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High-Volume Fly Ash Concrete
Basics
In general, concrete is a mixture of four to six components i.e. coarse aggregate, fine aggregate, water, portland cement, supplementary cementing materials (SCMs), and chemical admixtures. The last two components are sometimes optional. A chemical reaction called hydration occurs between water and portland cement. When SCMs are used, then a second chemical reaction called pozzolanic reaction happens between some of the hydration products and the SCMs. These two reactions bind the aggregate particles to produce a very hard building material that is concrete.
Concrete is the most used material in construction. More than a tonne of concrete is produced every year for each person in the planet, approximately six billion tonnes per year. Concrete is strong, inexpensive, versatile, and easy to make.
Coarse aggregate
Coarse aggregates are particles of gravel or crushed stone retained
on the 10 mm sieve and ranging up to 150 mm. The most commonly used
maximum aggregate size is 20 mm.
Fine aggregate
Fine aggregates are particles of natural or synthetic sand passing
the 5 mm sieve.
Portland cement
A cementing material obtained by pulverizing clinker, consisting
essentially of hydraulic calcium silicate that hardens by reacting with
water.
Supplementary cementing materials
Supplementary cementing materials (SCMs) are materials that when
used with portland cement contribute to the properties of the hardened
concrete through hydraulic or pozzolanic activity or both. Typical examples
are fly ash, ground granulated blast-furnace slag and silica fume.
Chemical admixtures
Chemical admixtures are those ingredients mainly used in concrete to
achieve certain properties in concrete more effectively than by other
means. Chemical admixtures can be classified by function as follows:
- Air-entraining admixtures (mainly used in concrete exposed to freezing and thawing cycles)
- Water-reducing admixtures (reduce the dosage of water while maintaining the workability)
- Retarding admixtures (mainly used in hot weather to retard the reaction of hydration)
- Accelerating admixtures (mainly used in cold weather to accelerate the reaction of hydration)
- Superplasticizer or high range water-reducer (significantly reduce the dosage of water while maintaining the workability)
- Miscellaneous admixtures such as corrosion inhibiting, shrinkage reducing, coloring, pumping etc.
Fly ash is a by-product from coal-fired electricity generating power plants. The coal used in these power plants is mainly composed of combustible elements such as carbon, hydrogen and oxygen (nitrogen and sulfur being minor elements), and non-combustible impurities (10 to 40%) usually present in the form of clay, shale, quartz, feldspar and limestone. As the coal travels through the high-temperature zone in the furnace, the combustible elements of the coal are burnt off, whereas the mineral impurities of the coal fuse and chemically recombine to produce various crystalline phases of the molten ash. The molten ash is entrained in the flue gas and cools rapidly, when leaving the combustion zone (e.g. from 1500°C to 200°C in few seconds), into spherical, glassy particles. Most of these particles fly out with the flue gas stream and are therefore called fly ash. The fly ash is then collected in electrostatic precipitators or bag houses and the fineness of the fly ash can be controlled by how and where the particles are collected.
The use of fly ash in concrete
Fly ash can be used in concrete as a partial replacement for ordinary portland cement (opc). Fly ash can be introduced in concrete directly, as a separate ingredient at the concrete batch plant or, can be blended with the opc to produce blended cement, usually called portland-pozzolana cement (ppc) in India. Fly ash blended cements are produced by several cement companies in India.
Generally speaking, currently in the concrete industry, the percentage of fly ash as part of the total cementing materials in concrete normally ranges from 15 to 25%, although it can go up to 30-35% in some applications. The use of fly ash in concrete will improve some aspects of the performance of the concrete provided the concrete is properly designed. The main aspects of the concrete performance that will be improved by the use of fly ash are increased long-term strength and reduced permeability of the concrete resulting in potentially better durability. The use of fly ash in concrete can also address some specific durability issues such as sulphate attack and alkali silica reaction. However, a few additional precautions have to be taken to insure that the fly ash concrete will meet all the performance criteria.
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