The Authors are Research Scholars and Senior Lecturers at the Departmet of Chemistry of the National Institute of Technology in Rourkela, India, one of the premier national level institutions for technical education in the country.
Key Words: - Fly ash, Thermal Power Plant, Conventional clay bricks, Curing, water bearing capacity, comprehensive modalities
Fly ash is a major by product of thermal power plant. It is a very fine material about 60-70% of which has a size below 0.076 mm. as it is formed by the burning of pulverized coal. The disposal of such fly ash is creating a serious problem as per its storage space and cost involved in its storage. At the same time there is a lot of pollution of the environment due to the fineness of the fly ash. The effects for its utilization for many gainful purposes have been made since late sixties of this century by various research institutions and public enterprises, but nothing spectacular has really been seen yet in commercial utilities. The present utilization of the fly ash is about 2% of the total generation. For the present development scenario of India, one of the thrust area is infrastructure where generation of power holds major share. It is estimated that at present about 125 million tonnes of fly ash is generated every year from 82 thermal power plants. This amount will reach 200 million tonnes very soon. The fly ash disposal is going to be a major problem in near future.
The fly ash or pulverised fuel ash is formed as a result of burning pulverised coal. The principal contents of fly ash are normally silica (30-60%), alumina (15-30%), iron oxide and carbon in the form of unburnt fuel up to 20%, lime 7% and small quantities of magnesium oxide and sulphate. Indian coals normally contain 25 to 40% ash. The main problem of utilization of fly ash comes from the unburnt carbon as it has no binding force or any other properties which can be utilize other than the fuel. The fly ash samples are collected from Nandira seam (Talcher), Bharatpur seam (NALCO) and power plant (Rourkela Steel Plant). The fly ash are analysed by standard methods and the data are summerised in Table - 1
Compounds | Nandira seam, Talcher | Bharatpur seam, NALCO | Power plant, RSP |
SiO2 | 53.6 | 55.7 | 59.2 |
Al2O3 | 18.3 | 18.2 | 17.9 |
Fe2O3 | 12.7 | 11.2 | 9.5 |
CaO | 3.8 | 2.5 | 3.2 |
MgO | 1.2 | 1.9 | 1.3 |
Sulphate | 1.3 | 0.9 | 1.2 |
Unburnt carbon | 7.2 | 7.5 | 7.0 |
Other oxides | 1.9 | 2.1 | 0.7 |
The fly ash can be used for making a verity of building materials, some using simple low cost processes and others high investment processes producing high quality products. They can also be used as fertilizer. The fly ash is processed to increase the surface area by grinding and to remove and reduce the unburnt carbon. This fly ash so produced is activated fly ash and gives superior engineering properties like higher crushing strength and are more reactive with lime or cement. It can be blended uniformly to give portlant pozzolana cement. It can be mixed directly with lime to give desired properties of mortar or concrete, as required. Fly ash can be mixed in varied proportions with lime or portland cement to give blends for better engineering properties.
Through it may give only 60% comprehensive strength of protland cement it will get to be on par with portland cement after 28 days and exceed its strength after three months. Due to its silicate base it encumbrance hydration of cement. Considering its affinity towards lime, it eliminates alkali aggregate reaction which is often the base of ordinary portland cement. It is highly resistant to sulphate attack hence advantageous along the sea shore. The activated pozzolona has finer surface structure resulting in low porosity. Its lower water demand makes it safer bet in mass concrete constructions such as dams. Since it liberates low heat on hydration, it gives better strength and lasts longer. It is known as low heat cement and because of this property, it accounts for its affinity to be free from creaks, provided curing process is faultless. Amongst building materials made out of fly ash, the pride of place goes to bricks. No fuel, no water or steam is required in producing fly ash bricks. The compressive strength of bricks made from fly ash is twice that of burnt clay bricks. In addition, this will prevent the precious top soil being destroyed as in the case of clay bricks. The chances of breakage while handling are as low as 1 to 2% as against 12 to 15% in the case of clay bricks. The fly ash bricks are more resistant to salinity, water seepate, have minimal air and liquid pollution, ingress hazards and are more economical. These cost only Rs. 650/- per thousand bricks as against Rs.800/- to Rs.1000/- per thousand conventional clay bricks. Cable tiles are being sold @ Rs. 1.80 against Rs. 4/- burnt brick tile. The fly ash can be used for making a variety of building products, some using simple low cost processes and others high investment processes, producing high quality product such as
For production of good quality fly ash bricks, the quality of fly ash should be as under:
Raw Materials:
A mix of the ingredients is prepared by intimate mixing in suitable blender/mixer. Manual mixing will not give the desired results and hence hand mixing should be avoided. This mix ultimately gives comprehensive strength of 80 - 110 kg/cm² fly ash bricks. The water, bricks mix ratio be maintained between 6 to 7 %. This percentage changes with different mix raw material ratio. For moulding the bricks, many types of machineries of indigenous make are available. They are :
Selection of machinery depends on the bricks mix contents. For manufacturing fly ash lime stabilised bricks, the best suited machinery is virbo - press machine, which is an indigenous low cost machine and can be run by ordinary semiskilled worker. Its production capacity is 1000 bricks per shift and can be operated for two shifts without any operation/maintenance load. The maintenance cost is so low that it can be ignored. 15 lakh bricks can be produced for each machine in its life cycle.
The stabilized bricks after moulding are further hardened by curing. The chemical changes occur in the bricks mix contents after moulding and heat of hydration is evolved. The rate of the effect of heat of hydration is mitigated and lowered with sufficient water in alkali solution is provided to accelerate pozzolanic reaction. There are different process of curing.
The cost of curing in all the processes varies and minimum cost involvement is in "water curing in open air" and maximum cost involvement is in "autoclaved pressurised curing". Water is heated by low cost solar collector and further increase in temperature of water is made by covering the brick stack by black tarpaulin, after watering the stack by hot water from solar collector. Unpressurised hot water vapours are produced and the vapours are allowed to pass through the whole stacks between individual bricks. It accelerates the pozzolanic reaction and reduced final time.
It is observed that the fly ash bricks produced are found to be superior then that of conventional Red burnt clay bricks. The fly ash bricks confirm to the Indian standard IS : 3495 - 1966. The technical comparison of fly ash bricks verses red burnt clay bricks are given in Table - 2.
Index | Fly Ash Bricks | Red Burned Clay Bricks |
Size (mm) | 225 x 112.5x75 | 225 x 112.5x75 |
Dry Density (kg/m³) | 1570 | 1700 |
Cold Crushing | 170 | 100 |
Strength (kg/m²) | ||
Water Absorption (%) | 13 to 15 | 20 |
More over they can also be used in the manufacture of mosaic tiles, plain tiles, prestressed roofing steps, thermal insulation bricks and road sub-grades. The fly ash can also be used as fertilizer to increase the production of crops particularly rice, wheat and cereals. But they can not be used in any quantity for better production. Depending on the type of crops, an optimum amount of fly ash can be used for better production. Field trails has been made in this regard. (Table 3 and 4)
Sl. No | Items | A | B | C | D |
1 | Name of the crops | Paddy | Paddy | Paddy | Paddy |
2 | Variety | Parijat | Parijat | Parijat | Parijat |
3 | Quantity of seeds | 100gm | 100gm | 100gm | 100gm |
4 | Area of land | 20 sq.mt | 20 sq.mt | 20 sq.mt | 20 sq.mt |
5 | Date of sowing | 20.1.2003 | 20.1.2003 | 20.1.2003 | 20.1.2003 |
6 | Use of fly ash | 10kg | 15kg | 20kg | |
7 | Use of fertilizer | 285 gm grommer 133gm potash | 285 gm grommer 133gm potash | 285 gm grommer 133gm potash | 285gm grommer 133gm potash |
8 | Top dressing | 90gm urea | 90gm urea | 90gm urea | 90gm urea |
9 | No. of irrigation | 13no | 13no | 13no | 13no |
10 | Presticide use 1stdose {04.03.01} 2nddose {12.3.01} | 60gm of furadon 60gm of furadon | 60gm of furadon 60gm of furadon | 60gm of furadon 60gm of furadon | 60gm of furadon 60gm of furadon |
11 | Date of yield | 28.4.2003 | 28.4.2003 | 28.4.2003 | 28.4.2003 |
12 | Yield per sq.mt. | 390gm | 840gm | 950gm | 750gm |
The increase in production may be due to the presence of Iron Oxide, calcium Oxide. Silica made the soil sandy and because of this air circulation is easier and water bearing capacity increases. The fly ash can be used to increase the agricultural production which is evident from the table - 3 & 4 is seen from the data when the amount of fly ash increases, the productivity of rice increases but at the same time when the amount increases from certain amount the production decreases. So the amount of fly ash depends on the quality of soil where it is used. However it requires further investigations before any conclusion is drawn.
Sl. No | Items | A | B | C | D |
1 | Name of the crops | Wheat | Wheat | Wheat | Wheat |
2 | Variety | RR-21 | RR-21 | RR-21 | RR-21 |
3 | Quantity of seeds | 100gm | 100gm | 100gm | 100gm |
4 | Area of land | 20 sq.mt | 20 sq.mt | 20 sq.mt | 20 q.mt |
5 | Date of sowing | 15.12.2002 | 15.12.2002 | 15.12.2002 | 15.12.2002 |
6 | Use of fly ash | 10kg | 15kg | 20kg | |
7 | Use of fertilizer | 250 gm grommer 130gm potash | 250 gm grommer 130gm potash | 250 gm grommer 130gm potash | 250 gm grommer 130 gm potash |
8 | Top dressing | 80 gm urea | 80 gm urea | 80 gm urea | 80 gm urea |
9 | No.of irrigation | 5 no | 5 no | 5 no | 5 no |
10 | Presticide use 1stdose {2.2.2001} 2nddose{2.3.2001) | 60gm of furadon 60gm of furadon | 60gm of furadon 60gm of furadon | 60gm of furadon 60gm of furadon | 60gm of furadon 60gm of furadon |
11 | Date of yield | 10.04.03 | 10.04.03 | 10.04.03 | 10.04.03 |
12 | Yield per sq.mt. | 300gm | 800gm | 910gm | 650gm |
The results of the present study indicate that Fly ash can be used for different useful purposes. It can be used for manufacture of a varieties of building materials. Proper know how and motivation can make uses of fly ash which reduces the pollution load. It can also be used for growing production of food crops but it require a comprehensive modalities.
The authors are thankful to Prof. S. K. Sarangi, Director, Prof. K. M. Purohit, HOD, Dept of Chemistry, N.I.T Rourkela for providing necessary facilities. The authors are also thankful to Prof. B. Pradhan, Dept. of Chemistry, NIT, Rourkela for his valuable suggestion and encouragement and also grateful to the staff in the Dept. for their valuable help.
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