Rice hull

The hull is hard to eat or swallow (unless finely ground) and mostly indigestible to humans because of its enriched fibre components.

However, during times of food scarcity in ancient China, a common daily meal was a pastry made from rice husks, wild vegetables, and soybean powder.

This led to the idiom "meals of cereal, hulls, and vegetables for half a year", indicating poverty and food insecurity.

[citation needed] A number of possible uses for RHA include absorbents for oils and chemicals, soil ameliorants, a source of silicon, insulation powder in steel mills, as repellents in the form of "vinegar-tar" release agent in the ceramics industry, as an insulation material.

[6] In southern India, charred rice hull, known as Umikkari in Malayalam and Tamil, was traditionally used for cleaning teeth before the advent of toothpaste.

It is popular as a cooking oil in the Indian subcontinent and East Asian countries, including India, Nepal, Bangladesh, Indonesia, Japan, Southern China, and Malaysia.

[citation needed] Rice hulls can be composted, but their high lignin content can make this a slow process.

[citation needed] Rice hulls that are parboiled (PBH) are used as a substrate or medium for gardening, including certain hydrocultures.

[8] Rice hulls are coated with fine-grained gunpowder and used as the main bursting charge in aerial fireworks shells.

[citation needed] With proper techniques, rice hulls can be burned and used to power steam engines.

[citation needed] Rice hulls themselves are a class A thermal insulating material because they are difficult to burn and less likely to allow moisture to propagate mold or fungi.

[14] Due to high amorphous silica content, the RHA (rice husk ash) can be used as a precursor material for geopolymer concrete.

[15] To achieve the best pozzolanic properties the combustion of the husks has to be carefully controlled by keeping the temperature below 700 °C (973 K) and to create conditions to minimize carbon formation by feeding sufficient air.