Uses

Rice husk ash can be used as a partial substitute for cement which alters the following properties of concrete:

- The heat of hydration is reduced to benefit drying shrinkage and facilitate durability of the concrete mix
- Reduced permeability of the concrete structure - avoiding the disintegration
- Increased resistance against chloride and sulfate

In addition to cement and concrete products, the rice husk ash can be used to create: 

  • Structural fills and embankments
  • Flowable fill
  • Waste stabilisation, absorbents
  • Agriculture and horticulture — soil amendment, fertiliser
  • Manufactured products.