Rice Husk Ash in Concrete -Properties, Advantages and Uses in Construction

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Rice husk ash is used in concrete construction as an alternative of cement. The types, properties, advantages and uses of rice husk in construction is discussed. The rice paddy milling industries give the by-product rice husk. Due to the increasing rate of environmental pollution and the consideration of sustainability factor have made the idea of utilizing rice husk. The reasons behind the usage of rice husk as an alternative for cement in concrete manufacturing are explained in the following sections. To have a proper idea on the performance of rice husk in concrete, a detailed study on its properties must be done. About 100 million tons of rice paddy manufacture by-products are obtained around the world. They have a very low bulk density of 90 to 150kg/m³. This results in a greater value of dry volume. The rice husk itself has a very rough surface which is abrasive in nature. These are hence resistant to natural degradation. This would result in improper disposal problems. So, a way to use these by-products to make a new product is the best sustainable idea. Among all industries to reuse this product, cement, and concrete manufacturing industries are the ones who can use rice husk in a better way.

Rice Husk Ash as a Supplementary Binder

The rice husk ash has good reactivity when used as a partial substitute for cement. These are prominent in countries where the rice production is abundant. The properly rice husk ashes are found to be active within the cement paste. So, the use and practical application of rice husk ash for concrete manufacturing are important.

Rice Husk Ash Classification and Chemical Composition

The rice husk ash possesses a chemical composition similar to many of the organic fibers. Rice husk ash consists of the following:

• Cellulose (C₅H₁₀O₅)
• Lignin (C₇H₁₀O₃)
• Hemicellulose
• SiO₂
• Holocellulose

These are compounds within them in common. The rice husk ash may vary depending upon the source as well as the type of treatment. Treatment in the sense the rice husk is burned to have proper properties. So, the method of heating can also bring changes in the overall chemical composition of the ash. The silicates are one of the primary components of the rice husk ash. During the burning process, the components that can evaporate are evaporated and the only component left are the silicates. The rice husk ash to be more precise have characteristics based on the components, the temperature of burning and the time of burning. The silicates are the component that gives the pozzolanic reactivity capacity for rice husk ash. So to gain this, the silica must remain in its non-crystalline form. They should gain a highly porous structure within their microstructure. So, this makes it clear that a proper quality burning of rice husk to get rice husk ash would remove the cellulose and rice husk components preserving the original cellular structure of the rice husk particles. The chemical composition of rice husk ash, in general, is given in Table-1 Table.1: Chemical Composition of Rice Husk Ash

Sl. No	Particulars	Proportion
1	Silicon Dioxide	86.94%
2	Aluminium Oxide	0.2%
3	Iron Oxide	0.1%
4	Calcium Oxide	0.3 - 2.25%
5	Magnesium Oxide	0.2 - 0.6%
6	Sodium Oxide	0.1 -0.8%
7	Potassium Oxide	2.15 - 2.30%

Burning are done in open air traditionally. This method causes large air pollution that causes lung and eye diseases.

Burning Process of Rice Husk

As mentioned above the quality of rice husk is greatly dependent upon the type of burning process undergone. Different types of burning are carried out, which are mentioned below:

a) Open - Field Burning

This method of open burning of rice husk ash produces poor quality rice husk ashes. These produces highly crystalline form structure which is of lower reactivity.

b) Fluidized -Bed Furnace Burning

This is a controlled method of burning the rice husk. The combustion heat of rice husk is used for the generation of electricity. The controlled operation is carried out by maintaining the time -temperature parameter. A combustion temperature between 500 to 700 degrees Celsius. This is carried out for a longer time so that complete removal of the carbon is taken. Or the temperature is increased from 700 to 800 degree Celsius for a time of 1 minute. The fluidized bed furnace method produced rice husk ashes have 80 - 95% of silicon dioxide, 1-2% of K₂O and 3-18% of unburned carbon. The rice husk ash produced by this method have a higher cellular structure.

c) Industrial Furnace

This method is put forward to facilitate environmental and economic reasons. The efficiency of combustion would bring up a silica dioxide in the range of 90 - 95%. This method helps in producing the rice husk ashes with amorphous silica and cellular structure products in an easier way. The rice husk ash produced by this method is highly pozzolanic.

Variation of Properties of Concrete with Rice Husk

The incorporation of rice husk ash in concrete convert it into an eco-friendly supplementary cementitious material. The following properties of the concrete are altered with the addition of rice husk:

• The heat of hydration is reduced. This itself help in drying shrinkage and facilitate durability of the concrete mix.
• The reduction in the permeability of concrete structure. This will help in penetration of chloride ions, thus avoiding the disintegration of the concrete structure.
• There is a higher increase in the chloride and sulfate attack resistance

The rice husk ashes in the concrete react with the calcium hydroxide to bring more hydration products. The consumption of calcium hydroxide will enable lesser reactivity of chemicals from the external environment.

Applications of Rice Husk Ash

The rice husk ash is a green supplementary material that has applications in small to large scale. It can be used for waterproofing. It is also used as the admixture to make the concrete resistant against chemical penetration. The main applications of rice husk ash in the construction are:

• High-performance Concrete
• Insulator
• Green concrete
• Bathroom floors
• Industrial factory floorings
• Concreting the foundation
• Swimming pools
• Waterproofing and rehabilitation

Read More: Characteristics of Rice Husk Ash Concrete – Workability and Strength