WHICH MATERIAL CAN BE USED AS SUBSTITUTE OF CEMENT FOR MAKING CONCRETE

PARTIAL REPLACEMENT OF CEMENT

Ground granulated blast furnace slag (GGBS)
Ground granulated blast furnace slag (GGBS) is a non-metallic product obtained as a by product from the blast furnaces used to make iron. Ground granulated blast furnace slag consists of silicates and aluminates of calcium and other bases. The size of its particles is less than 45 microns and specific surface area is about 300 to 400 m2/kg.
In simple words, ground granulated blast furnace slag is a waste product in the manufacture of pig iron. Approximately 300kg of slag is produced for each tonne of pig iron. Chemically, it’s a mixture of lime, silica and alumina which are also found in most of the cementitious substances. Blast furnace slag varies in composition and physical structure depending on the method of cooling of the slag.
Pulverised fuel ash (PFA) or Fly ash
 Fly ash or pulverized fuel ash (PFA) is the residue from the combustion of pulverized coal collected by mechanical or electronic separators from the power plants. The properties and compositions of fly ash vary widely, not only between different plants but from hour to hour in the same plant. Its composition depends upon the type of fuel burnt and on the variation of load in the boiler. Fly ash obtained from cyclone separators is comparatively coarse and contains a large proportion of unburnt fuel, Whereas that obtained from electrostatic precipitators is relatively fine. The specific surface area is about 350 to 500 m2/kg. It is more finer than portland cement. It may vary in color from light grey o dark grey or even brown.
Carbon content in fly ash is important consideration for use with cement. It should be as low as possible. The fineness of fly ash should be as high as possible. The silica contained in fly ash should be present in finely divided state since it combines slowly over a long period with the lime liberated during the hydration of the cement. 

Effects of fly ash on cement concrete :

  • On amount of mixing water : The use of fly ash in limited amounts as a replacement for cement or as an addition to cement requires a little more water for the same slump because of fineness of the fly ash. It is generally agreed that the use of fly ash, particularly as an admixture rather than as a replacement of cement, reduces, segregation and bleeding. If the sand is coarse the addition of fly ash produces beneficial results; for fine sands, its addition may increase the water requirement for a given workability.
  • On compressive strength : Since the puzzolanic action is very slow, an addition of fly ash up to 30 per cent may result in lower strength at 7 and 28 days, but may be about equal at 3 months and may further increase at ages greater than 3 months provided curing is continued.
  • On modulus of elasticity : It is lower at early ages and higher at later ages.


It is similar to Portland cement concrete.


Coarser fly ashes and those having a high carbon content are more

liable to increase drying shrinkage than the finer fly ashes and those having a low carbon content

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