Fly ash report

7/23/2019 Fly ash report

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Institute of Technology,Nirma University.

M.Tech CASAD Semester I

CL1105 Advanced Materials 2014-2015

Term Assignment IV

Application and Case studies of Advanced

Materials in Civil Engineering

Fly Ash usage in various civil engineering

applications.

Yash Khandol(14MCLC06)Neeraj Khatri (14MCLC12)

Pragnesh Patel (14MCLC17)Ravi Patel (14MCLC18)

Sachin Patel (14MCLC19)Tejas Patil (14MCLC22)

M. Tech. 1st Year

November 17, 2014



Contents

1 Introduction 3

1.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2 Applications In Civil Engineering . . . . . . . . . . . . . . . . . . 3

1.2.1 Portland cement . . . . . . . . . . . . . . . . . . . . . . . 31.2.2 Soil stabilization . . . . . . . . . . . . . . . . . . . . . . . 41.2.3 Roller compacted concrete . . . . . . . . . . . . . . . . . . 51.2.4 Bricks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Avoid fly ash for, . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4 Use fly ash for, . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Properties and Composition 8

2.1 Size, Shape and Colour . . . . . . . . . . . . . . . . . . . . . . . . 82.2 Fineness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3 Specific Gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.4 Pozzolanic Activity . . . . . . . . . . . . . . . . . . . . . . . . . . 92.5 Particle Morphology . . . . . . . . . . . . . . . . . . . . . . . . . 92.6 Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.7 Chemical Composition . . . . . . . . . . . . . . . . . . . . . . . . 102.8 Mineralogical Characteristics . . . . . . . . . . . . . . . . . . . . 10

3 Quality of fly ash 12

3.1 Chemical Requirements . . . . . . . . . . . . . . . . . . . . . . . 123.2 Physical Requirements . . . . . . . . . . . . . . . . . . . . . . . . 123.3 Classification of Fly Ash . . . . . . . . . . . . . . . . . . . . . . . 13

3.3.1 Class F fly ash . . . . . . . . . . . . . . . . . . . . . . . . 133.3.2 Class C fly ash . . . . . . . . . . . . . . . . . . . . . . . . 13

4 Fly Ash : Mechanism 16

5 Effect of fly ash incorporation in concrete 175.1 Reduced Heat of Hydration . . . . . . . . . . . . . . . . . . . . . 175.2 Workability of Concrete . . . . . . . . . . . . . . . . . . . . . . . 175.3 Permeability and corrosion protection . . . . . . . . . . . . . . . 175.4 Effect of fly ash on Carbonation of Concrete . . . . . . . . . . . . 18

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5.5 Sulphate Attack . . . . . . . . . . . . . . . . . . . . . . . . . . . 185.6 Corrosion of steel . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.7 Reduced alkaliaggregate reaction . . . . . . . . . . . . . . . . . 195.8 Bleeding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.9 Setting time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.10 Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.11 Water Demand . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

6 Fly Ash : Usage and Mix Proportions 21

6.1 Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216.1.1 Simple replacement method . . . . . . . . . . . . . . . . . 216.1.2 Addition Method . . . . . . . . . . . . . . . . . . . . . . 226.1.3 Modified replacement method . . . . . . . . . . . . . . . . 22

6.2 Mix Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226.2.1 Workability and Consistency . . . . . . . . . . . . . . . . 22

6.2.2 Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236.2.3 Durability . . . . . . . . . . . . . . . . . . . . . . . . . . . 236.2.4 Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246.2.5 Heat of Hydration . . . . . . . . . . . . . . . . . . . . . . 24

6.3 Proportioning of Concrete . . . . . . . . . . . . . . . . . . . . . . 246.3.1 Selection of slump for requirement of consistency . . . . 246.3.2 Selection of maximum size of aggregates . . . . . . . . . . 246.3.3 Estimation of mixing water and air content . . . . . . . . 256.3.4 Selection of water cementitious materials [w /(c+p)] or

water cement ratio . . . . . . . . . . . . . . . . . . . . . . 276.3.5 Calculation of cementitious material content: . . . . . . . 286.3.6 Estimation of coarse aggregate content . . . . . . . . . . . 286.3.7 Estimation of fine aggregate content . . . . . . . . . . . . 296.3.8 Adjustments for aggregate moisture . . . . . . . . . . . . 296.3.9 Trial batch adjustment . . . . . . . . . . . . . . . . . . . . 29

7 Fly Ash : Industrial Overview 30

7.1 Deposition of fly-ash . . . . . . . . . . . . . . . . . . . . . . . . . 307.2 Flyash Disposal in Ash Ponds . . . . . . . . . . . . . . . . . . . 317.3 Flyash as Fill Material . . . . . . . . . . . . . . . . . . . . . . . . 337.4 Environmental Considerations . . . . . . . . . . . . . . . . . . . . 337.5 Fly ash transportation . . . . . . . . . . . . . . . . . . . . . . . . 34

7.5.1 Airslide-airlift systems . . . . . . . . . . . . . . . . . . . . 357.5.2 Airslide channel . . . . . . . . . . . . . . . . . . . . . . . 357.5.3 Airlift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

7.5.4 Dense phase pneumatic transport . . . . . . . . . . . . . . 367.5.5 Combination of airslides and pressure vessel system . . . 377.6 Packing of fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

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Chapter 1

Introduction

Electricity is the key for development of any country. Coal is a major sourceof fuel for production of electricity in many countries in the world. In the pro-cess of electricity generation large quantity of fly ash get produced and becomesavailable as a byproduct of coal-based power stations. It is a fine powder re-sulting from the combustion of powdered coal - transported by the flue gases of the boiler and collected in the Electrostatic Precipitators (ESP).

1.1 History

In the past, fly ash was generally released into the atmosphere, but pollutioncontrol equipment mandated in recent decades now requires that it be capturedprior to release. In the US fly ash is generally stored at the coal power plants

or placed in landfills. About 43% is recycled often used to supplement Portlandcement in concrete production. Fly ash was first used in large scale in construc-tion of Hungry Horse dam in America in the approximate amount of 30% byweight of cement. Later on it was used in Canyon and Ferry dams etc. In In-dia, Fly ash was used in Rihand dam construction replacing cement up to about15%. In recent times, the importance and use of fly ash in concrete has grown somuch that it has almost become a common ingredient in concrete, particularlyfor making high strength and high performance concrete. Extensive researchhas been done all over the world on the benefits that could be accrued in theutilization of fly ash as a supplementary cementitious material. High volume flyash concrete is a subject of current interest all over the world.

1.2 Applications In Civil Engineering1.2.1 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 in-

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gredient was recognized as early as 1914, although the earliest noteworthy studyof its use was in 1937.Roman structures such as aqueducts or the Pantheon in

Rome used volcanic ash or pozzolana (which possesses similar properties to flyash) as pozzolan in their concrete. As pozzolan greatly improves the strengthand 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 particularlysuitable but not limited to Class C fly ashes. Class ”F” fly ashes can have volatileeffects on the entrained air content of concrete, causing reduced resistance tofreeze/thaw damage. Fly ash often replaces up to 30% by mass of Portland ce-ment, but can be used in higher dosages in certain applications. Fly ash can addto the concrete’s final strength and increase its chemical resistance and dura-bility. Fly ash can significantly improve the workability of concrete. Recently,techniques have been developed to replace partial cement with high-volume flyash (50% cement replacement). For roller-compacted concrete (RCC)[used indam construction], replacement values of 70% have been achieved with pro-cessed fly ash at the Ghatghar dam project in Maharashtra, India. Due to thespherical shape of fly ash particles, it can increase workability of cement whilereducing water demand. Proponents of fly ash claim that replacing Portlandcement with fly ash reduces the greenhouse gas ”footprint” of concrete, as theproduction of one ton of Portland cement generates approximately one ton of CO2, compared to no CO2 generated with fly ash. New fly ash production, i.e.,the burning of coal, produces approximately 20 to 30 tons of CO2 per ton of flyash. Since the worldwide production of Portland cement is expected to reachnearly 2 billion tons by 2010, replacement of any large portion of this cement byfly ash could significantly reduce carbon emissions associated with construction,as long as the comparison takes the production of fly ash as a given.

1.2.2 Soil stabilizationSoil stabilization is the permanent physical and chemical alteration of soils toenhance their physical properties. Stabilization can increase the shear strengthof a soil and/or control the shrink-swell properties of a soil, thus improving theload-bearing capacity of a sub-grade to support pavements and foundations.Stabilization can be used to treat a wide range of sub-grade materials from ex-pansive clays to granular materials. Stabilization can be achieved with a varietyof chemical additives including lime, fly ash, and Portland cement. Proper de-sign and testing is an important component of any stabilization project. Thisallows for the establishment of design criteria as well as the determination of theproper chemical additive and admixture rate to be used to achieve the desiredengineering properties. Benefits of the stabilization process can include: Higher

resistance (R) values, Reduction in plasticity, Lower permeability, Reductionof pavement thickness, Elimination of excavation - material hauling/handling- and base importation, Aids compaction, Provides ”all-weather” access ontoand within projects sites. Another form of soil treatment closely related to soilstabilization is soil modification, sometimes referred to as ”mud drying” or soilconditioning. Although some stabilization inherently occurs in soil modifica-

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tion, the distinction is that soil modification is merely a means to reduce themoisture content of a soil to expedite construction, whereas stabilization can

substantially increase the shear strength of a material such that it can be incor-porated into the project’s structural design. The determining factors associatedwith soil modification vs soil stabilization may be the existing moisture content,the end use of the soil structure and ultimately the cost benefit provided. Equip-ment for the stabilization and modification processes include: chemical additivespreaders, soil mixers (reclaimers), portable pneumatic storage containers, watertrucks, deep lift compactors, motor graders.

1.2.3 Roller compacted concrete

The upper reservoir of Ameren’sTaum Sauk hydroelectric plant was constructedof roller-compacted concrete which included fly ash from one of Ameren’s coalplants. Another application of using fly ash is in roller compacted concretedams. Many dams in the US have been constructed with high fly ash contents.Fly ash lowers the heat of hydration allowing thicker placements to occur. Datafor these can be found at the US Bureau of Reclamation. This has also beendemonstrated in theGhatghar Dam Project in India.

1.2.4 Bricks

There are several techniques for manufacturing construction bricks from fly ash,producing a wide variety of products. One type of fly ash brick is manufacturedby mixing fly ash with an equal amount of clay, then firing in a kiln at about1000 C. This approach has the principal benefit of reducing the amount of clayrequired. Another type of fly ash brick is made by mixing soil, plaster of paris,fly ash and water, and allowing the mixture to dry. Because no heat is required,this technique reduces air pollution. More modern manufacturing processes usea greater proportion of fly ash, and a high pressure manufacturing technique,which produces high strength bricks with environmental benefits. In theUnited Kingdom, fly ash has been used for over fifty years to make concretebuilding blocks. They are widely used for the inner skin of cavity walls. Theyare naturally more thermally insulating than blocks made with other aggregates.

Ash bricks have been used in house construction in Windhoek, Namibia sincethe 1970s. There is, however, a problem with the bricks in that they tend to failor produce unsightly pop-outs. This happens when the bricks come into con-tact with moisture and a chemical reaction occurs causing the bricks to expand.

In India, fly ash bricks are used for construction. Leading manufacturers usean industrial standard known as ”Pulverized fuel ash for lime-Pozzolana mix-

ture” using over 75% post-industrial recycled waste, and a compression process.This produces a strong product with good insulation properties and environ-mental benefit. American civil engineer Henry Liu announced the invention of a new type of fly ash brick in 2007. Liu’s brick is compressed at 27.58 MPa(272 atm) and cured for 24 hours in a 66 C steam bath, then toughened withan air entrainment agent, so that it lasts for more than 100 freeze-thaw cycles.

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Owing to the high concentration of calcium oxide in class C fly ash, the brickcan be described as self-cementing. Since this method contains no clay and uses

pressure instead of heat, it saves energy, reduces mercury pollution, and costs20% less than traditional manufacturing techniques.This type of brick is nowmanufactured under license in the USA.

1.3 Avoid fly ash for,

• Elevated beams and slabs - where formwork often needs to be removedquickly.

• Cold weather pours - may not be appropriate for fly ash concrete whenearly strength is needed.

• Face mixes of architectural or precast concrete - due to the effect on colorcontrol and uniformity.

• Below-grade concrete support structures for utility pipes ,avoid using flyash for concrete in contact with metal or ductile iron pipes (as fly ash canbe corrosive to metals).

1.4 Use fly ash for,

• Poured-in-place concrete walls and columns, mat slabs and poured footingsin earth.

• Lightweight concrete on metal deck - an ideal application for fly ash be-cause the metal deck acts as a permanent formwork.

• Drilled piers and piles - fly ash concrete can perform well in water con-ditions due to decreased permeability. Also building piles are often notloaded to full capacity for some time after pouring. This allows for the56-day curing period typically required to meet strength requirements forhigh volume flyash applications.

• Grouting of concrete block.

• Precast concrete elements - This application is dependent on the pre-caster’s ability and willingness to allow for early strength gain before re-moval of the formwork. Conversations with several fabricators yielded arange of responses:

– Typical range of 15 - 25% replacement for Portland cement in themix.

– Certain fabricators were reluctant to use fly ash, citing concerns thatit would change the rheological behavior of the mix (rheology is the

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study of the flow of matter), add cost and complicate the mix oper-ations (which are computer controlled whereas fly ash may need to

be added manually to the mix)

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Chapter 2

Properties and Composition

Fly ash is part of coal ash, or the ’total residue’,created during the combustion of coal in electrical power plants. The coal that is not incinerated either settles atthe bottom of the boiler (’bottom ash’) or rises in the flue (’fly ash’). In short, flyash is the dust collected in the smokestacks as a result of combustion. Dependingon the source and properties of the coal being burned, the components of fly ashvary considerably, but all fly ash includes substantial amounts of silicon dioxide(SiO2) and calcium oxide or lime (CaO).

2.1 Size, Shape and Colour

1. Fly ash particle size is finer than ordinary Portland cement. Fly ashconsists of silt sized particles which are generally spherical in nature and

their size typically ranges between 10 and 100lm .

2. These small glass spheres improve the fluidity and workability of freshconcrete. Fineness is one of the important property contributing to thepozzolanic reactivity of fly ash. Fly ash colour depends upon its chemicaland mineral constituents. It can be tan to dark gray.

3. ” Tan and light colours are generally associated with higher lime content,and brownish colour with the iron content. A dark gray to black color isattributed to elevated unburned carbon (LOI) content. Fly ash color isusually very consistent for each power plant and coal source.

2.2 Fineness

1. Fineness of fly ash is most closely related to the operating condition of thecoal crushers and the grindability of the coal itself. Fineness of fly ash isrelated to its pozzolanic activity.

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2. Generally, a large fraction of ash particle is smaller than 3 m in size. Inbituminous ashes, the particle sizes range from less than 1 to over 100 m.

A coarser gradation can result in a less reactive ash and could containhigher carbon content.

2.3 Specific Gravity

• The specific gravity of fly ash is related to shape, color and chemicalcomposition of fly ash particle. In general, specific gravity of fly ash mayvary from 1.3 to 4.8.

• Canadian fly ashes have specific gravity ranging between 1.94 and 2.94,whereas American ashes have specific gravity ranging between 2.14 and2.69.

2.4 Pozzolanic Activity

1. property of fly ashes, possessing little or no cementing value to react withcalcium oxide in the presence of water, and produce highly cementitiouswater insoluble products, is called pozzolanic reactivity.

2. The meta-stable silicates present in self-cementitious fly ash react withcalcium ions in the presence of moisture to form water insoluble calcium-alumino-silicate hydrates.

3. The pozzolanic activity of a fly ash depends upon its (1) fineness; (2)calcium content; (3) structure; (4) specific surface; (5) particle size dis-

tribution; and (6) and LOI content.Several investigators have reportedthat when fly ash is pulverized to increase fineness, its pozzolanic activityincreases significantly.

4. However, the effect of increase in specific surface area beyond 6,000 cm2/gis reported to be insignificant.

2.5 Particle Morphology

1. Fly ash particles consist of clear glassy spheres and a spongy aggre-gate. Several morphological investigations have been carried out on par-ticle shape and surface characteristics of various types of fly ashes usingscanning electron microscope (SEM) and energy dispersive x-ray analysis

(EDXA).

2. Scanning electron micrographs of different fly ashes show the typical spher-ical shape of fly ash particles, some of which are hollow. The hollow spher-ical particles are known as cenospheres or floaters as they are very lightand tend to float on water surface.

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3. Cenospheres are unique free flowing powders composed of hard shelled,hollow, minute spheres. Cenospheres are made up of silica, iron and alu-

mina. Cenospheres have a size range from 1 to 500 lm. Colors range fromwhite to dark gray.

4. Sometimes fly ashes may also contain many small spherical particles withina large glassy sphere, called pherospheres. The exterior surfaces of thesolid and hollow spherical particles of low-calcium oxide fly ashes are gen-erally smooth and better defined than those of high-calcium oxide fly asheswhich may have surface coatings of material rich in calcium.

2.6 Moisture

1. Any amount of moisture in Class C fly ash will cause hardening from hy-

dration of its cementitious compounds. Even surface spraying may causecaking.

2. To prevent caking and packing of the fly ash during shipping and storageand to control uniformity of fly ash shipments, a 3.0% limit on moisturecontent is specified in ASTM C618. Therefore, it is important that suchashes have to be kept dry before being mixed with cement.

2.7 Chemical Composition

1. Chemical composition of fly ashes include silica (SiO2), alumina (Al2O3),and oxides of calcium (CaO), iron (Fe2O3), magnesium (MgO), titanium(TiO2), sulfur (SO3), sodium (Na2O), and potassium (K2O), and un-

burned carbon (LOI).

2. Amongst these SiO2 and Al2O3 together make up about 45-80% of thetotal ash. The sub-bituminous and lignite coal ashes have relatively higherproportion of CaO and MgO and lesser proportions of SiO2, Al2O3 andFe2O3 as compared to the bituminous coal ashes.

2.8 Mineralogical Characteristics

1. X-ray diffraction study of the crystalline and glassy phases of a fly ashis known as mineralogical analysis. Mineralogical characterization deter-mines the crystalline phases that contain the major constituents of fly

ash.2. Generally, fly ashes have 15-45% crystalline matter. The high-calcium

ashes (Class C) contain larger amounts of crystalline matter ranging be-tween 25 and 45%. Table 1.3 presents crystalline phases in fly ashes iden-tified by XRD analysis .

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3. Although high-calcium Class C ashes may have less glassy or amorphousmaterial, they do contain certain crystalline phases such as anhydride

(CaSO4), tricalcium aluminate (3CaOAl2O3), calcium sulpho-aluminate(CaSAl2O3) and very small amount of free lime (CaO) that participate inproducing cementitious compounds. Also, glassy phase in Class C ashesis usually more reactive.

4. The glassy particles in Class C fly ashes contain large amount of calciumwhich possibly makes the surface of such particles highly strained, andprobably, it is because of highly reactive nature of Class-C fly ashes.

5. Anhydrite (CaSO4) is formed from the reaction of CaO, SO2 and O2 inthe furnace or flue. Quantity of anhydrite increases with the increase inSO3 and CaO contents. It plays a significant role in fly ash hydrationbehavior because it.

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Chapter 3

Quality of fly ash

3.1 Chemical Requirements

1. Pulverized fuel ash, shall conform to the chemical requirements given intable

2. The fly ash may be supplied in dry or moist condition as mutually agreed.However, in case of dry condition, the moisture content shall not exceed2 percent. All tests for the properties specified in 6.1 shall, however, becarried out on oven dry samples.

3.2 Physical Requirements

1. Pulverized fuel ash, when tested in accordance with the methods of testspecified in IS 1727, shall conform 10 the physical requirements given inTable.

2. Uniformity Requirements In tests on individual samples, the specific sur-face, particles retained on 45 micron IS Sieve (wet sieving) and lime re-activity -value shall not vary more than 15 percent from the average es-tablished from the tests on the 10 preceding samples or of all precedingsamples if less than 10.

3. Notwithstanding the strength requirements specified in Table, mixes inwhich pulverized fuel ash is incorporated shall show a progressive increaseFly ash of fineness 250 m2/kg (Min) is also permitted to be used in the

manufacture of Portland pozzolana cement by intergrinding it with Port-land cement clinker if the flyash when ground to fineness of 320 m2/kg orto the fineness of resultant Portland pozzolana cement whichever is lower,meets all the requirements specified in physical and chemical requirementsof the standard.

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Figure 3.1: Chemical Properties

3.3 Classification of Fly Ash

Two classes of fly ash are defined by ASTM C618: Class F fly ash and ClassC fly ash. The chief difference between these classes is the amount of calcium,silica, alumina, and iron content in the ash.

3.3.1 Class F fly ash

The burning of harder, older anthracite and bituminous coal typically producesClass 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 as Portland cement, quicklime,or hydrated lime, with the presence of water in order to react and producecementitious compounds. Alternatively, the addition of a chemical activatorsuch as sodium silicate (water glass) to a Class F ash can lead to the formationof a geopolymer.

3.3.2 Class C fly ash

Fly ash produced from the burning of younger lignite or sub-bituminous coal,in addition to having pozzolanic properties, also has some self-cementing prop-erties. In the presence of water, Class C fly ash will harden and gain strengthover time. Class C fly ash generally contains more than 20% lime (CaO). UnlikeClass F, self-cementing Class C fly ash does not require an activator. Alkali and

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Figure 3.2: Physical Properties

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sulfate (SO4) contents are generally higher in Class C fly ashes. At least oneUS manufacturer has announced a fly ash brick condtaining up to 50% Class

C fly ash. Testing shows the bricks meet or exceed the performance standardslisted in ASTM C 216 for conventional clay brick; it is also within the allowableshrinkage limits for concrete brick in ASTM C 55, Standard Specification forConcrete Building Brick. It is estimated that the production method used in flyash bricks will reduce the embodied energy of masonry construction by up to90%. Bricks and pavers were expected to be available in commercial quantitiesbefore the end of 2009.

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Chapter 4

Fly Ash : Mechanism

Ordinary Portland Cement (OPC) is a product of four principal mineralogi-cal phases. These phases are Tricalcium Silicate- C S (3CaO.SiO ), DicalciumSilicate -CS (2CaO.SiO ), Tricalcium Aluminate- C A (3CaO.Al O ) and Tetra-calcium alumino-ferrite - C AF(4CaO. Al O Fe O ). The setting and hardeningof the OPC takes place as a result of reaction between these principal com-pounds and water.The reaction between these compounds and water are shownas under:

2C3S + 6H = C3S2H3 + 3CH

2C2S (dicalcium silicate) + 4H (Water) = + C3S2H3 (C-S-H gel) +CH(Calcium hydroxide)

The hydration rod s from C S and C S are similar but quantity of calcium

hydroxide (lime) released is higher in C S as compared to C S .The reaction of C A with water takes place in presence of sulphate ions supplied by dissolutionof gypsum present in OPC. This reaction is very fast and is shown as under:

C3A + 3(CSH2) + 26H = C3 A(CS)3 H32

C3A + CSH2 + 10H = C3ACSH12

Tetracalcium alumino-ferrite forms hydration product similar to those of C A,with iron substituting partially for alumina in the crystal structures of ettringiteand monosulpho-aluminate hydrate. Above reactions indicate that during thehydration process of cement, lime is released out and remains as surplus in thehydrated cement. This leached out surplus lime renders deleterious effect toconcrete such as make the concrete porous, give chance to the development of

micro- cracks, weakening the bond with aggregates and thus affect the durabilityof concrete. If fly ash is available in the mix, this surplus lime becomes thesource for pozzolanic reaction with fly ash and forms additional C-S-H gel havingsimilar binding properties in the concrete as those produced by hydration of cement paste. The reaction of fly ash with surplus lime continues as long aslime is present in the pores of liquid cement paste.

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Chapter 5

Effect of fly ashincorporation in concrete

5.1 Reduced Heat of Hydration

In concrete mix, when water and cement come in contact, a chemical reactioninitiates that produces binding material and consolidates the concrete mass.The process is exothermic and heat is released which increases the temperatureof the mass When fly ash is present in the concrete mass, it plays dual rolefor the strength development. Fly ash reacts with released lime and producesbinder as explained above and render additional strength to the concrete mass.The unreactive portion of fly ash act as micro aggregates and fills up the ma-trix to render packing effect and results in increased strength. The large

temperature rise of concrete mass exerts temperature stresses and can lead mi-cro crackes. When fly ash is used as part of cementitious material, quantum of heat liberated is low and staggers through pozzolanic reactions and thus reducesmicro-cracking and improves soundness of concrete mass.

5.2 Workability of Concrete

Fly ash particles are generally spherical in shape and reduces the water require-ment for a given slump. The spherical shape helps to reduce friction betweenaggregates and between concrete and pump line and thus increases workabilityand improve pumpability of concrete. Fly ash use in concrete increases finesvolume and decreases water content and thus reduces bleeding of concrete.

5.3 Permeability and corrosion protection

Water is essential constituent of concrete preparation. When concrete is hard-ened, part of the entrapped water in the concrete mass is consumed by cement

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mineralogy for hydration. Some part of entrapped water evaporates, thus leav-ing porous channel to the extent of volume occupied by the water. Some part

of this porous volume is filled by the hydrated products of the cement paste.The remaining part of the voids consists capillary voids and give way for ingressof water. Similarly, the liberated lime by hydration of cement is water-solubleand is leached out from hardened concrete mass, leaving capillary voids for theingress of water. Higher the water cement ratio, higher will be the porosityand thus higher will be the permeability. The permeability makes the ingressof moisture and air easy and is the cause for corrosion of reinforcement. Higherpermeability facilitate ingress of chloride ions into concrete and is the maincause for initiation of chloride induced corrosion.

Additional cementitious material results from reaction between liberatedsurplus lime and fly ash, blocks these capillary voids and also reduces the riskof leaching of surplus free lime and thereby reduces permeability of concrete.

5.4 Effect of fly ash on Carbonation of Concrete

Carbonation phenomenon in concrete occurs when calcium hydroxides (lime)of the hydrated Portland Cement react with carbon dioxide from atmospheresin the presence of moisture and form calcium carbonate. To a small extent,calcium carbonate is also formed when calcium silicate and aluminates of thehydrated Portland cement react with carbon dioxide from atmosphere. Car-bonation process in concrete results in two deleterious effects (i) shrinkage mayoccur (ii) concrete immediately adjacent to steel reinforcement may reduce itsresistance to corrosion. The rate of carbonation depends on permeability of con-crete, quantity of surplus lime and environmental conditions such as moistureand temperature. When fly ash is available in concrete; it reduces availability of

surplus lime by way of pozzolanic reaction, reduces permeability and as a resultimproves resistance of concrete against carbonation phenomenon.

5.5 Sulphate Attack

Sulphate attacks in concrete occur due to reaction between sulphate from exter-nal origins or from atmosphere with surplus lime leads to formation of etrringite,which causes expansion and results in volume destabilization of the concrete.Increase in sulphate resistance of fly ash concrete is due to continuous reac-tion between fly ash and leached out lime, which continue to form additionalC-S-H gel. This C-S-H gel fills in capillary pores in the cement paste, reducingpermeability and ingress of sulphate ions.

5.6 Corrosion of steel

Corrosion of steel takes place mainly because of two types of attack. One is dueto carbonation attack and other is due to chloride attack. In the carbonation

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attack, due to carbonation of free lime, alkaline environment in the concretecomes down which disturbs the passive iron oxide film on the reinforcement.

When the concrete is permeable, the ingress of moisture and oxygen infuse tothe surface of steel initiates the electrochemical process and as a result-rust isformed. The transformation of steel to rust increases its volume thus resultingin the concrete expansion, cracking and distress to the structure.

In the chloride attack, Chloride ion becomes available in the concrete eitherthrough the dissociation of chlorides-associated mineralogical hydration or infu-sion of chloride ion. The sulphate attack in the concrete decomposes the chloridemineralogy thereby releasing chloride ion. In the presence of large amount of chloride, the concrete exhibits the tendency to hold moisture. In the presenceof moisture and oxygen, the resistivity of the concrete weakens and becomesmore permeable thereby inducing further distress. The use of fly ash reducesavailability of free limes and permeability thus result in corrosion prevention.

5.7 Reduced alkaliaggregate reaction

Certain types of aggregates react with available alkalis and cause expansionand damage to concrete. These aggregates are termed as reactive aggregates.It has been established that use of adequate quantity of fly ash in concretereduces the amount of alkali aggregate reaction and reduces/ eliminates harmfulexpansion of concrete. The reaction between the siliceous glass in fly ash andthe alkali hydroxide of Portland cement paste consumes alkalis thereby reducestheir availability for expansive reaction with reactive silica aggregates.

5.8 Bleeding

Generally fly ash will reduce the rate and amount of bleeding primarily dueto the reduced water demand (Gebler 1986). Particular care is required todetermine when the bleeding process has finished before any final finishing of exposed slabs. High levels of fly ash used in concrete with low water contentscan virtually eliminate bleeding. Therefore, the freshly placed concrete shouldbe finished as quickly as possible and immediately protected to prevent plasticshrinkage cracking when the ambient conditions are such that rapid evaporationof surface moisture is likely. The guidance given in ACI 305, Hot WeatherConcreting should be followed. An exception to this condition is when fly ashis used without an appropriate water reduction, in which case bleeding (andsegregation) will increase in comparison to Portland cement concrete.

5.9 Setting time

The impact of fly ash on the setting behaviour of concrete is dependent notonly on the composition and quantity of fly ash used, but also on the type andamount of cement, the water-to-cementitious materials ratio (w/cm), the type

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and amount of chemical admixtures, and the concrete temperature. It is fairlywell-established that low-calcium fly ashes extend both the initial and final set

of concrete as shown in Figure.During hot weather the amount of retardation due to fly ash tends to be

small and is likely to be a benefit in many cases. During cold weather, theuse of fly ash, especially at high levels of replacement, can lead to very significantdelays in both the initial and final set. These delays may result in placementdifficulties especially with regards to the timing of finishing operations for floorslabs and pavements or the provision of protection to prevent freezing of theplastic concrete. Practical considerations may require that the fly ash content islimited during cold-weather concreting. The use of set-accelerating admixturesmay wholly or partially offset the retarding effect of the fly ash. The settingtime can also be reduced by using ASTM C150 Type III (or ASTM C1157Type HE) cement or by increasing the initial temperature of the concrete duringproduction (for example, by heating mix water and/or aggregates).

Higher-calcium fly ashes generally retard setting to a lesser degree than low-calcium fly ashes, probably because the hydraulic reactivity of fly ash increaseswith increasing calcium content. However, the effect of high-calcium fly ashesis more difficult to predict because the use of some of these ashes with certaincement-admixture combinations can lead to either rapid (or even flash) settingor to severely retarded setting (Wang 2006 and Roberts 2007).

5.10 Strength

In conventional concrete the flexural strength reaches its maximum value be-tween 14 to 28 days. In case of concretes with fly ash as a supplement thestrength keeps on increasing with age because of pozzolanic reaction of fly ash,

and the strengthening of interfacial bond between cement paste and aggregate.The strength properties are strongly dependent on the quality of cement andfly ash used.

5.11 Water Demand

The water demand and workability are controlled by particle size distribution,particle packing effect, and smoothness of surface texture. As mentioned abovethe fly ash replacing some of the cement will increase the paste volume. The flyash concrete is more workable and less water is needed for the same slump.

Although increased fineness usually increases the water demand, the spher-ical particle shape of the fly ash lowers particle friction and offsets such effects.

The use of fly ash as a partial replacement for Portland cement will usuallyreduce water demand.

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Chapter 6

Fly Ash : Usage and MixProportions

6.1 Usage

The main objective of using fly ash in most of the cement concrete applicationsis to get durable concrete at reduced cost, which can be achieved by adoptingone the following two methods :

1. Using Fly ash based Portland Pozzolana Cement (PPC) conforming toIS:1489 Part-1 in place of Ordinary Portland Cement

2. Using fly ash as an ingredient in cement concrete.

The first method is most simple method, since PPC is factory-finished prod-uct and does not requires any additional quality check for fly ash during pro-duction of concrete. In this method the proportion of fly ash and cement is,however, fixed and limits the proportioning of fly ash in concrete mixes. Theaddition of fly ash as an additional ingredients at concrete mixing stage as partreplacement of OPC and fine aggregates is more flexible method. It allows formaximum utilization of the quality fly ash as an important component (cemen-titious and as fine aggregates) of concrete. There are three basic approaches forselecting the quantity of fly ash in cement concrete:

6.1.1 Simple replacement method

In this method a part of the OPC is replaced by fly ash on a one to one basisby mass of cement. In this process, the early strength of concrete is lower andhigher strength is developed after 56-90 days. At early ages fly ash exhibitsvery little cementing value. At later ages when liberated lime resulting fromhydration of cement, reacts with fly ash and contributes considerable strengthto the concrete. This method of fly ash use is adopted for mass concrete works

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where initial strength of concrete has less importance compared to the reductionof temperature rise.

6.1.2 Addition Method

In this method, fly ash is added to the concrete without corresponding reductionin the quantity of OPC. This increases the effective cementitious content of theconcrete and exhibits increased strength at all ages of the concrete mass.This method is useful when there is a minimum cement content criteria due tosome design consideration.

6.1.3 Modified replacement method

This method is useful to make strength of fly ash concrete equivalent to thestrength of control mix (without fly ash concrete) at early ages i.e. between 3

and 28 days. In this method fly ash is used by replacing part of OPC by massalong with adjustment in quantity of fine aggregates and water. The concretemixes designed by this method will have a total weight of OPC and fly ash higherthan the weight of the cement used in comparable to control mix i.e. withoutfly ash mix. In this method the quantity of cementitious material (OPC + Flyash) is kept higher than quantity of cement in control mix (without fly ash) tooffset the reduction in early strength.

6.2 Mix Design

Cement Concrete is principally made with combination of cement (OPC / PPC/Slag), aggregate and water. It may also contain other cementitious materials

such as fly ash, silica fumes etc. and / or chemical admixture. Use of Flyash along with cement helps to provide specific properties like reduced earlyheat of hydration, increased long term strength, increased rsistance to alkaliaggregate reaction and sulphate attack, reduced permeability, rsistance to theintrusion of aggressive solutions and also economy. Chemical admixture are usedto accelerate, retard, improve workability, reduce mixing water requirement,increase strength or alter other properties of the concrete.

Criteria for Mix Design

The selection of concrete proportions involves a balance between economyand requirements for workability and consistency, strength, durability, densityand appearance for a particular application. In addition, when mass concreteis being proportioned, consideration is also given to heat of hydration.

6.2.1 Workability and Consistency

Workability is considered to be that property of concrete, which determines itscapacity to be placed, compacted properly and finished without segregation.

Workability is affected by: the grading, particle shape, proportions of aggre-gate, the quantity qualities of cement + cementitious materials, the presence

22



of entrained air and chemical admixtures, and the consistency of the mixture.Consistency is defined as the relative mobility of the concrete mixture. It is mea-

sured in terms of slump. The higher the slump the more mobile the mixture-and it affects the ease with which the concrete will flow during placement. Itis related with workability. In properly proportioned concrete, the unit watercontent required to produce a given slump will depend on several factors. Waterrequirement increases as aggregates become more angular and rough textured.It decreases as the maximum size of well- graded aggregate is increased. It alsodecreases with the entrainment of air.Certain water- reducing admixtures reduce mixing water requirement signifi-cantly.

6.2.2 Strength

Although strength is an important characteristics of concrete, other character-istics such as durability, permeability and wear resistance are often equally ormore important. Strength at the age of 28 days is generally used as a parame-ter for the structural design, concrete proportioning and evaluation of concrete.Strength mainly depends on water - cement or water - cementitious materialratio [w/c or w/(c+p)]. For a given set of materials and conditions, concretestrength is determined by the net quantity of water used per unit quantity of cement or total cementitious materials. The net water content excludes waterabsorbed by the aggregates. Difference in strength for a given water- cement(w/c) ratio or water- cementitious materials w/(c+p) ratio (p indicates poz-zolana or supplementary cementitious materials) may result from changes in:maximum size of aggregate; grading, surface texture, shape, strength, stiffnessof aggregate particles, differences in cement types and sources, air content, and

the use of chemical admixtures that affect the cement hydration process or de-velop cementitious properties themselves.

6.2.3 Durability

Concrete must be able to endure those exposures that may deprive it of itsserviceability- heating cooling, wetting drying, freezing thawing in cold coun-tries, chemicals, de-icing agents etc. Resistance to some of these may be en-hanced by use of special ingredients, low-alkali cement, fly ash, Ground Granu-lated Blast Furnace (GGBF) slag, and silica fume. The durability of concreteexposed to seawater or sulfate- bearing or aggregate composed of minerals andfree of excessive soft particles where resistance to surface abrasion is requiredcan also be enhanced substantially by using above special ingredients. Use of

low water-cement or water cementitious materials ratio [w/c or w/(c+p)] willprolong the life of concrete by reducing the penetration of aggressive liquids.

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6.2.4 Density

For certain applications, concrete may be used primarily for its weight char-acteristics. Examples of such applications are counterweights on lift bridges,dams, weights for sinking oil pipelines under water, shielding from radiationand insulation from sound.

6.2.5 Heat of Hydration

A major concern in proportioning mass concrete is the size and shape of thecompleted structure or portion thereof. If the temperature rise of the concretemass is not controlled a minimum and the heat is allowed to dissipate at a rea-sonable rate, or if the concrete is subjected to severe temperature differential orthermal gradient, cracking is likely to occur. Thermal cracking of foundation,floor slabs, beams, columns, bridge piers and other massive structure such as

dams can or may reduce the service life of a structure by promoting early de-terioration and may need excessive maintenance. Utilization of fly ash providesa partial replacement of cement with material, which generates considerableless heat at early ages. The early age heat contribution of a pozzolana mayconservatively be estimated to a range between 15 to 50 percent of that of anequivalent weight of concrete. The required temperature control measures canthus be suitably reduced.

6.3 Proportioning of Concrete

The selection of concrete proportions involves a balance between economy andvarious criteria defined in para 8.1 above. Proportioning or mix design of con-

crete involves a sequence of logical, straight forward steps which in effect fit thecharacteristics of the available materials into a mixture suitable for the work.Steps to be followed for proportioning of concrete utilizing fly ash are givenbelow. These guidelines are based on Standard Practice for Selecting Propor-tions for Normal, Heavyweight, and Mass Concrete (ACI 211.1-91) of AmericanConcrete Institute (ACI).

6.3.1 Selection of slump for requirement of consistency

If slump is not specified, a value appropriate for the work can be selected fromTable (a). The slump ranges shown apply when vibration is used to consolidatethe concrete. The maximum value of slump may be increased by 25 mm if themethodof consolidation adopted is other than vibration.

Recommended slumps in ACI 211.1-91 for various types of constructions-

6.3.2 Selection of maximum size of aggregates

Large nominal maximum sizes of well-graded aggregates have less voids thansmaller size aggregates. This results in, concrete with the larger sized aggre-

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Figure 6.1: Slumps

gates require less mortar per unit volume of concrete. Therefore, the nominalmaximum size of aggregates should be the largest that is economically availableand consistent with the dimensions of the structure. However, nominal maxi-mum size of aggregates should not be more than (i) one fifth of the narrowestdimension between sides of forms.(ii) one third of the depth of slab (iii) threefourths of the minimum clear spacing between individual reinforcing bars/ bun-dles of bars or pretensioning strands. These limitations are sometimes can berelaxed if workability and methods of consolidation are such that the concretecan be placed without honeycombs or voids.

6.3.3 Estimation of mixing water and air content

The quantity of water per unit volume of concrete required to produce a givenslump is dependent on the nominal maximum size, particle shape, grading of theAggregates, the concrete temperature; the amount of entrained air and use of chemical admixture. Slump is not greatly affected by the quantity of cement orcementitious material. Estimates of required mixing water for concrete, with orwithout air entrainment recommended by ACI (American Concrete Institutes)

Approximate Mixing Water and Air Content Requirements for DifferentSlumps and Nominal Maximum Sizes of Aggregates is shown in table.

The quantity of mixing water given for air entrained concrete are based ontypical total air content requirement and as shown for ”moderate exposure”in The table above. reasonably well-shaped angular aggregates grading withinlimits of accepted specification. Rounded course aggregate will generally require18 kg less water for non-air-entrained and 15 kg less for air-entrained concrete.

The use of water reducing chemical admixture may also reduce mixing water by5% or more. The volume of liquid admixture is included as part of total volumeof mixing water.

These quantities of mixing water are for use in computing cement factors fortrial batches when 75 mm or 150 mm nominal maximum size aggregate is used.

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Figure 6.2: Approximate Mixing Water and Air Content Requirements for Dif-ferent Slumps and Nominal Maximum Sizes of Aggregates

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They are average for reasonably well-shaped coarse aggregate, well graded fromcoarse to fine.

When using large aggregate in low cement factor concrete, air entrainmentneed not to be detrimental to strength. In most cases, mixing water requirementis reduced sufficiently to improve the water cement ratio and to thus compensatefor the strength reducing effect of entrained air concrete. Generally, therefore,fo these large nominal maximum sizes of aggregate, air contents recommendedfor extreme exposure should be considered even though there may be a little orno exposure to moisture and free water.

6.3.4 Selection of water cementitious materials [w /(c+p)]or water cement ratio

The approximate values corresponding to compressive strength at 28 days understandard laboratory conditions are given in table (c), which can be used forselection of water cementitious materials [w /(c+p)] or water cement (w/c)ratio for concrete proportioning.

Figure 6.3: Relationship between water cementitious materials ratio and com-pressive strength of Cement

* Values are estimated average strength for concrete containing not morethan 2% air for non-air-entrained concrete and 6% total air content for air-entrained concrete. For constant water-cement ratio, the strength of concrete isreduced as the air content is increased.

Strength is based on 152 x305 mm cylinder moist-cured for 28 days in ac-cordance with standard norms specified in relevant ASTM code.

The relationship given in the above table is based on the nominal maximumsize of about 19 to 25 mm. For given source of aggregate, strength produced at

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given water -cement ratio will increase as nominal maximum size of aggregatesdecreases. Fly ash 15-35% by weight of total cementitious material can

be used as part replacement of Ordinary Portland cement. When high earlystrength is required, the total weight of cementitious material (Cement + flyash) may be kept greater than the quantity that would be need if PortlandCement were the only cementitious material. When high early strength is notrequired higher percentage of fly ash can be used. When fly ash is used inconcrete, a water-to- cement + fly ash ratio by weight must be considered inplace of the traditional water-cement ratio (w/c) by weight.

6.3.5 Calculation of cementitious material content:

The amount of cementitious material (c + p) per unit volume of concrete can bedetermined by selecting the mixing water content and the water to cementitiousmaterial ratio as described in step 3 4. However, if minimum cementitiousmaterial requirement is specified for strength and durability criteria, in thatcase higher quantity of cementitious content will be used in the mix.

6.3.6 Estimation of coarse aggregate content

Nominal maximum size and grading will produce concrete of satisfactory worka-bility when a given volume of coarse aggregates, on an oven dry-rodded basis, isused per unit volume of concrete. The approximate value of dry mass of coarseaggregate required for a cubic meter of concrete can be worked out by takingvalue corresponding to nominal maximum size of aggregate from table (d) andmultiplying by the dry- rodded unit mass of aggregates in kg.

Figure 6.4: Volume of Coarse Aggregate Per Unit of Volume of Concrete

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6.3.7 Estimation of fine aggregate content

The fine aggregate content can be worked out from the formula given below:Wet density of concrete (kg/m3) - weight of (cement + fly ash + water + coarseagreegates) in kg.Normally wet density of concrete is taken as 2400 kg/m3

6.3.8 Adjustments for aggregate moisture

The aggregate quantities actually to be weighted out for the concrete mustallow for moisture in the aggregates. Generally, aggregates are moist and theirdry weights should be increased by the percentage of water they contain (bothabsorbed and surface). The mixing water to be added in a batch must bereduced by an amount equal to the free moisture contributed by aggregate i.e.total moisture minus absorption.

6.3.9 Trial batch adjustment

The estimated mixture proportion is to be checked by trial batches preparedand tested according to standard practice for compressive strength, slump, unitweight etc. In the trial batch sufficient water should be used to produce therequired slump regardless of the amount assumed in selecting the trial propor-tions. The trial batches should be carefully observed for proper workability,freedom from segregation and finishing properties. Appropriate adjustmentshould be made in the proportions for subsequent batches. By followingabove-mentioned steps, designing of cement concrete mix using fly ash as a ce-mentitious material for partly replacing cement can be carried out for desiredstrength and durability.

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Chapter 7

Fly Ash : IndustrialOverview

COAL-based thermal power plants have been a major source of power generationin India, where 75% of the total power obtained is from coal-based thermal powerplants. The coal reserve of India is about 200 billion ton-nes (bt) and its annualproduction reaches 250 million tonnes (mt) approximately. About 70% of thisis used in the power sector. In India, unlike in most of the deve-loped countries,ash content in the coal used for power generation is 30-40%. High ash coalmeans more wear and tear of the plant and machinery, low thermal efficiency of the boiler, slogging, choking and scaling of the furnace and most serious of themall, generation of a large amount of fly ash. India ranks fourth in the world inthe production of coal ash as by-product waste after USSR, USA and China,

in that order. Fly ash is defined in Cement and Concrete Terminology (ACICommittee 116) as the ’finely divided residue resulting from the combustionof ground or powdered coal, which is trans-ported from the fire box throughthe boiler by flue gases’. Fly ash is fine glass powder, the particles of whichare generally spherical in shape and range in size from 0.5 to 100 ?m. Fly ashis classified into two types according to the type of coal used. Anthracite andbituminous coal produces fly ash classified as class F. Class C fly ash is producedby burning lignite or sub-bituminous coal. Class C fly ash has self-cementingproperties.

7.1 Deposition of fly-ash

1. Thermal Power stations using pulverized coal or lignite as fuel generatelarge quantities of ash as a by-product. There are about 82 power plantsin India, which form the major source of flyash in the country. With thecommissioning of super thermal power plants and with the increasing useof low grade coal of high ash content, the current production of ash isabout 85 million tonnes per year. This figure is likely to go upto 100

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million tonnes per year by the year 2000 AD and pose serious ecologicalproblems.

2. Although the scope for use of ash in concrete, brick making, soil-stabilizationtreatment and other applications has been well recognized, only a smallquantity of the total ash produced in India is currently utilized in suchapplications. Most of the ash generated from the power plants is dis-posed off in the vicinity of the plant as a waste material covering severalhectares of valuable land. The bulk utilization of ash is possible in two ar-eas, namely, ash dyke construction and filling of low-lying areas. Coal ashhas been successfully used as structural fills in many developed countries.However, this particular bulk utilization of ash is yet to be implemented inIndia. Since most of the thermal power plants in India are located in areaswhere natural materials are either scarce or expensive, the availability of flyash is bound to provide an economic alternative to natural soils

7.2 Flyash Disposal in Ash Ponds

1. Primarily, the flyash is disposed off using either dry or wet disposal scheme.In dry disposal, the flyash is transported by truck, chute or conveyor atthe site and disposed off by constructing a dry embankment (dyke). Inwet disposal, the flyash is transported as slurry through pipe and disposedoff in impoundment called ”ash pond”. Most of the power plants in Indiause wet disposal system, and when the lagoons are full, four basic optionsare available: (a) constructing new lagoons using conventional construc-tional material, (b) hauling of flyash from the existing lagoons to anotherdisposal site, (c) raising the existing dyke using conventional construc-

tional material, and (d) raising the dyke using flyash excavated from thelagoon (”ash dyke”). The option of raising the existing dyke is very costeffective because any fly ash used for constructing dyke would, in additionto saving the earth filling cost, enhance disposal capacity of the lagoon.The constructional methods for an ash dyke can be grouped into threebroad categories: (a) Upstream method, (b) Downstream method and (c)Centerline method. Fig.1 shows typical configurations of embankmentsconstructed using the different methods. The construction procedure of an ash dyke includes surface treatment of lagoon ash, spreading and com-paction, benching and soil cover.

2. An important aspect of design of ash dykes is the internal drainage sys-tem. The seepage discharge from internal surfaces must be controlled with

filters that permit water to escape freely and also to hold particles in placeand the piezometric surface on the downstream of the dyke. The inter-nal drainage system consists of construction of rock toe, 0.5m thick sandblanket and sand chimney. After completion of the final section includingearth cover the turfing is developed from sod on the downstream slope.

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Figure 7.1: Fly Ash Deposition

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7.3 Flyash as Fill Material

1. Large scale use of ash as a fill material can be applied where (a) fly-ash replaces another material and is therefore in direct competition withthat material, (b) flyash itself is used by the power generating companyproducing the flyash to improve the economics of the overall disposal of surplus flyash; and (c) at some additional cost, flyash disposal is combinedwith the rehabilitation and reclamation of land areas desecrated by otheroperations.

2. Fills can be constructed as structural fills where the flyash is placed in thinlifts and compacted. Structural flyash fills are relatively incompressibleand are suitable for the support of buildings and other structures. Non-structural flyash fill can be used for the development of parks, parkinglots, playgrounds and other similar lightly loaded facilities. One of the

most significant characteristics of flyash in its use as a fill material is itsstrength. Well-compacted flyash has strength comparable to or greaterthan soils normally used in earth fill operations. In addition, lignite flyashpossesses self-hardening properties which can result in the development of shear strengths. The addition of illite or cement can induce hardening inbituminous flyash which may not self-harden alone. Significant increasesin shear strength can be realized in relatively short periods of time and itcan be very useful in the design of embankments.

7.4 Environmental Considerations

1. The environmental aspects of ash disposal aim at minimizing air and wa-

ter pollution. Directly related to these concerns is the additional envi-ronmental goal of aesthetically enhancing ash disposal facilities. The ashproduced in thermal power plants can cause all three environmental risks- air, surface water and groundwater pollution. The pathways of pollutantmovement through all these modes are schematically represented in Fig.

2. ” Air pollution is caused by direct emissions of toxic gases from the powerplants as well as wind-blown ash dust from ash mound/pond. The air-borne dust can fall in surface water system or soil and may contaminatethe water/soil system. The wet system of disposal in most power plantscauses discharge of particulate ash directly into the nearby surface watersystem. The long storage of ash in ponds under wet condition and humidclimate can cause leaching of toxic metals from ash and contaminate theunderlying soil and ultimately the groundwater system. However, mostof these environmental problems can be minimised by incorporating engi-neering measures in the design of ash ponds and continuous monitoring of surface and groundwater water systems.

3. Air pollution is caused by direct emissions of toxic gases from the powerplants as well as wind-blown ash dust from ash mound/pond. The air-

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Figure 7.2: Pollution

borne dust can fall in surface water system or soil and may contaminatethe water/soil system. The wet system of disposal in most power plantscauses discharge of particulate ash directly into the nearby surface water

system. The long storage of ash in ponds under wet condition and humidclimate can cause leaching of toxic metals from ash and contaminate theunderlying soil and ultimately the groundwater system. However, mostof these environmental problems can be minimised by incorporating engi-neering measures in the design of ash ponds and continuous monitoring of surface and groundwater water systems.

7.5 Fly ash transportation

For the fly ash collection and transportation we can design and supply differentpneumatic transportation system such as:

1. Airslide-Airlift system2. Dense phase pneumatic transport

3. Vacuum type pneumatic transport

4. Mechanical ash collection transport

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5. High concentration ash slurry disposal system

6. Positive pressure lean phase pneumatic transport.

7. Silo technologies.

7.5.1 Airslide-airlift systems

The Airslide-Airlift material transport is usually used together. It is a simpleand well proven design and suitable to transport large quantities for short dis-tances. Typical application when the fly ash has to be collected from the largenumber of ESP hoppers and has to be transported into a near-by fly ash storagesilo.

7.5.2 Airslide channel

Technical parameters:

1. Standard transport capacity: 1 - 500 t/h

2. Standard transport distance: 10 - 200 m

3. Specific air demand: 100 - 500 m3/h/m2

Advantages of the application:

operational safety, simplicity, flexibility and high transport capacity dependingon channel size. It has no moving part and transports the material with lowspeed, in a protecting operating mode. There is a stable working point in widerange according to change of the loading, that is, the same channel is able to

operate even for a considerably changed material

7.5.3 Airlift

The Airlift is used for vertical transportation of fly ash, cement and fine-grainedmaterials. Technical parameters

1. Standard transport capacity: 10 - 150 t/h

2. Standard vertical lifting height: 10 - 80 m

Advantages of the application:

The Airlift has simple structural constructions, contains no moving part, itsoperating cost is low and can be operated safety in wide operating range. The

airlift has a stable working point and the equipment is self-adjusting.The most frequent application fields:

In fly ash handling systems, cement works chemical plants vertical transporta-tion of bulk material in large quantities into large-size silos, intermediate storagetanks.

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7.5.4 Dense phase pneumatic transport

In the last 15-20 years it is a trend world-wide to use dense phase pneumaticconveying for fly ash collection and transportation.Why dense phase?

1. High material to air ratio, big quantities can be transported for long dis-tances with less air.

2. Due to lower velocity less wear

3. Long conveying distance in one stage up to 1500 m.

4. No storage in ESP hoppers, fly ash is collected in the transporting vessels.

5. No sucking of flue gas, therefore danger of condensation is minimized whilefilling and during conveying no danger of plugging.

6. Due to dense phase smaller transport pipes required, consequently lessstructure and erection works

7. Lower power consumption compared to other methods.

Typical arrangements usually offering:

1. Direct, multi dense phase transport from each ESP hoppers to storagesilo. This simple solution can be used for silo distance up to 500 m.For this direct transportation system we have developed pair and groupoperation of the transport vessels. It means that up to 4 Nos. of vessels

can be operated simultaneously is such a way that the transport vesselsare working onto one common delivery pipeline. It results simpler andmaintenance friendly operation since the number of valves is reduced.This arrangement is usually recommended under the 3rd , 4th and 5throw of the E-precipitator hoppers.

2. Two stage dense phase transport. In the first stage the fly ash is collectedby individual vessels into a transfer bin and the long distance transporta-tion from transfer bin to storage silo (up to 1500 m distance) is made byJumbo transport vessel.

3. Two stage transportation, where the fly ash collection is done by airslide,mechanical or vacuum system and the long distance transport by single

Jumbo transport vessels.Advantages of the application:

The equipment can be operated in wide operating range. Main operating pa-rameters of working point belonging to quantitative and qualitative changes of the material to be transported can be changed, resp., adjusted flexibly. Low

36



power consumption and operating cost. With PLC control, full automatic oper-ation of the transport equipment of transport vessel can be realized. Because of

the low delivery air demand, on venting of the receiving silos we need filters of smaller size compared to the conventional, thin-phase flow pneumatic transportequipment. Additional advantages against vacuum type systems in case of bigfly ash quantities: The fly ash is collected in the transporting vessel, thereforeESP hoppers have no storage function, consequently the plugging of hopperoutlet is eliminated. Several transport vessels can be connected to a commonpipeline, thereby a possibility is afforded to construct more complex systems aswell.

7.5.5 Combination of airslides and pressure vessel system

For power plants over 300 MW where big size ESP-s with large number of hoppers are required and silos are far from units there is an alternative solution

to individual pressure vessels. This is combination of airslides and pressurevessels. A typical 600 MW unit with two ESP including 32 NOS of collectinghoppers each working as follows:

1. From the four hoppers belong to one path, fly ash is conveyed by airslideinto big size conveying vessels instead of 4 NOS of smaller ones. Resultingeight (8) pressure vessels instead of thirty-two (32).

2. Out of the total eight (8) pressure vessels two (2) or four (4) can evenreceive boiler ash via short distance pneumatic conveying, making easierto transport the boiler ash to long distance by mixing it with the finer flyash from ESP.

3. Moreover four pneumatic conveying vessels belong to one ESP can be

connected to one common ash conveying line in such a way that two-twovessels can be coupled and connected to one common ash outlet valveforming conveying pairs

7.6 Packing of fly ash

1. Single spot fly ash packing machine

The air filler type single spout fly ash packing machine is designed to fillfly ash under ESP hopper of power plant and fill valve type bag. Materialis pushed by the pneumatic feeder and filled in the bag through the spoutwhich is connected to machine with a flexible hose pipe. The materialis fluidized and easily flows into the bag through the nozzle. Weighing

is provided through mechanical weighing beam. This is an economicalmachine to pack low cost powdered materials in valve type bags.

2. Double spot fly ash packing machine

Double spout fly ash packing machine is capable of filling and packing atleast 400 bags of fly as per hour. Compressed air requirements for this

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machine are 750 lpm @ 7 kg / cm2. Height and other specifications, fea-tures of both the machines are same, both uses pneumetic filling mathodto fill the bags.

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Figure 7.3: Double spot fly ash packing machine

Figure 7.4: Packing parameters

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Fly ash report

Documents

Transcript of Fly ash report