Use of Rice husk ash in Concrete

Rice husk ash

Rice milling generates a byproduct know as husk. This surrounds the paddy grain. During milling of paddy about 78 % of weight is received as rice, broken rice and bran .Rest 22 % of the weight of paddy is received as husk. This husk is used as fuel in the rice mills to generate steam for the parboiling process. This husk contains about 75 % organic volatile matter and the balance

25 % of the weight of this husk is converted into ash during the firing process, is known as rice husk ash (RHA). This RHA in turn contains around 85 % - 90 % amorphous silica.

So for every 1000 kgs of paddy milled, about 220 kgs (22 %) of husk is produced, and when this husk is burnt in the boilers, about 55 kgs (25 %) of RHA is generated.

India is a major rice producing country, and the husk generated during milling is mostly used as a fuel in the boilers for processing paddy, producing energy through direct combustion and / or by gasification. About 20 million tons of RHA is produced annually. This RHA is a great environment threat causing damage to the land and the surrounding area in which it is dumped. Lots of ways are being thought of for disposing them by making commercial use of RHA

Rice husk ash is produced by incinerating the husks of rice paddy. Rice husk is a by-product of rice milling industry. Controlled incineration of rice husks between 500ºC and 800ºC produces non-crystalline amorphous RHA.

The physical properties of RHA largely depend on burning conditions. Particularly, the period and temperature of burning affect the microstructure and characteristics of RHA. The partial burning of rice husks produces black RHA whereas the complete burning results in either white or grey RHA. In addition, the uncontrolled burning at high temperature produces crystalline RHA, which possesses poor pozzolanic property. Conversely, the controlled burning at about 500ºC to 8000C results in non-crystalline or amorphous silica, which shows very high pozzolanic activity. The burning condition also affects the relative density of RHA. The relative density of grey RHA obtained from complete burning is generally 2.05 to 2.11.

The role of RHA in SCC is the same as that in any other concretes. In concrete, the RHA mainly serves as a micro filler, pozzolan, and viscosity modifier. The RHA particles can fill the voids between the larger cement grains because of their smaller size. However, I had used unprocessed rice husk ash particle size may varying. Although RHA is not very fine in particle size, it behaves as a very reactive pozzolanic material because of its extreme surface fineness and high silica content. In the presence of water, the RHA actively reacts with Ca(OH)2 liberated during cement hydration (pozzolanic reaction) and produces additional calcium silicate hydrate (CSH),

Specific gravity of rice husk ash was 1.96

Disposal of rice husk ash

Most of the large units dispose their boiler ash on part of their own land, converted as landfills. After disposal of boiler ash on the land it is covered with a layer of soil and plantation developed over. Recycled wastewater is periodically sprinkled on the ash disposal site to avoid wind blowing of fly ash in the nearby areas.

Most of the small and medium mills not having their own disposal facilities generally dispose their

husk ash, through contractors, who dispose it to available low lying areas. Most mills give this job on contract basis where the contactors collect the boiler ash from the unit premises and dispose it on payment basis. In this case even the generator of ash is not aware of the ultimate disposal site of its boiler ash. In the areas where use of boiler ash is common in fields by farmers, the mills or contractors also sell boiler ash for use in the farms.

Potential for use of rice husk ash in India and Gujarat

Worldwide over 700 million tonnes of rice produced annually, in India approximately 27 million tonnes RHA is been produced per year. it has been estimated that some 120 million tonnes of husk could be available annually on a global basis for pozzolana production. As the ash content by weight is about 20%, there are potentially 24 million tonnes of RHA available as a pozzolana. Rice is grown in large quantities in many Third World countries including China, the Indian sub-continent, South-east Asia and, in smaller quantities, in some regions of Africa and South America. Traditionally, rice husk has been considered a waste material and has generally been disposed of by dumping or burning, although some has been used as a low-grade fuel. Nevertheless, RHA has been successfully used as a pozzolana in commercial production in a number of countries including Columbia, Thailand and India. In Gujarat around more than 500 rice husk mills are producing rice husk ash.

Effect of rice husk ash in concrete

The financial gain obtained by changing a substantial part of the Portland cement by inexpensive, pollution free, natural pozzolans or industrial waste, RHA is industrial waste. The dropping of the blended cement environmental cost linked with the greenhouse gases produced during Portland cement construction. It is improved durability of the concrete.

The properties of hardened blended cements are powerfully linked to the development of the binder microstructure, i.e., to the distribution, type, shape and dimensions of both reaction products. The useful properties of pozzolan addition in terms of higher compressive strength and better durability are mostly credited to the pozzolanic reaction in which calcium hydroxide is consumed to produce additional C-S-H and C-A-H reaction products. These pozzolanic reaction yields fill in pores and result in a refilling of the pore size distribution or pore structure. This results in a lowered permeability of the binder.