Economy of Using Fly Ash in Concrete Essay Example

down the depletion of raw material (mainly limestone) used for cement production. All these objectives can be easily achieved if the fly ash is used appropriately to partially replace Portland cement in concretes.

NECESSITY TO MAKE CONCRETE ECO-FRIENDLY

Under utilization Concrete industry in the world is the largest consumer of natural resources, such as rock and sand (8 billion tones), and also one of the largest consumer of water (600 to 700 billion gallons).Cement forming binder of the concrete has very high energy content (1. 3 kwH/kg) and its usage in concrete has to be as less as possible so that the final energy content of concrete is less. Portland cement being a very high-tech material can produce strength up to 800 MPa as against the conventional strength levels of about 30 to 60 MPa in construction.

Thus, Portland cement is grossly under-utilized in normal constructions. Current Scenario The cement industry is the India’s second highest payer of Central excise and major contributor to GDP.With infrastructure development growing and the housing sector booming, the demand for cement is likely to increase. The industry is extremely energy intensive - after thermal power plants and the iron and steel sector, the Indian cement industry is the third largest user of coal in the country.

In 2003-04, 11,400 million kWh of power was consumed by the Indian cement industry. India is the second largest producer of cement in the world. In 2003, India produced 115 million metric tons (Mt) of cement, behind China (750 Mt), but ahead of the U. S. (93 Mt) and Japan (72 Mt).India’s cement industry – both installed capacity and actual production – has grown significantly over the

past three decades, with production increasing at an average rate of 8. 1% per year between 1981 and 2003.

Environmental Impacts of Cement production Producing one tonne of cement requires about 2 tonnes of raw materials (shale and limestone) and releases approximately 1 tonne of CO2, about 3 kg of NOX (an air contaminant that contributes to ground level smog and 0. 4 kg of PM 10 (an air borne particulate matter that is harmful to the respiratory tract when inhaled).After aluminium and steel the manufacturing of Portland cement is the most energy intensive process as it consumes 4GJ of energy in electricity, process heat and transfer.

The global release of CO2 from all sources is estimated at 23 billion tonnes a year and the Portland cement production accounts for about 7% of total carbon emissions. The cement industry has made significant progress in reducing CO2 emissions through improvements in process and efficiency, but further improvements are limited because CO2 production is inherent to the basic process of calcinating limestone.

The cement industry does not fit the contemporary picture of a sustainable industry because it uses raw materials and energy that are non-renewable; extracts its raw materials by mining and manufactures a product that cannot be recycled. Limestone mining has impact on land-use patterns, local water regimes and ambient air quality. Blasting causes problems of vibrations cracks and fly rocks. The impact of mining is especially high in ecologically sensitive areas.

There is poor mine management and poor planning for rehabilitation of exhausted land.Mining is one of the reasons for the high environmental impact of the industry. Dust emissions during cement manufacturing have long been accepted as one of

the main issues facing the industry. The industry handles millions of tonnes of dry material.

Even if 0. 1 per cent of this is lost to the atmosphere, it can cause havoc environmentally. Fugitive emissions are therefore a huge problem, compounded by the fact that there is neither an economic incentive nor regulatory pressure to prevent emissions.

AVAILABILITY OF FLY ASH

Fly Ash is unburnt (i. e. incombustible) portion of coal in a thermal power plant and collected from the flue gases in electrostatic precipitators installed from pollution control point of view. In thermal power plant, when powdered coal passes through the high-temperature zone of the furnace, the volatile matter and carbon portions of the coal are burned off and most of the non-combustible mineral portion (such as clay, quartz, feldspar etc) of the coal gets fused due to prevailing high temperature.The fused material is transported to the lower temperature zones where it gets solidified as spherical particles and some agglomerates are formed to fall down as 'furnace bottom ash'.

Most of the spherical particles fly out with flue gas system and these particles are removed, as 'fly ash', from the flue gas by electrostatic precipitator. A 200 MW unit power generation plant based on coal produces 50-60 tonnes of ash per hour and about 100 million tonnes of FA per year are being produced at present in India and land occupied by ash ponds is about 65000 acres.Due to planned addition of capacity for increased power generation, the estimated ash generation by year 2012 is 170 million tonnes needing about 1 lakh hectares land if the conventional methods of disposal are adopted. Though, measures such as coal washing

before burning would reduce the ash generation, there is urgent need for gainful use of this vast quantity of FA produced annually. Moreover, about 1000 million tonnes of FA is already dumped in ash ponds of various thermal power plants in India.

However, in case of new thermal plants, cent per cent utilisation of FA is needed to be planned and achieved over a total 9 years of operation starting with 30% within 3 years and later on 10% every year to reach 100% by next 6 years. It may be noted here that ash production per unit of power in India is more than that in advanced countries due to higher ash content of coal (average of about 34%) and lower calorific value of coals in India.

PROPERTIES OF FLY ASH AND CONTRIBUTION TO CONCRETE

Spherical shape: Fly ash particles are almost totally spherical in shape, allowing them to flow and blend freely in mixtures.Ball bearing effect: The "ball-bearing" effect of fly ash particles creates a lubricating action when concrete is in its plastic state.

Higher Strength: Fly ash continues to combine with free lime, increasing structural strength over time. Decreased Permeability: Increased density and long term pozzolanic action of fly ash, which ties up free lime, results in fewer bleed channels and decreases permeability. Increased Durability Dense fly ash concrete helps keep aggressive compounds on the surface, where destructive action is lessened.Fly ash concrete is also more resistant to attack by sulphate, mild acid, soft (lime hungry) water, and seawater.

Reduced Sulphate Attack: Fly ash ties up free lime that can combine with sulphate to create destructive expansion. Reduced Efflorescence: Fly ash chemically binds free lime and

salts that can create efflorescence and dense concrete holds efflorescence producing compounds on the inside. Reduced Shrinkage: The largest contributor to drying shrinkage is water content. The lubricating action of fly ash reduces water content and drying shrinkage.Reduced Heat of Hydration: The pozzolanic reaction between fly ash and lime generates less heat, resulting in reduced thermal cracking when fly ash is used to replace Portland cement.

Reduced Alkali Silica Reactivity: Fly ash combines with alkalis from cement that might otherwise combine with silica from aggregates, causing destructive expansion. Workability: Concrete is easier to place with less effort, responding better to vibration to fill forms more completely. Ease of Pumping. Pumping requires less energy and longer pumping distances are possible.Improved Finishing: Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity. Reduced Bleeding: Fewer bleed channels decreases porosity and chemical attack.

Bleed streaking is reduced for architectural finishes. Improved paste to aggregate contact results in enhanced bond strengths. Reduced Segregation: Improved cohesiveness of fly ash concrete reduces segregation that can lead to rock pockets and blemishes. Reduced Slump Loss: More dependable concrete allows for greater working time, especially in hot weather.

STRENGTHS OF FLY ASH CONCRETES

Compressive Strength Concrete is primarily meant to withstand the compressive stresses. Use of fly ash as a replacement in the case of Fly Ash Concrete decreases the compressive strength of concrete at early ages and increases it later due to pozzolanic action and thus strength development in this case is more than conventional concrete. With FAC, adequate early age and high later age strengths can be achieved if proper selection of cement type and amount is done. The strength increases

with time.

A judiciously proportioned FAC can give an acceptable value of early age strength. Pozzolanic ActionIf fly ash is added during the hydration reaction where cement and water react to solidify, it causes a chemical reaction over a long period on glassy phase such as silica and aluminum with cement hydrate (calcium hydroxide), which makes up about 70 to 80 percent of the fly ash. This is called a “Pozzolanic reaction''. The two-layered pozzolanic reaction phase surrounding the fly ash particles grows with time to fill a micro pore (capillary porosity) of tens to hundreds of nanometers generated after cement mixing, and that the resultant decrease in distance between hydrates improves adhesion force to increase the concrete strength. .

Flexural Strength The ratio of flexural and splitting tensile strength to compressive strength at 28 days is either comparable or higher than those for Portland cement concrete. In conventional concretes flexural strength reaches a maximum value between 14 and 28 days and beyond this there is no significant increase. On the contrary, In FAC the flexural strength keeps on increasing with age because of the pozzolanic reaction of fly ash and strengthening of interfacial bond between cement paste and aggregate.This gives FAC a significant advantage over conventional concrete for use in pavements.

Young’s modulus of elasticity The modulus of elasticity of FAC will be lower at early ages and higher in value later as compared to conventional concrete. The high value later is achieved because a considerable portion of the unreacted fly ash consisting of glassy spherical particles acts as a fine aggregate. Also, the low porosity of transition zone between cement mortar and coarse aggregates leads to

higher values.

BENEFITS OF FLY ASH CONCRETES

Economical benefit 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. The cost incurred is mainly that of transportation from the power plant to the construction site.

Cost of fly ash with in 200 km from a thermal power plant is as low as 10% to 20% of the cost of cement. This offers a certain economic advantage.Concrete requiring 400 kg / m3 binding material, may result in saving of 15% to 18% in overall cost of concrete with 45% to 50% replacement by fly ash. 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.

Water Demand Reduction Water demand and workability depends on particle size distribution, particle packing and voids present. The conventional concrete mixes do not have

an optimum particle size distribution and hence their water requirement is high.Moreover, the Portland cement particles have surface charges, which form flocs, thus trapping large volumes of water.

Thus the water that is actually added to concrete mixes to achieve the desired consistency is much higher than actually required for the normal hydration reactions. By use of fly ash we can achieve a 20 % reduction in amount of water added.

Reduction of Energy input For enhanced eco-friendliness of cement concretes (CCs), the energy input required for production of concrete should be as much less as possible.Among the ingredients of CC, the cement is the most energy intensive material.

Hence, quantity of cement should be minimised in making of concrete and this is achieved by partial replacement of cement by FA. The energy input required to produce 1 m of concrete lies in the range of 440-770 kWh. The energy input for FA can be taken as negligibly small from concrete production point of view (since FA is a waste product from Thermal Power Plants). The energy input in terms of cement is taken as about 1. 3 kWh per kg of cement.

It may be noted here that each ton of FA used to replace a ton of cement saves the equivalent of one barrel of oil required to produce the cement and Cement and lime are the third most energy-intensive materials to produce on a per-ton basis, after to steel and aluminium The energy input of Portland cement forms the major portion of the energy requirement per m3 of any concrete. Therefore, addition of FA as a replacement material for cement reduces the energy input for production

of concrete. The energy input for concrete can reduce by 30% to 60% depending upon the replacement level of cement by FA.

Protection of Embedded Steel against Corrosion Corrosion of steel, being an electrochemical reaction, involves flow of ions and higher electric resistance (ER) of concrete can decrease this flow thereby protecting the steel embedded in concrete.

Addition of FA can increase the ER by 50% to 200% depending upon the replacement of cement by FA. Since, the movement of electric charge is the basic activity required to initiate the corrosion of the steel reinforcement, a concrete with higher electrical resistivity is obviously more resistive to corrosion of steel.

Radioactivity Heavy Metal Content of Fly Ash Most of the heavy elements (such as Arsenic, Boron, Cobalt, Cadmium, Mercury etc) are present in very small quantities. It is found that FA when used in Portland cement based applications, leachates from FA are very much reduced due to increased impermeability of hydrated cement matrix on account of pozzolanic activity of FA. It is reported that there is no significant risk for use of fly ash containing concretes in construction of structures and products which are to be in contact with drinking water and there is no threat to ground water quality.EC European Water Catalogue and the UN-ECE list the FA as non-hazardous. The available information on various environmental aspects of FA indicate that it is, in general, a safe material to handle, use and stock; common-sense precautions are sufficient to prevent any potential risk, if there are any.

ACHIEVMENTS OF FACs

The Fly Ash story begins 2000 years ago... When the Romans built the Colosseum in the year 100 A. D. -

that still stands the test of time!! The ash generated from Volcanoes was used extensively in the construction of Roman structures.Colosseum is a classic example of durability achieved by using volcanic ash. This is a building constructed 2000 years ago and still standing today! The Ghatghar Roller Compacted Concrete Dam was a unique project in all respects. For the upper dam, 65% fly ash was used with 35% Portland cement. The result was much better than expected.

So Fly Ash was increased to 70% with only 30% of Portland cement!! Off corse, this was possible due to the roller compacting technology. Hungry Horse Dam, Montana, is a thick-arch structure that was built between 1948 and 1953 with concrete containing 120,000 metric tons of fly ash.The use of coal fly ash in cement and concrete displaces Portland cement. Photograph from U. S. Bureau of Reclamation. Iraivan Temple, Kauai, Hawaii, 2001, 60% fly ash replacement used in the unreinforced mat slab foundation, with no cracking and better than – specified strength/

CONCLUDING REMARKS

Fly ash (FA) is a pozzolanic material which improves the properties of cement concretes (CC) due to its reaction with free CH produced during hydration of Portland cement (PC). This reaction improves micro structural properties of concrete resulting in improved durability properties.This is accepted by Indian Standard IS: 456-2000 by suggesting use of FA on site for blending with PC and also recommending specifically in many severe environmental conditions.

Other countries have also published standard specifications. Chemical and physical requirements of fly ash for use in cement concrete have been specified. Research publications have established the utility of FA in CCs and many applications have been already carried

out yielding long-term properties.The test data shows that FACs can easily replace cement concretes from considerations of strength, economy, superior durability, environmental friendliness and ecology. Apprehensions on use of fly ash in cement concrete can not be justified. An emphasis on design of concrete mixes to achieve actual desired properties of concrete in fresh and hardened stages of concrete followed by proper quality control system during construction will ensure the realisation of enormous technical and economical advantages of fly ash.

The following major advantages of use of FA in concretes can be expected: More efficient use of cement for strength development (better utilisation of cement) Substantial reduction in cost of concretes (higher economy) Strength-Weight Ratios (SWRs) higher than conventional concrete (enhanced weight efficiency of concretes) Increased Strength-Energy Ratios (SER) (more eco-friendly and less energy-intensive concretes) Enhanced protection of steel embedded in concrete (due to higher electrical resistivity) Higher durability (due to reduced permeability to aggressive agents such as acids, water, moisture, vapours, chlorides, oxygen, etc)

REFERENCES

1. Annie J Peter, M. Neelamegam, J. K. Dattatreya, N. P. Rajamane and S. Gopalakrishnan, [Feb,1998], Utilisation of Fly ash as Cement Replacement Material to Produce High Performance Concrete, Proceeding of the National Seminar on Fly ash Utilisation for Metallurgical Laboratory, Jamshedpur, India.
2. N. P. Rajamane, Scientist, Making Concrete 'Green' Through Use of Fly Ash, Concrete Composites Laboratory Structural Engineering Research Centre, CSIR,Chennai.