Management of Fly Ash, in the Context of its Growing Production

By P. C. Mishra and Dr. R. K. Patel
December 2004

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.

Fly ash is a major byproduct of Thermal power plant, which makes a lot of pollution to the environment due to its fineness. As a large amount of fly ash is dumped in the nearby places of power plants it mixes in all segment of environment like water, air as well as soil. Though it can be utilized for different useful purposes due to lake of technical know how and proper motivation no progress has been made in its utilization. It has been observed that the fly ash can be used for making a varieties of building materials by using simple low cost or high investment processes. The strength of the bricks increases with increasing time. More over it can be use in its optimal quantity for better production of crops like rice, wheat etc.

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.

Materials & Methods

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

Table - 1
Compounds Nandira seam, Talcher Bharatpur seam, NALCO Power plant, RSP
SiO2 53.655.759.2
Al2O3 18.318.217.9
Fe2O3 12.711.29.5
Unburnt carbon
Other oxides

Results and Discussion

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

  1. Clay fly ash bricks
    20 to 50% fly ash depending upon the quality of soil, can be mixed with clay to produce burnt clay fly ash bricks. The green bricks are dried and fired in conventional kilns to obtain red clay fly ash bricks. Logistic problems of getting fly ash at the brick kilns or getting land near power stations to make bricks there have prevented commercialization of the technology.
  2. Compacted mud fly ash bricks.
    Compacted mud fly ash bricks stabilised with lime, cement and other chemicals can be made easily. Adoption of this technology has been found to be somewhere difficult due to the problem of dry fly ash at the site.
  3. Calcium silicate bricks.
    Utilisation of fly ash in place of quartz sands in sandlime bricks produces the calcium silicate bricks. The process involves compaction either by low pressure or high pressure followed by autoclaving under elevated hydrothermal conditions. The product from high pressure compacting is much superior but requires larger capital investment.
  4. Portland Pozzolona Cement (PPC)
    10 to 20% dry fly ash can be mixed with clinker during manufacture of cement or blended with finished ordinary portland cement to produce portland pozzolona cement. PPC requires more setting time than ordinary portland cement but strength is comparable with it.
  5. Cellular cement
    Lightweight aerated concrete or cellular concrete can be manufactured by a process involving mixing of fly ash, quick lime or cement or gypsum in a high-speed mixer to form a slurry. A small amount of foaming agents such as aluminium powder is added and mixed into slurry liberating hydrogen gas. The aerated slurry is poured in steel moulds and allowed to set. The blocks are removed after the initial setting and autoclaved at steam pressure. After autoclaving the blocks are allowed to cool and stacked for use. They are considered as excellent products for wall blocks and prefab floor slabs. The initial heavy investment and resistance to adoption by users has inhibited the commercialization of this technology.
  6. Sintered lightweight aggregate.
    This is produced by pelletisation or nodulisation of fly ash and sintering of the pallets or nodules at 1000 - 1300 degree centigrade. Unburnt fuel in fly ash nodules supports ignition. Sintered light weight aggregate substitutes stone chips in concrete, reducing dead weight.

Production of Fly ash bricks: Technology of manufacture

For production of good quality fly ash bricks, the quality of fly ash should be as under:

  1. It should be either dry or moist {containing moisture not more than 5 %}
  2. Visual appearance should be light steel grey or smoky grey in colour. The brownish or light yellowish grey colour fly ash is of inferior quality.
  3. The fly ash should be very fine and can pass through 200 mesh sieve.
  4. The unburnt carbon in fly ash with negligible fraction is tolerable for use.


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 :

  1. Manual press (with power)
  2. Vibro press (with power)
  3. Hydraulic press, with or without vibration.
  4. Screw press with or without wire cutting arrangement.
  5. Tampering hand moulding machines

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.

  1. Steam curing under high pressure {normally called autoclaved curing}
  2. Steam curing under normal pressure
  3. Hot water dip curing
  4. Hot water air curing
  5. Water tank curing
  6. Water curing in open air.

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.


  1. Various raw materials of brick mix in desired proportion are blended intimately in dry or wet form. Water/brick-mix ratio is maintained as explained above.
  2. The wet brick-mix is fed into the machine mould. The vibration is given for a while and the mould is again fed. The striper head is pressed and vibration is given simultaneously for about 8 seconds. The mould is lifted and bricks produced pallet is removed and kept on the platform for air drying.
  3. Next day the bricks produced on the previous day are put in the stack. The stack is formed with care to see that curing water and air for drying reach to every brick.
  4. After 3 days the hot water from the solar collector in small quantity is poured on the fresh stack without any pressure.
  5. After 5 days the solar collector water is poured on the bricks stack for 2 times a day.
  6. The bricks in stack after each watering are immediately covered with black PVC tarpaulin, with a clear space of 250 mm form the layers of the bricks, inside the closed cover.
  7. The curing is continued for 15 days and the tarpaulin cover is removed. The bricks are then left in the stack for drying or heating the bricks stack.
  8. The bricks are ready for despatch after 22 days from the date of manufacture.
  9. The comprehensive strength of the bricks produced from the brick-mix and the manufacturing process suggested here in, will be 80kg/cm² to100 kg/cm².

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.

Table - 2
Index Fly Ash Bricks Red Burned Clay Bricks
Size (mm)225 x 112.5x75225 x 112.5x75
Dry Density (kg/m³)15701700
Cold Crushing170100
Strength (kg/m²)  
Water Absorption (%)13 to 1520

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)

Table - 3 (Result seat of Paddy Trail, Demonstration)
Sl. No Items A B C D
1Name of the cropsPaddyPaddyPaddyPaddy
3Quantity of seeds100gm100gm100gm100gm
4Area of land20 sq.mt20 sq.mt20 sq.mt20
5Date of sowing20.1.200320.1.200320.1.200320.1.2003
6Use of fly ash 10kg15kg20kg
7Use of fertilizer285 gm grommer
133gm potash
285 gm grommer
133gm potash
285 gm grommer
133gm potash
285gm grommer
133gm potash
8Top dressing90gm urea90gm urea90gm urea90gm urea
9No. of irrigation13no13no13no13no
10Presticide 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
11Date of yield28.4.200328.4.200328.4.200328.4.2003
12Yield per

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.

Table - 4 (Result seat of Wheat Trail, Demonstration)
Sl. No Items A B C D
1Name of the cropsWheatWheatWheatWheat
3Quantity of seeds100gm100gm100gm100gm
4Area of land20 sq.mt20 sq.mt20 sq.mt20
5Date of sowing15.12.200215.12.200215.12.200215.12.2002
6Use of fly ash 10kg15kg20kg
7Use of fertilizer250 gm grommer
130gm potash
250 gm grommer
130gm potash
250 gm grommer
130gm potash
250 gm grommer
130 gm potash
8Top dressing80 gm urea80 gm urea80 gm urea80 gm urea
9No.of irrigation5 no5 no5 no5 no
10Presticide use
1stdose {2.2.2001}
60gm of furadon
60gm of furadon
60gm of furadon
60gm of furadon
60gm of furadon
60gm of furadon
60gm of furadon
60gm of furadon
11Date of yield10.04.0310.04.0310.04.0310.04.03
12Yield per


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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  2. Chandra D., Gupta R.L. and jain S.K. Auto calcium silicate bricks from different mining and industrial wastes
  3. Indian Standard: 3495 - 1966
  4. Indian Patent no. 128684
  5. Indian patent no. 139230
  6. Kaul H.M. -Eighth plan power strategy, Uria Feb, 1989
  7. Mukherjee S.M., Mitra B. and Majumdar S.K.-problem of thermal power station including waste disposal FST, Vol.2 Oct 1983,149.
  8. Report on "Changing Trends in Key Building Materials in Urban Housing Activities":- Centre of symbiosis of Technology, Environment and Management, Bangalore Ministry of science and Technology, Govt of India New Delhi - 1989.
  9. Seshagiri Rao M.V., Janadhana M and Swaroopa Rani M., International conference on civil engineering, Bangalore, July 2001


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