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Background | The CIDA/NRCan/CII Project

Background

• Cement Usage and Greenhouse Gas Emissions
• A solution -- the use of fly ash in concrete
• Cost Benefits of Fly Ash
  

Cement Usage and Greenhouse Gas Emissions

The current rapid development in India has created a large demand for construction for years to come, requiring increased production of cast-in-place concrete and concrete products. These will be essential to major infrastructure construction projects such as transportation (roads, bridges, airports), water distribution (embankments and water channels), energy development (nuclear power generation), and for buildings in general.

Typically, conventional concrete is made of fine and coarse aggregates, water, portland cement and chemical admixtures. Materials used in the manufacture of portland cement must contain appropriate proportions of lime, silica, alumina, and iron components. In the manufacturing process, the raw materials are pulverized and blended in the desired proportions, and fed into the upper end of a kiln. The mixture passes through the kiln at a rate controlled by the slope of the kiln and the speed at which the kiln rotates. Heat produced by the combustion of powdered coal, fuel oil or gas, is forced into the lower end of the kiln producing temperatures of 1400 to 1650°C. During this process, several reactions occur, which result in the formation of portland cement clinker. The clinker is cooled and then pulverized. During this operation, a small amount of gypsum is added to regulate the setting of the cement.

The production of clinker is very energy intensive (approximately 4 GJ of energy per tonne of conventional cement produced) and furthermore, large amounts of CO2, a greenhouse gas, are emitted during the calcination of limestone used as a source of lime, resulting in approximately 0.9 tonne of CO2 for every tonne of portland cement produced.

The impact is significant in India with the current annual production of about 100 million tonnes of cement, and the consequent release of over 90 million tonnes of CO2.

Therefore, it is essential to introduce new technologies and practices for alternative cement and concrete compositions in order to curtail the rising CO2 emissions caused by increased Portland cement production. Furthermore, these new technologies will slow down the depletion of raw material (mainly limestone) used for cement production.

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A solution -- the use of fly ash in concrete

Fortunately, a portion of the portland cement could be replaced in concrete by supplementary cementing materials (SCMs) such as fly ash, a byproduct from coal-burning in thermal power stations. The replacement can be done either directly at the concrete batch plant, or during the production of blended cements.

Given this scenario, it is imperative that every effort is made in the extensive use of fly ash, which is abundant in India, to meet the current construction demands and to decrease the environmental damage.

In terms of public good, the greater use of fly ash in concrete contributes to reduced greenhouse gas (GHGs) emission without negative impacts on the economy. The overall quantity of CO2 emitted will be reduced in one important sector of the economy without affecting it. Such changes in the construction sector should provide certain flexibility to the governments to meet emission targets with reduced pressure on other sectors bound to be more affected by measures of CO2 reduction.

In addition to CO2 emission reductions, there are other potential benefits of using larger amounts of fly ash in concrete including reduced landfills, lower-cost concrete, increased durability, and reduced fly ash disposal costs.

Approximately 100 million tonnes of fly ash are produced in India annually from the combustion of coal, and this is increasing rapidly due to the growth in demand for energy. It is predicted that the amount of fly ash produced in India will double in the next ten to fifteen years. While this ensures that there will be more than sufficient supply of fly ash in India to meet its use as a supplementary cementing material for years to come, currently, it is creating a waste disposal problem. Most of the fly ash produced is currently disposed of in landfills, requiring large tracts of land and water. Therefore, it is essential that the utilization of fly ash in useful applications increase dramatically, to reduce the use of land for waste disposal, land that then becomes potentially available for other uses such as housing, agriculture or industry.

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Cost Benefits of Fly Ash

Since it is a byproduct, the initial cost of fly ash is minimal compared to that of portland cement. There are some costs associated with the handling of the fly ash and possibly with any special operations required to ensure proper quality control of the material. However, for the cement or concrete producer, the cost of fly ash will depend mainly on the cost of its transportation from the thermal power plant. Consequently, in areas close to power stations producing good quality fly ash, there should be cost benefits associated with the use of fly ash in concrete or in the production of blended cement. Thus, by making use of appropriate technologies, fly ash concrete of equivalent quality to that of conventional concrete could be produced at a lower cost. Therefore, there is some immediate economical benefit potential in using fly ash in concrete.

For the power generation industry, the disposal of fly ash in landfills is costly. In the event that large amounts of fly ash are used by the concrete industry, the disposal costs would be reduced by a corresponding amount. Also, it is well established that the proper use of fly ash in concrete improves the durability of concrete, translating into increased service life of concrete structures, resulting in considerable savings in repair and replacement costs. Consequently, there are potential indirect economical benefits of using fly ash in concrete.

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