Reciept No. 133

Utilisation of Fly Ash in Cement Concrete Pramey M. Zode

S.E. (Civil), Sinhgad Academy of Engineering, Kondhwa(Bk.), Pune-48 (India) E-mail: pramey.17@gmail.com

Abstract - To meet the ever increasing demand of electricity, Thermal Power Plants (TPPs) are being set up all over the world, thereby resulting into more consumption of the coal in these plants. The disposal of ash derived from combustion has become a major issue now-a-days. The study of Fly Ash, as it is called, has found that it can be used in various civil engineering applications such as bricks and concrete making. This paper reviews the utilisation of Fly Ash as the admixture in partial replacement of ordinary Portland cement to upto 35%, and even more upto 50% in High-Volume Fly Ash (HVFA) concrete which reduces the water demand, improves the workability, minimizes cracking due to thermal and drying shrinkage, and enhances durability to reinforcement corrosion, sulphate attack, and alkali-silica expansion. This admixing proves to be a best filler material which also reduce overall cost of construction and act as an eco-friendly material.

I.INTRODUCTION

Ash is a residue resulting from combustion of pulverised coal or lignite in Thermal Power Plants (TPPs). About 80% of total ash is in finely divided form which is carried away with flue gases and is collected by electrostatic precipitator or other suitable technology. This ash is called Fly Ash or chimney Ash or Hopper Ash. In an industrial context, fly ash usually refers to ash produced as an industrial by-product during combustion of coal in TPPs.

Fly ash is a fine (85% of its mass passing through a 45µm screen), pozzolaneous or a siliceous and/or aluminous glassy powdered material having micron-sized earth elements, which in the presence of water and lime, will react to form a cementitous material. It consists of inorganic materials mainly silica and alumina with some quantum of organic material in the form of unburnt carbon. Fly ash also contains environmental toxins in significant amounts, including arsenic (43.4 ppm); barium (806 ppm); beryllium (5 ppm); boron (311 ppm); cadmium (3.4 ppm); chromium (136 ppm); chromium VI (90 ppm); cobalt (35.9 ppm); copper (112 ppm); fluorine (29 ppm); lead (56 ppm); manganese (250 ppm); nickel

(77.6ppm); selenium (7.7 ppm); strontium (775 ppm); thallium (9 ppm); vanadium (252 ppm); and zinc (178 ppm).

II.SOURCES OF FLY ASH IN INDIA

According to National Thermal Power Corporation (NTPC), coal is used for approximately 62.3% of electric power generation in India. According to Central Electricity Authority of India, there are around 83 major coal fired thermal power plants in India. As per the Ministry of Power Statistics, the total installed generating capacity (TPPs) is about 79838 MW. In addition to this, there are more than 1800 selected industrial units which have TPPs of >1MW capacity. These are the chief sources of fly ash in India.

III.ASH CONTENT IN INDIAN COAL

The quality of coal depends upon its rank and grade. The coal rank arranged in an ascending order of carbon contents is:

Lignite --> sub-bituminous coal --> bituminous coal --> anthracite

Indian coal is of mostly sub-bituminous rank, followed by bituminous and lignite (brown coal). Thus, the ash content in Indian coal ranges from 35% to 50%.

IV.CURRENT FLY ASH GENERATION IN INDIA

The current electricity generation in India is about 1,12,058 MW, 65-70% of which is thermal (mostly coal based). According to an estimate 100,000 MW capacity or more would be required in the next 10 years due to continually increasing demand for electricity. Thus, the present fly ash generation in India is around 110 million tonnes / year and is set to continue at a high rate into the foreseeable future.

V.CURRENT FLY ASH UTILISATION

According to the MOEF Gazette Notification dated Sept. 14, 1999, the then existing power stations were to achieve 20% ash utilization within three years and 100% utilization in 15 years from the date of notification. New Stations were to achieve 30% ash utilization within 9 years at the rate of 10%

ash utilization within 3 years. Presently, out of 110 million tonnes of total ash generated, only about 30% is being utilized. Therefore thermal power stations are under great pressure to find useful applications of fly ash. The technology utilizing fly ash in high volume fly ash concrete can provide an avenue for utilization of fly ash on a bulk scale.

VI. PROBLEMS DUE TO FLY ASH

Fly ash is a very fine powder and tends to travel far in the air. When not properly disposed, it is known to pollute air and water, and causes respiratory problems when inhaled. When it settles on leaves and crops in fields around the power plant, it lowers the yield. The conventional method used for disposal of both fly ash and bottom ash is to convert them into slurry for impounding in ash ponds around the thermal plants. This method entails long-term problems. The severe problems that arise from such dumping are: 1) The construction of ash ponds requires vast tracts of land. This depletes land available for agriculture over a period of time. 2) When one ash pond fills up, another has to be built, at great cost and further loss of agricultural land. 3) Huge quantities of water are required to convert ash into slurry. During rains, numerous salts and metallic content in the slurry can leach down to the groundwater and contaminate it. Taking into account these facts, fly ash is being used in various construction activities as a raw material. In this paper, detailed study of use of fly ash in raw materials like Portland cement is considered. Further this paper reviews the use of high volume fly ash in cement making for better yield.

FLY ASH BASED POZZOLANA PORTLAND CEMENT

I.POZZOLANS

Pozzolans are defined as silicious and aluminous materials which in themselves possess little or no cementitious value but in finely divided form and in the presence of moisture, it chemically react with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties.

II.CLASS F FLY ASH

The burning of harder, older anthracite and bituminous coal typically produces Class F fly ash. This fly ash is pozzolanic in nature, and contains less than 20% lime (CaO).

Possessing pozzolanic properties, the glassy silica and alumina of Class F fly ash requires a cementing agent, such Portland cement, quicklime, or hydrated lime, with the presence of water in order to react and produce cementitious compounds.

Most of the state and federal specifications allow, and even encourage, the use of Fly Ash; especially, when specific durability requirements are needed. Fly Ash has a long history of use in concrete. Fly Ash is used in about 50% of ready mixed concrete. Class C Fly Ash is used at dosages of 15 to 40% by mass of the cementitious materials in the concrete. Class F is generally used at dosages of 15 to 30%.

III.FLY ASH IN PORTLAND CEMENT

Owing to its pozzolanic properties, fly ash is used as a replacement for some of the Portland cement content of concrete. The use of fly ash as a pozzolanic ingredient was recognized as early as 1914, although the earliest noteworthy study of its use was in 1937.Before its use was lost to the Dark Ages, Roman structures such as aqueducts or the Pantheon in Rome used volcanic ash (which possesses similar properties to fly ash) as pozzolan in their concrete. As pozzolan greatly improves the strength and durability of concrete, the use of ash is a key factor in their preservation.

Use of fly ash as a partial replacement for Portland cement is generally limited to Class F fly ashes. It can replace up to 30% by mass of Portland cement, and can add to the final strength of concrete and increase its chemical resistance and durability. Recently concrete mix design for partial cement replacement with High Volume Fly Ash (50 % cement replacement) has been developed. For Roller Compacted Concrete (RCC) [used in dam construction] replacement values of 70% have been achieved with processed fly ash at the Ghatghar Dam project in Maharashtra, India. Due to the spherical shape of fly ash particles, it can also increase workability of cement while reducing water demand. The replacement of Portland cement with fly ash is considered by its promoters to reduce the greenhouse gas "footprint" of concrete, as the production of one ton of Portland cement produces approximately one ton of CO2 as compared to zero CO2 being produced using existing fly ash. New fly ash production, i.e., the burning of coal, produces approximately twenty to thirty tons of CO2 per ton of fly ash. Since the worldwide production of Portland cement is expected to reach nearly 2 billion tons by 2012, replacement of any large portion of this cement by fly ash can significantly reduce carbon emissions associated with construction.

Inclusion of Fly Ash in Portland cement based plastic concrete mixes improves concrete workability by reducing the water content for a given consistency. The spherical particles create a ‘ball bearing’ effect in the mix – thus improving workability. Fly Ash particles also fill voids in the mix which reduces the water requirement for a given plastic consistency. Workable Fly Ash concrete, places easier, finishes better and produces better ‘off-form’ surfaces than plain Portland cement concrete. For use in concrete, Fly Ash is referred to as a ‘supplementary cementitious material’.

IV.CHEMICAL COMPARISION OF FLY ASH AND PORTLAND CEMENT

The chemical composition of fly ash is very similar to that of portland cement.

TABLE I TYPICAL CHEMICAL COMPOUNDS

IN POZZOLANIC CLASS F FLY ASH AND PORTLAND CEMENT

Chemical compound

Class F fly ash Cement

SiO 54.90 2.60 Al2O3 25.80 4.30 Fe2O3 6.90 2.40 CaO 8.70 64.40 MgO 1.80 2.10 SO2 0.60 2.30 Na2O & K2O 0.60 0.60

The table above shows typical compound analysis for Class F fly ash and ordinary portland cement. A glance at the table reveals:

1. The same compounds exist in fly ash and portland cement. Those of fly ash are amorphous (glassy) due to rapid cooling; those of cement are crystalline, formed by slower cooling.

2. The major difference between fly ash and portland cement is the relative quantity of each of the several compounds in them. Portland cement is rich in lime (CaO) while fly ash is low. Fly ash is rich in reactive silicates while Portland cement has smaller amounts.

Portland Cement + Water Calcium Silicate Hydrate

Free Lime (CaOH)

Portland Cement + Water

+ Fly Ash Calcium Silicate Hydrate

Portland cement is manufactured with CaO, some of which is released in a free state during hydration. As much as 20 pounds of free lime is released during hydration of 100 pounds of cement. This liberated lime forms the necessary ingredient for reaction with fly ash silicates to form strong and durable cementing compounds no different from those formed during hydration of ordinary Portland cement. A review of the chemistry of both materials makes it apparent that a blend of the two will enhance the concrete product and efficiently utilize the properties of both.

V.ADVANTAGES OF FLY ASH BASED PORTLAND CEMENT

A.Fly Ash improves concrete workability and lowers water demand

Fly Ash particles are mostly spherical tiny glass beads. Ground materials such as Portland Cement are solid angular particles. Fly Ash particles provide a greater workability of the powder portion of the concrete mixture which results in greater workability of the concrete and a lowering of water requirement for the same concrete consistency. Pump ability is greatly enhanced.

B.Fly Ash generally exhibit less bleeding and segregation than plain concretes

This makes the use of Fly Ash particularity valuable in concrete mixtures made with aggregates deficient in fines.

C.Sulphate and Alkali Aggregate Resistance

Class F and a few Class C Fly Ashes impart significant sulphate resistance and alkali aggregate reaction resistance to the concrete mixture.

D.Fly Ash has a lower heat of hydration

Portland cement produces considerable heat upon hydration. In mass concrete placements the excess internal heat may contribute to cracking. The use of Fly Ash may greatly reduce this heat build up and reduce external cracking.

F.Fly Ash generally reduces the permeability and adsorption of concrete

By reducing the permeability of chloride ion, corrosion of embedded steel is greatly decreased. Also, chemical resistance is improved by the reduction of permeability and adsorption.

G.Fly Ash is economical

The cost of Fly Ash is generally less than Portland Cement depending on transportation. Significant quantities may be substituted for Portland Cement in concrete mixtures and thus increase the long term strength and durability. Thus, the use of Fly Ash may impart considerable benefits to the concrete mixture over a plain concrete for less cost.

HIGH-VOLUME FLY ASH (HVFA) CONCRETE

Fly Ash has a vast potential for use in High Volume Fly Ash (HVFA) concrete especially due to its physic-chemical properties. Considerable amount of research has already been done in India and abroad on its strength and other requisite parameters. In commercial practice, the dosage of fly ash is limited to 15%-20% by mass of the total cementitious material. Usually, this amount has a beneficial effect on the workability and cost economy of concrete but it may not be enough to sufficiently improve the durability to sulphate attack, alkali-silica expansion, and thermal cracking. Thus, from theoretical considerations and practical experience it is established that, with 50% or more cement replacement by fly ash, it is possible to produce sustainable, high performance concrete mixtures that show high workability, high ultimate strength, and high durability.

I.CHARACTERISTICS DEFINING HVFA CONCRETE MIXTURE

The characteristics defining a HVFA concrete mixture are as follows:

1) Minimum of 50% of fly ash by mass of the cementitious materials must be maintained.

2) Low water content, generally less than 130 kg/m3 is mandatory.

3) Cement content, generally no more than 200kg/m3 is desirable.

4) For concrete mixtures with specified 28-day compressive strength of 30 MPa or higher, slumps greater than 150 mm, and water-to-cementitious materials ratio of the order of 0.30, the use of high range water-reducing admixtures (superplasticizers) is mandatory.

5) For concrete exposed to freezing and thawing environments, the use of an air-entraining admixture resulting in adequate air-void spacing factor is mandatory.

6) For concrete mixtures with slumps less than 150 mm and 28-day compressive strength of less than 30 MPa, HVFA concrete mixtures with a water-to-cementitious materials ratio of the order of 0.40 may be used without superplasticizers.

II.MECHANISMS BY WHICH FLY ASH IMPROVES THE PROPERTY OF CONCRETE

A good understanding of the mechanisms by which fly ash improves the rheological properties of fresh concrete and ultimate strength as well as durability of hardened concrete is helpful to insure that potential benefits expected from HVFA concrete mixtures are fully realized. These mechanisms are discussed next:

A.Fly ash as a water reducer

There are two reasons why typical concrete mixtures contain too much mixing-water. Typical concrete mixtures do not have an optimum particle size distribution, and this accounts for the undesirably high water requirement to achieve certain workability. Secondly, to plasticize a cement paste for achieving a satisfactory consistency, much larger amounts of water than necessary for the hydration of cement have to be used because portland cement particles, due to the presence of electric charge on the surface, tend to form flocs that trap volumes of the mixing water. It is generally observed that a partial substitution of portland cement by fly ash in a mortar or concrete mixture reduces that water requirement for obtaining a given consistency. Experimental studies have shown that with HVFA concrete mixtures, depending on the quality of fly ash and the amount of cement replaced, up to 20% reduction in water requirements can be achieved. This means that good fly ash can act as a superplasticizing admixture when used in high-volume. The phenomenon is attributable to three mechanisms. First, fine particles of fly ash get absorbed on the oppositely charged surfaces of cement particles and prevent them from flocculation. The cement particles are thus effectively dispersed and will trap large amounts of water, that means that the system will have a reduced water requirement to achieve a given consistency. Secondly, the spherical shape and the smooth surface of fly ash particles help to reduce the interparticle friction and thus facilitates mobility. Thirdly, the “particle packing effect” is also responsible for the reduced water demand in plasticizing the system. It may be noted that both portland cement and fly ash contribute particles that are mostly in the 1 to 45 µm size range, and therefore serve as excellent fillers for the void space within the aggregate mixture. In fact, due to its lower density and higher volume per unit mass, fly ash is a more efficient void-filler than portland cement.

B.Drying shrinkage

Perhaps the greatest disadvantage associated with the use of neat portland-cement concrete is cracking due to drying shrinkage. The drying shrinkage of concrete is directly influenced by the amount and the quality of the cement paste present. It increases with an increase in the cement paste-to-aggregate ratio in the concrete mixture, and also increases with the water content of the paste. Clearly, the water-reducing property of fly ash can be advantageously used for achieving a considerable reduction in the drying shrinkage of concrete mixtures. Table 2 shows mixture proportions of a conventional 25 MPa concrete compared to a superplasticized HVFA concrete with similar strength but higher slump. Due to a significant reduction in the water requirement, the total volume of the cement paste in the HVFA concrete is only 25% as compared to 29.6% for the conventional portland-cement concrete which represents a 30% reduction in the cement paste-to-aggregate volume ratio.

TABLE 2

COMPARISION OF CEMENT PASTE VOLUMES

Conventional concrete

HVFA concrete

kg/m3 m3 kg/m3 m3 Cement 307 0.098 154 0.149 Fly ash - - 154 0.065 Water 178 0.178 120 0.120 Entrapped air (2%)

- 0.020 - 0.020

Course aggregate 1040 0.385 1210 0.448 Fine aggregate 825 0.305 775 0.287 Total 2350 0.986 2413 0.989 w/cm 0.58 - 0.39 - Paste volume - 0.296 - 0.254 Paste percent - 30.0% - 25.7%

C.Thermal cracking

Thermal cracking is of serious concern in massive concrete and reinforced concrete structures. For unreinforced mass-concrete construction, several methods are employed to prevent thermal cracking, and some of these techniques can be successfully used for mitigation of thermal cracks in massive reinforced-concrete structures. For instance, a 40-MPa concrete mixture containing 350 kg/m3 portland cement can raise the temperature of concrete by approximately 55-60oC within a week if there is no heat loss to the environment. However, with a HVFA concrete mixture containing 50%

cement replacement with a Class F fly ash, the adiabatic temperature rise is expected to be 30-35oC.

D.Water-tightness and durability

In general, the resistance of a reinforced-concrete structure to corrosion, alkali aggregate expansion, sulphate and other forms of chemical attack depends on the water-tightness of the concrete. The water-tightness is greatly influenced by the amount of mixing-water, type and amount of supplementary cementing materials, curing, and cracking resistance of concrete. High-volume fly ash concrete mixtures, when properly cured, are able to provide excellent water-tightness and durability. The mechanisms responsible for this phenomenon are discussed briefly below.

When a concrete mixture is consolidated after placement, along with entrapped air, a part of the mixing-water is also released. As water has low density, it tends to travel to the surface of concrete. However, not all of this “bleed water” is able to find its way to the surface. Due to the wall effect of coarse aggregate particles, some of it accumulates in the vicinity of aggregate surfaces, causing a heterogeneous distribution of water in the system. Obviously, the interfacial transition zone between the aggregate and cement paste is the area with high water/cement and therefore with more available space that permits the formation of a highly porous hydration product containing large crystals of calcium hydroxide and ettringite. Microcracks due to stress are readily formed through this product because it is much weaker than the bulk cement paste with a lower water/cement. It has been suggested that microcracks in the interfacial transition zone play an important part in determining not only the mechanical properties but also the permeability and durability of concrete exposed to severe environmental conditions. This is because the rate of fluid transport in concrete is much larger by percolation through an interconnected network of microcracks than by diffusion or capillary suction. The heterogeneities in the microstructure of the hydrated portland-cement paste, especially the existence of large pores and large crystalline products in the transition zone, are greatly reduced by the introduction of fine particles of fly ash. With the progress of the pozzolanic reaction, a gradual decrease occurs in both the size of the capillary pores and the crystalline hydration products in the transition zone, thereby reducing its thickness and eliminating the weak link in the concrete microstructure. In conclusion, a combination of particle packing effect, low water content, and pozzolanic reaction accounts for the eventual disappearance of the interfacial transition zone in HVFA concrete, and thus enables the development of a highly crack-resistant and durable product.

III.PROPERTIES OF HVFA CONCRETE

Based on field experience and laboratory tests, the properties of HVFA concrete, when compared to conventional portland cement concrete, can be summarized as follows:

1) Easier flowability, pumpability, and compactability.

2) Better surface finish and quicker finishing time when power finish is not required.

3) Slower setting time, which will have a corresponding effect on the joint cutting and lower power-finishing times for slabs.

4) Early-strength up to 7 days, which can be accelerated with suitable changes in the mix design when earlier removal of formwork or early structural loading is desired.

5) Much later strength gain between 28 days and 90 days or more. (With HVFA concrete mixtures, the strength enhancement between 7 and 90-day often exceeds 100%, therefore it is unnecessary to over design them with respect to a given specified strength.)

6) Superior dimensional stability and resistance to cracking from thermal shrinkage, autogenous shrinkage, and drying shrinkage.

7) After three to six months of curing, much higher electrical resistivity, and resistance to chloride ion penetration, according to ASTM Method C1202.

8) Very high durability to the reinforcement corrosion, alkali-silica expansion, and sulphate attack.

9) Better cost economy due to lower material cost and highly favorable lifecycle cost.

10) Superior environmental friendliness due to ecological disposal of large quantities of fly ash, reduced carbon-dioxide emissions, and enhancement of resource productivity of the concrete construction industry.

CONCLUSION

The study of Fly Ash has shown that owing to its numerous advantageous physical and chemical properties, the material is found to be one of the best admixtures in Portland cement concrete and High Volume Fly Ash (HVFA) concrete making which improves not only the quality but also its workability subjected to various parameters. Moreover, the fly ash concrete offers a holistic solution to the problem of fly ash disposal which is one of the major issues now-a-days that too

in a sustainable manner, at a reduced or no additional cost and at the same time reducing the environmental impact of two industries that are vital to economic development namely the cement industry and the coal-fired power industry. Thus it also favours the Green Technology and waste management which in turn helps in sustainable development.

ACKNOWLEDGMENT

I wish to acknowledge the instructive guidance of Prof. R.B. Bajare, Asst. Professor and Prof. Dr. S. R. Parekar, HOD, Dept. of Civil Engineering, Sinhgad Academy of Engineering.

REFERENCES

[1] Parisara, ENVIS Newsletter (Vol.2 No. 6, January 2007) by State Environment Related Issues, Department of Forests, Ecology and Environment, Government of Karnataka

[2] 'High Volume Fly-Ash Concrete Technology', Fly Ash Summary Report in India by Canadian International Development Agency (CIDA)

[3] P. Kumar Mehta, "HIGH-PERFORMANCE, HIGH-VOLUME FLY ASH CONCRETE FOR SUSTAINABLE DEVELOPMENT", International Workshop on Sustainable Development and Concrete Technology, University of California, Berkeley, USA

[4] http://www.ashgroveresources.com/

[5] Fly ash, From Wikipedia, the free encyclopedia

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[7] Headwaters Resources, Chemical Comparison of Fly Ash and Portland Cement, Bulletin No. 2