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close this bookJournal of the Network of African Countries on Local Building Materials and Technologies - Volume 3, Number 1 (HABITAT, 1994, 44 p.)
View the document(introduction...)
View the documentThe aim of the network and its journal
View the documentForeword
View the documentUganda: Follow-up actions with regard to the recommendations of the Workshop of the Network of African Countries on Local Building Materials and Technologies
View the documentUnited Nations Conference on Human Settlements (HABITAT II) - Istanbul, Turkey 3-14 June 1996 - “The City Summit”1
View the documentA strategy for effective participation of the African region in the preparatory process for the United Nations Conference on Human Settlements (Habitat II)2
View the documentHabitat II Preparatory Process and the Construction Sector
View the documentTechnology profile No. 1: Blended cements*
View the documentTechnology profile No. 2: Phosphogypsum as building material**
View the documentTechnology profile No. 3: Utilization of fly ash in the production of building materials***
View the documentEvents
View the documentPublications review - Published by UNCHS (Habitat)

Technology profile No. 3: Utilization of fly ash in the production of building materials***

*** Submitted by the Central Building Research Institute (CBRI), Roorkee, India

About 25 million tons of fly ash is produced per annum at 60 thermal power stations in India, at present. Since early 1960’s CBRI has been carrying out R and D work on characteristics of Indian fly ashes and their utilization in building industry.

The results of physical and chemical tests, such as X-ray, DTA and microscopic examination have shown that Indian fly ashes fall within the range of the average values for fly ashes produced abroad.

Table below shows the average constituents of common fly ashes




44.13 - 57.96


20.81 - 26.99


4.31 - 17.49


3.06 - 5.95


1.12 - 2.36


0.34 - 1.63

The Indian fly ashes, however, contain relatively higher amount of SiO2, Al2O3 and unburnt fuel and lower amounts of Fe2O3, SO3 and spheroidal glass. The crystalline phases present were identified as mullite, magnetite, haematite and quartz.

The important building materials, produced from fly ash by CBRI technical know-how are:

Portland-pozzolana cement

The Portland-pozzolana cement conforming to IS: 1489-1976 can be manufactured using fly ash either by intergrinding Portland cement clinker, fly ash and gypsum or by intimately blending together Portland cement and fly ash in suitable proportions. It is suitable for use wherever ordinary Portland cement is usable under normal conditions. It is low-heat giving and sulphate-resistant cement and therefore, is suitable for marine and hydraulic structures and mass concrete constructions.

Ready-mixed fly-ash concrete

Portland cement concrete in which a part of the cement has been replaced with fly ash is termed as fly-ash concrete. When it is prepared in plastic, unhardened, ready-for-the-use state, it is known as ready mixed fly ash concrete. It has a 28-days compressive strength equal to that of corresponding plain cement concrete. Batching and mixing of different ingredients is done at a central batching and mixing plant.

The ready mixed concrete has the advantages of better quality control, reduction in wastage and pilferage of materials, labour and supervision, which are normally associated with concrete prepared at site.

Precast fly-ash concrete building units

Work done at CBRI has shown that fly-ash concrete with 20 per cent less cement (by weight), having a one-day strength equal to corresponding plain cement concrete could be used for producing building blocks, flooring and roofing units (such as cored units, channel and cellular units).

Sintered fly-ash light-weight aggregate

The sintered fly ash light-weight aggregate (SFALA) is produced by:

(i) palletization of the fly-ash;
(ii) sintering the fly ash pellets in 1100°-1200°C in a vertical shaft kiln; or
(iii) on a moving grate sintering strand.

The production of SFALA from the Indian fly-ashes has been successfully carried out on a pilot plant moving grate sintering strand at CBRI (power: 12500 kwh/t, fuel: 10 per cent). The aggregate is suitable for use in the light-weight concrete and precast building units.

Table below shows some physical characteristics of light-weight concrete produced by SFALA:

Bulk density

1376 - 1776 kg/m3

Comprehensive strength (28 days)

152 - 400 kg/cm2

Bending strength

25 - 54 kg/cm2

Drying shrinkage

0.059 - 0.084 per cent

Lime fly ash cellular-concrete

Lime fly ash cellular-concrete consists of fine grained silicate structure having small and non-communicating air cells. It has a buck density of 400 - 1442 kg/m3 and good fire resistance and, like wood, it can be sawn, chiselled, screwed and nailed. Its building units having bulk density of 700 kg/m3 or higher are suitable for load bearing walls for 2 to 3 storey houses and partition walls in multistoreyed buildings.

Flay-ash building bricks

Good quality, high strength building bricks can be produced from fly ash using sodium silicate, cement or lime as binder. The mixture of fly ash, binder and coarse fillers such as bottom ash, sand etc. in suitable proportions, is moulded into bricks under pressure. While the sodium silicate bonded bricks are burnt at 1060° - 1100°C, the lime bonded fly ash bricks are autoclaved under saturated steam at a pressure of about 14 kg/cm2. The cement bonded fly-ash bricks are water cured at ambient temperature and dried before use. The process for manufacturing clay fly-ash bricks is similar to that of normal clay building bricks production except that certain amount of fly-ash (40 - 50 per cent) is mixed in the clay before moulding bricks. The fly-ash building bricks can be used like burnt bricks for all types of brick masonry.

Table below shows some physical properties of fly-ash building bricks

Water absorption

20 - 24 per cent

Bulk density

1317 - 1225 kg/m3

Crushing strength

140 - 150 kg/cm2

Portland cement clinker using fly-ash

Cement clinker was produced at 135°C (compared with traditional cement production which is at 145°C) by firing the nodules of raw mix containing fly ash instead of clay. Besides a substantial saving in fuel consumption, the use of fly ash in the raw mix has been found to enable production of cement clinker with MgO content of 6 per cent without causing unsoundness in the final cement.

Masonry cement

Masonry cement is mainly intended for use in place of ordinary Portland cement in masonry mortars. Work carried out at CBRI has shown that masonry cement conforming to the standard requirements can be produced by intergrinding 2 parts of fly-ash, 2 parts of hydrated lime, 1 part of Portland cement and 5 parts of granulated blast furnace slag with suitable quantity of gypsum and air entraining admixture.