Rice husk ash

Millions of tonnes of rice husk is produced every year that, when burnt, produces a pozzolanic ash that can be added to cement in place of clinker.

Rice husk is the outer covering of the grain of rice. This covering is removed when paddy rice is processed, leaving a waste product that contains 15%-20% silica, as well as cellulose and lignin. When burnt, the cellulose and lignin is removed, leaving a pozzolanic ash that can be added to cement as a replacement for the clinker.

Research in the 1970s found that the pozzolanic reactivity of rice husk ash was influenced by the environment in which it was heated. This resulted in the development of a fluidised bed furnace that enabled the controlled combustion of rice husk and the production of a highly-pozzolanic material.

Its use in concrete is associated with the range of potential benefits common to all pozzolans: improved long-term strength and durability, including reduced permeability and increased resistance to chemically-aggressive environments and alkali-silica reactivity.

Research has suggested that the addition of rice husk increases setting time, however, although this can be offset by a reduction in the size of rice husk ash particles.