Concrete can get cheap and green

(India)  —  Reinforcement of concrete continues to be a hot topic the world over. From steel to synthetic plasticisers, many kinds of material have been tried and used to enhance the properties and quality of concrete.

A popular material that is emerging now is rice husk ash (RHA). Usually used for repair and rehabilitation of existing structures, RHA is emerging as a cost-effective, energy-efficient and eco-friendly addition to conventional concrete.

“RHA is an agro by-product that finds application in the chemical industry and as fuel in kilns. In Kerala, RHA or ‘umikari’ has been used over generations for cleaning teeth.

However, very few industries in India have found applications for RHA. The disposal of the large quantities of RHA produced in rice mills has emerged as an environmental issue. However, if used in the construction sector as an economical concrete admixture, the problem of disposal can be solved,” said Helvin Antony, final-year Civil Engineering student at Sun College of Engineering and Technology, Nagercoil.

Antony, along with classmate Salahudheen Ayubi, recently did a project on the application of RHA as a concrete admixture under the guidance of their head of department S.U. Kannan.

They found that the optimum addition of RHA to the concrete mixture can increase the compressive strength, durability and workability of concrete blocks, while bringing down the amount of cement required, making it more economical.

“We referred literature on RHA research as part of this project and we found that the property of RHA that makes it a good concrete admixture is its high silica content. When burnt in a specific manner at an optimum temperature, RHA precipitates silica, which makes it pozzalonic (a material which, when combined with calcium hydroxide, exhibits cementitious properties),” Antony said.
Prevents corrosion

RHA forms a calcium silicate hydrate gel around the cement particles, which is highly dense and less porous. This prevents cracking of concrete blocks and protects it from corrosion, consequently increasing its corrosion resistance.

The students used ordinary M-20 grade concrete mix for the experiment. They found that by replacing 5 per cent of the cement with RHA, the compressive strength of each block could be increased by around four Newtons per square mm.

“M-20 is a comparatively low-grade mixture. If a higher-grade concrete mixture is used, it is possible to achieve up to 30 per cent more compressive strength by replacing 10-12 per cent of cement with RHA,” said Antony.

However, if a higher amount of RHA is mixed, the compressive strength goes down. This is probably because of the water-absorbing character of RHA.

“We found that an optimum amount of water absorption is good for the cohesive bond within the concrete mixture, but a higher RHA component leads to weakening of the bond,” said Antony.
Improves strength

RHA also helps in increasing the flexural (bend) strength of concrete, resistance to chemical attack, durability and workability. It also reduces shrinkage, apart from being an excellent thermal insulation. The concrete is less likely to allow moisture in that propagates mould or fungi.

“Being locally available, renewable and non-toxic, RHA contributes to the concept of green concrete,” said Sixtus Paulson, a civil engineer at Travancore Titanium Products.

“In Kerala, the concept still needs to be explored, but RHA is available in abundance in Tamil Nadu and is widely used as a concrete admixture.

In fact, rice mills in Tamil Nadu sell RHA to concrete-mixing units and cement companies for this purpose,” pointed out Paulson . The two students are now planning to repeat their experiment by using a combination of RHA and super plasticiser in the concrete mixture.

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