chapter-3

Assessment of Soil Stabilization Using Fly Ash and Rice Husk Ash

3 .1 FLY ASH

Fly ash is a by-product of burning coal at electric power plants. It is a fine residue

composed of unburned particles that solidifies while suspended in exhaust gases. Fly ash

is carried off in stack gases from a boiler unit, and is collected by mechanical methods or

electrostatic precipitators. Fly ash is composed of fine spherical silt size particles in the

range of 0.074 to 0.005 mm. Fly ash collected using electrostatic precipitators usually has

finer particles than fly ash collected using mechanical precipitators. Fly ash is one of the

most useful and versatile industrial by-products. When geotechnical Engineers are faced

Stabilization of black cotton soil using lime and fly ash with clayey or expansive soils,

the engineering properties of those soils may need to be improved to make them suitable

for construction. Waste materials such as fly ash or pozzolanic materials are a source of

silica and alumina with high surface area has been used for soil improvement. Fly ash is

generated in huge quantities as a by-product of burning coal at electric power plants . The

potential for using fly ash in soil stabilization has increased significantly in many

countries since it is environmentally safe.

Fly ash is classified into two classes: F and C. Class F fly ash which is not self-

cementing fly ash is produced from burning anthracite and bituminous coals and contains

small amount of lime (CaO) to produce cementitious products. An activator such as

Portland cement or lime must be added. This fly ash has siliceous and aluminous

material, which itself possesses little or no cementitious value but it reacts chemically

with lime at ordinary temperature to form cementitious compounds.

Class C fly ash which is self-cementing fly ash is produced from lignite and sub-

bituminous coals and usually contains significant amount of lime .This type (class C) is

self-cementing because it contains a high percent of calcium oxide (CaO) ranging from

20 to 30%.

Formation of cementitious material by the reaction of lime with the pozzolans

(Al2O3, SiO2, and Fe2O3) in the presence of water is known as hydration of fly ash. The

DEPARTMENT OF CIVIL ENGINEERING 4 SVIST,KADAPA(YSR)


hydrated calcium silicate or calcium aluminate as cementitious material, can join inert

materials together.

The pozzolanic reactions for soil stabilization are as follow :

CaO + H2O Ca(OH)2 + Heat

Ca (OH)2 Ca ++ + 2 (OH)

Ca ++ + 2 (OH) + SiO2 CSH “Calcium silicate hydrate” (silica) (gel)

Ca ++ + 2 (OH) + Al2O3 CAH “Calcium aluminate hydrate” (alumina) (fibrous)

In case of the class C fly ash, the lime present in the fly ash reacts with the

siliceous and aluminous materials in the fly ash. A similar reaction can occur in class F

fly ash, but lime must be added because of the low lime content of the fly ash class F.

3.1.1 Chemical composition and classification

Component Bituminous Subbituminous Lignite

SiO2 (%) 20-60 40-60 15-45

Al2O3 (%) 5-35 20-30 20-25

Fe2O3 (%) 10-40 4-10 4-15

CaO (%) 1-12 5-30 15-40

LOI (%) 0-15 0-3 0-5

Fly ash material solidifies while suspended in the exhaust gases and is collected

by electrostatic precipitators or filter bags. Since the particles solidify rapidly while

suspended in the exhaust gases, fly ash particles are generally spherical in shape and

DEPARTMENT OF CIVIL ENGINEERING Page 15 SVIST,KADAPA(YSR)


range in size from 0.5 µm to 300 µm. The major consequence of the rapid cooling is that

only few minerals will have time to crystallize and that mainly amorphous, quenched

glass remains. Nevertheless, some refractory phases in the pulverized coal will not melt

(entirely) and remain crystalline. In consequence, fly ash is a heterogeneous material.

SiO2, Al2O3, Fe2O3 and occasionally CaO are the main chemical components present in

fly ashes. The mineralogy of fly ashes is very diverse. The main phases encountered are a

glass phase, together with quartz, mullite and the iron oxides hematite, magnetite and/or

maghemite.

Other phases often identified arecristobalite, anhydrite, free lime, periclase,

calcite, sylvite, halite, portlandite, rutile and anatase. The Ca-bearing minerals anorthite,

gehlenite, akermanite and various calcium silicates and calcium aluminates identical to

those found in Portland cement can be identified in Ca-rich fly ashes. The mercury

content can reach1 ppm , but is generally included in the range 0.01 - 1 ppm for

bituminous coal. The concentrations of other trace elements vary as well according to the

kind of coal combusted to form it. In fact, in the case of bituminous coal, with the notable

exception of boron, trace element concentrations are generally similar to trace element

concentrations in unpolluted soils Two classes of fly ash are defined by ASTM C618:

Class F fly ash and Class C fly ash. The chief difference between these classes is the

amount of calcium, silica, alumina, and iron content in the ash. The chemical properties

of the fly ash are largely influenced by the chemical content of the coal burned (i.e.,

anthracite, bituminous, and lignite).

Not all fly ashes meet ASTM C618 requirements, although depending on the

application, this may not be necessary. Ash used as a cement replacement must meet

strict construction standards, but no standard environmental regulations have been

established in the United States. 75% of the ash must have a fineness of 45 µm or less,

and have acarbon content, measured by the loss on ignition (LOI), of less than 4%. In the

U.S., LOI needs to be under 6%. The particle size distribution of raw fly ash is very often

fluctuating constantly, due to changing performance of the coal mills and the boiler

performance. This makes it necessary that, if fly ash is used in an optimal way to replace



cement in concrete production, it needs to be processed using beneficiation methods like

mechanical air classification. But if fly ash is used also as a filler to replace sand in

concrete production, unbeneficiated fly ash with higher LOI can be also used. Especially

important is the ongoing quality verification. This is mainly expressed by quality control

seals like the Bureau of Indian Standards mark or the DCL mark of the Dubai

Municipality.

3.1.2Class F fly ashThe 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 as Portland cement, quicklime, or hydrated

lime, with the presence of water in order to react and produce cementitious compounds.

Alternatively, the addition of a chemical activator such as sodium silicate (water glass) to

a Class F ash can lead to the formation of a geopolymer.

3.1.3 Class C fly ash

Fly ash produced from the burning of younger lignite or subbituminous coal, in

addition to having pozzolanic properties, also has some self-cementing properties. In the

presence of water, Class C fly ash will harden and gain strength over time. Class C fly

ash generally contains more than 20% lime (CaO). Unlike Class F, self-cementing Class

C fly ash does not require an activator. Alkali and sulfate (SO4) contents are generally

higher in Class C fly ashes.

At least one US manufacturer has announced a fly ash brick containing up to 50%

Class C fly ash. Testing shows the bricks meet or exceed the performance standards listed

inASTM C 216 for conventional clay brick; it is also within the allowable shrinkage

limits for concrete brick in ASTM C 55, Standard Specification for Concrete Building

Brick. It is estimated that the production method used in fly ash bricks will reduce the

embodied energy of masonry construction by up to 90%. Bricks and pavers were

expected to be available in commercial quantities before the end of 2009.



3.1.4 Disposal and market sourcesIn the past, fly ash produced from coal combustion was simply entrained in flue

gases and dispersed into the atmosphere. This created environmental and health concerns

that prompted laws which have reduced fly ash emissions to less than 1% of ash

produced. Worldwide, more than 65% of fly ash produced from coal power stations is

disposed of inlandfills and ash ponds.

The recycling of fly ash has become an increasing concern in recent years due to

increasing landfill costs and current interest in sustainable development. As of 2005, U.S.

coal-fired power plants reported producing 71.1 million tons of fly ash, of which 29.1

million tons were reused in various applications. If the nearly 42 million tons of unused

fly ash had been recycled, it would have reduced the need for approximately

27,500 acre·ft (33,900,000 m3) of landfill space. Other environmental benefits to

recycling fly ash includes reducing the demand for virgin materials that would

need quarrying and substituting for materials that may be energy-intensive to create such

as Portland cement.

As of 2006, about 125 million tons of coal-combustion byproducts, including fly

ash, were produced in the U.S. each year, with about 43% of that amount used in

commercial applications, according to the American Coal Ash Association Web site. As

of early 2008, the United States Environmental Protection Agency hoped that figure

would increase to 50% as of 2011.

3.1.5 Fly ash reuseThere is no U.S. governmental registration or labelling of fly ash utilization in the

different sectors of the economy - industry, infrastructures and agriculture. Fly ash

utilization survey data, acknowledged as incomplete, are published annually by the

American Coal Ash Association.

The ways of coal ash utilization include (approximately in order of decreasing

importance):

Concrete production, as a substitute material for Portland cement and sand



Embankments and other structural fills (usually for road construction)

Grout and Flowable fill production

Waste stabilization and solidification

Cement clinkers production - (as a substitute material for clay)

Mine reclamation

Stabilization of soft soils

Road subbase construction

As Aggregate substitute material (e.g. for brick production)

Mineral filler in asphaltic concrete

Agricultural uses: soil amendment, fertilizer, cattle feeders, soil stabilization in

stock feed yards, and agricultural stakes

Loose application on rivers to melt ice

Loose application on roads and parking lots for ice control

Other applications include cosmetics, toothpaste, kitchen counter tops, floor and

ceiling tiles, bowling balls, flotation devices, stucco, utensils, tool handles, picture

frames, auto bodies and boat hulls, cellular concrete, geopolymers, roofing tiles, roofing

granules, decking, fireplace mantles, cinder block, PVC pipe, Structural Insulated Panels,

house siding and trim, running tracks, blasting grit, recycled plastic lumber, utility poles

and crossarms, railway sleepers, highway sound barriers, marine pilings, doors, window

frames, scaffolding, sign posts, crypts, columns, railroad ties, vinyl flooring, paving

stones, shower stalls, garage doors, park benches, landscape timbers, planters, pallet

blocks, molding, mail boxes, artificial reef, binding agent, paints and

undercoatings, metal castings, and filler in wood and plastic products.

3.1.6 Portland cementOwing 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. Roman structures such as aqueducts or the Pantheon in Rome used volcanic ash



or pozzolana (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 particularly suitable

but not limited to Class C fly ashes. Class "F" fly ashes can have volatile effects on the

entrained air content of concrete, causing reduced resistance to freeze/thaw damage. Fly

ash often replaces up to 30% by mass of Portland cement, but can be used in higher

dosages in certain applications. Fly ash can add to the concrete’s final strength and

increase its chemical resistance and durability.

Fly ash can significantly improve the workability of concrete. Recently,

techniques have been developed to replace partial cement with high-volume fly ash (50%

cement replacement). 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

increase workability of cement while reducing water demand. Proponents of fly ash claim

that replacing Portland cement with fly ash reduces the greenhouse gas "footprint" of

concrete, as the production of one ton of Portland cement generates approximately one

ton ofCO2, 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 fly ash. Since the

worldwide production of Portland cement is expected to reach nearly 2 billion tons by

2010, replacement of any large portion of this cement by fly ash could significantly

reduce carbon emissions associated with construction, as long as the comparison takes

the production of fly ash as a given.

3.1.7 ContaminantsFly ash contains trace concentrations of heavy metals and other substances that

are known to be detrimental to health in sufficient quantities. Potentially toxic trace

elements in coal include arsenic, beryllium, cadmium, barium, chromium, copper, lead,

mercury,molybdenum, nickel, radium, selenium, thorium, uranium, vanadium, and zinc.

Approximately 10% of the mass of coals burned in the United States consists of



unburnable mineral material that becomes ash, so the concentration of most trace

elements in coal ash is approximately 10 times the concentration in the original coal. A

1997 analysis by the U.S. Geological Survey (USGS) found that fly ash typically

contained 10 to 30 ppm of uranium, comparable to the levels found in some granitic

rocks, phosphate rock, and blackshale.

In 2000, the United States Environmental Protection Agency (EPA) said that coal

fly ash did not need to be regulated as a hazardous waste. Studies by the U.S. Geological

Survey and others of radioactive elements in coal ash have concluded that fly ash

compares with common soils or rocks and should not be the source of alarm. However,

community and environmental organizations have documented numerous environmental

contamination and damage concerns.

A revised risk assessment approach may change the way coal combustion wastes

(CCW) are regulated, according to an August 2007 EPA notice in the Federal Register. In

June 2008, the U.S. House of Representatives held an oversight hearing on the Federal

government's role in addressing health and environmental risks of fly ash.

3.2 Black cotton soil

Black cotton soils are inorganic clays of medium to high compressibility and form

a major soil group in Maharashtra. They are characterized by high shrinkage and swelling

properties. Because of its high swelling and shrinkage characteristics, the Black cotton

soil has been a challenge to the highway engineers. The Black cotton soil is very hard

when dry, but loses its strength completely when in wet condition. It is observed that on

drying, the black cotton soil develops cracks of varying depth.The roads laid on Black

cotton soil (BC soil) bases develop undulations at the road surface due to loss of strength

of the sub grade through softening during monsoon. Around 40 to 60% of the Black

cotton soil (BC soil) has a size less than 0.001 mm. At the liquid limit, the volume change

is of the order of 200 to 300% and results in swelling



STRUCTURE OF SOIL

3.3 Rice ProductionPaddy rice (Oryza sativa) is grown on every continent except Antarctica and the

extent of paddy cultivation covers about 1 percent of the earth’s surface. More than half

of the world’s population depends on rice as a staple food and it ranks second to wheat in

terms of cultivation area and production. The quantum of global production of paddy is

close to 650 million tons per annum. Production of rice is dominated by Asia, where rice

is the only food crop that can be grown during the rainy season in the waterlogged

tropical areas. Asia generates over 90 percent of world rice production. Together, China

and India accounted for over half of the world’s rice supply. In India, Tamil Nadu is the

third ranking state in the production of paddy after Andhra Pradesh and West Bengal.

Paddy production is nearly 7 million tonnes in Tamil Nadu.

Paddy, on an average, consists of about 72 percent of rice, 5-8 percent of bran,

and 20-22 percent of husk. Of all the plant residues, the ash of rice husk contains the

highest proportion of silica. It is estimated that every tonne of paddy produces about 0.20

tonnes of husk and every tonne of husk produces about 0.18 to 0.20 tonnes of ash,

depending on the variety, climatic conditions and geographical location. The total global



ash production could be as high as about 23 million tonnes per year.pressure as high as 8

kg/cm2/ to 10 kg/cm2. Average Plasticity Index of black cotton soil was found as 60 %

As such Black cotton soil (BC soil) has very low bearing capacity and high

swelling and shrinkage characteristics. Due to its peculiar characteristics, it forms a very

poor foundation material for road construction. Soaked laboratory CBR values of Black

Cotton soils are generally found in the range of 2 to 4%. Due to very low CBR values of

Black cotton soil (BC soil), excessive pavement thickness is required for designing for

flexible pavement.

3.3.1 Properties of Rice HuskRice husk is a potential material, which is amenable for value addition. The usage

of rice husk either in its raw form or in ash form is many. Most of the husk from the

milling is either burnt or dumped as waste in open fields and a small amount is used as

fuel for boilers, electricity generation, bulking agents for composting of animal manure,

etc.

The exterior of rice husk are composed of dentate rectangular elements, which

themselves are composed mostly of silica coated with a thick cuticle and surface hairs.

The mid region and inner epidermis contain little silica. confirmed that the presence of

amorphous silica is concentrated at the surfaces of the rice husk and not within the husk

itself.

The chemical composition of rice husk is similar to that of many common organic

fibers and it contains of cellulose 40-50 percent, lignin 25-30 percent, ash 15-20 percent

and moisture 8- 15 percent. After burning, most evaporable components are slowly lost

and the silicates are left. The typical properties of rice husk are indicated in other plant

except paddy husk is able to retain such a huge proportion of silica in it. Plants absorb

various minerals and silicates from earth into their body. Inorganic materials, especially

silicates are found in higher proportions in annually grown plants, such as rice, wheat,

sunflower, etc, than in long-lived trees. Inorganic materials are found in the form of free

salts and particles of cationic groups combined with the anionic groups of fibres into the

plants. A combined study using back scattered electron and X-ray images of the husk



showed that the silica is distributed mostly under the husk’s outer surface. This Confirms

the general concept of a soluble form of silica transported through the plant, and

concentrated at the outer surface of straw and husk through evaporation, whereupon it

polymerizes into an opaline cellulose-silica.

3.3.2 Thermal Decomposition Of Rice HuskThere are two distinct stages in the decomposition of rice husk - carbonization

and decarbonation. Carbonization is the decomposition of volatile matter in rice husk at

temperature greater than 300°C and releases combustible gas and tar. Decarbonation is

the combustion of fixed carbon in the rice husk char at higher temperature in the presence

of oxygen. The melting temperature of RHA is estimated as 1440°C, that is, the

temperature at which silica melts .

3.3.3 Forms of Silica in Rice Husk AshRice husk ash contains 87-97 percent of silica with small amount of alkalis and

other trace elements. Based on temperature range and duration of burning of the husk,

crystalline and amorphous forms of silica are obtained. The crystalline and amorphous

forms of silica have different properties and it is important to produce ash with correct

specifications for specific end use. Generally, the amorphous forms of silica are

composed of silica tetrahedral arranged in a random three-dimensional network without

regular lattice structures. Due to disordered arrangement, the structure is open with holes

in the network where electrical neutrality is not satisfied and the specific surface area is

also large. This helps to increase the reactivity, since large area is available for reaction to

take place. The structure of crystalline silica is built by repetition of a basic unit–the

silicon tetrahedron in an oriented three-dimensional framework. In framework type

structure, the silicon tetrahedrons are joined through the vertices by oxygen, each of

which is linked to two silicon atoms. The oxygen to silicon ratio equals to 2:1, thus

electrical neutrality is attained.

The silica occurs in several forms within the rice husks are at the molecular level

and it is associated with water. In nature, the polymorphs of silica are quartz, cristobalite,



tridymite, coestite, stishovite, lechatelerite and silica gel. It is this silica concentrated in

husk by burning which makes the ash so valuable.

3.3.4 Applications of Rice Husk AshRHA has got numerous applications in silicon based industries. Substantial

research has been carried out on the use of RHA as a mineral admixture in the

manufacture of concrete. RHA in amorphous form can be used as a partial substitute for

Portland cement and as an admixture in high strength and high performance concretes. A

review on the use of RHA by the construction industry, in particular on concrete

production has been separately dealt in Chapter-5. Apart from this specific use, it has

many other usages as given below

Due to its refractory properties, crystalline RHA is the most wanted material for

steel industries, ceramic industry and for the manufacture of refractory bricks.Examined

the possibilities of improving residual soil properties by mixing RHA and cement in

suitable proportions as stabilizing agent.

Indian Space Research Organization has successfully developed a technology for

producing high purity silica from RHA that can be used in silicon chip manufacture

Naito (1999) introduced a low cost technology for controlling insect pests in Soya beans

by using RHA. The insects are irritated by the high levels of silicon and the needle like

particles.

Attempts have been made to utilize RHA in vulcanizing rubber. RHA has been

shown to offer advantages over silica as a vulcanising agent for ethylenepropylene- diene

terpolymer (EPDM), and is recommended as diluents filler for EPDM rubber

Rice husk burnt for prolonged periods have been successfully used as oil

absorbent. RHA produced by maintaining temperatures 350-450°C for extended period of

time will be highly amorphous and porous. High porosity of RHA is essential to absorb

oil .


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