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UNIT-V

BUILDING MATERIALS

1. CEMENT

Introduction

Cement is a dirty greenish heavy powder which finds its importance as a building

material. It is described as a material which possesses adhesive and cohesive properties to

bind rigid masses like stones, bricks, building blocks etc. Cements are hydraulic in nature i.e.

it possesses the property of setting and hardening in the presence of water. Further the

essential constituents of cement used for constructional purpose are calcareous (compounds

of calcium) and argillaceous i.e. (Al + Si), materials.

Classification of CementBased on different chemical compositions, cement is classified into four types. They

are

1. Natural cement

2. Puzzolana cement

3. Slag cement and

4. Portland cement

1. Natural cement: This is obtained by calcining and pulverizing natural rocks

consisting of clay and limestone. Calcium silicates and aluminates are formed because of

the combination of silica and alumina with calcium oxide. Natural cement is usually used

for construction of big structures such as dams.

Properties:

a) It is hydraulic in nature with low strength and

b) Its setting time is very less.2. Puzzolana cement: It is one of the ancient cements in the world and was identified

by the Romans. It was used by them in making concrete for the construction of walls and

domes. This cement was prepared from volcanic ash of Mount Vesuvius situated around

the place called puzzouli in Italy. The volcanic ash consisting of silicates of calcium, iron

and aluminium mixed with lime and on heating result in puzzolanic cement.

Properties:

It is hydraulic in nature and is mixed with Portland cement and is then used for

different applications.

1A. Sri Charitha, Department of Chemistry. NRI Institute of Technology

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3. Slag cement: This cement is prepared by mixing hydrated lime and blast furnace slag,

which is a mixture of calcium and aluminium silicates, in a stream of cold water. It is dried

and then pulverized to fine powder. Sometimes accelerators like clay or caustic soda are

added to hasten the hardening process.

Properties:

a) It possesses low strength

b) The time required for setting and hardening is more i.e. one week.

Because of these properties it has very few applications and is usually used in making

concrete in bulk construction. It is also used as concrete in water logged areas, where the

tensile strength is less important.

4. Portland cement: William Aspdin (1824) is generally recognized as the father of the

modern Portland cement industry, as he produced improved cement by heating a mixture

of limestone and clay and crushing the resulting product to a fine powder. Portland cement

is most widely used non-metallic material of construction. It is also known as magic

powder and is a mixture of calcium silicates and calcium aluminates with small amount of

gypsum.

The name Portland cement was used because this powder, on mixing with water, sets

to give a hard, stone-like mass which resembles the Portland rock.

Manufacture of Portland cement

1. Raw materials

The raw materials used for manufacturing of Portland cement are:

a) Calcareous materials: Those which supply lime. Eg: Limestone, cement rock, chalk

and waste calcium carbonate from industrial processes. Limestone high in magnesia

(MgO) cannot be used, because it leads to cracking. Similarly chalk freed from flint is to

be used.

b) Argillaceous materials: Those which supply silica, alumina and iron oxide. Eg: Clay,

blast furnace slag ashes, shale and cement rock. Commonly used are clay and shale.

c) Gypsum: This is added during the final grinding and it controls the ratio of setting and

hardening.

2. Composition of Portland cement

A good sample of Portland cement has the composition of Calcium oxide or lime (CaO) = 60 70%


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Silica (SiO2) = 20 24%

Alumina (Al2O3) = 5 7.5%

Magnesia (MgO) = 2 3%

Ferric oxide (Fe2O3) = 1 2.5%

Sulphur trioxide (SO3) = 1 1.5%

Sodium oxide (Na2O) = 1%

Potassium oxide (K2O) = 1%

Gypsum (CaSO4.2H2O)

During the manufacture of Portland cement, great care should be taken, because

a) Excess lime in cement results in cracks during setting.

b) On the other hand if the lime content is less, the cement is low in strength and sets very

soon.

c) Excess silica produces a slow hardening cement.

d) Excess of alumina even though hastens the setting, it weakens the cement.

e) If iron is not present in cement it will be white and hard to burn. The presence of iron

imparts a grey colour and also strength to the cement.

f) Presence of excess of alkali oxides causes cement efflorescence.

g) If excess of sulphur trioxide is present, it will reduce the soundness of the cement.

(Volume change that takes place when cement is hydrated is called soundness. If

volume changes are less then cement is considered to be sound.

h) Gypsum helps to retard the setting action of cement. It enhances the initial setting time

of cement.

Methods of manufacturing process

The manufacture of cement involves the following steps:

1. Mixing of raw materials:

A mixture of finely ground limestone and clay (3:1) is made by anyone of the

following methods.

Dry process: The dry process produces a fine ground powder. This process is

employed if the limestone and clay are hard. In this process, initially limestone is crushed

into pieces and then it is mixed with clay in the proportion of 3:1. This mixture is

pulverized to a fine powder and is stored in storage bins (silos) and later on it is

introduced into the upper end of the rotary kiln.


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Wet process:The wet process takes place in the presence of water and usually results

in a slurry formation. This process is preferred if limestone and clay are soft.

In this process, the clay is washed with water in wash mills to remove any foreign

material, organic material etc. Powdered limestone is then mixed with the clay paste in a

proper proportion (3:1). The mixture is then finely ground and homogenized to form

slurry containing about 40% of water. This is also stored in the storage bins and can be

fed into the rotary kiln when necessary.

Merits and demerits of dry and wet processes

Dry process Wet process

1) This process is adopted when the

raw materials are quite hard.

2) It is a slow and costly process.

3) The fuel consumption is low as the

rotary kiln used is a smaller one.

4) Since the fuel consumption is less,

the cost of production of cement is less.

5) The quality of cement produced is

inferior.

6) This process is not suitable if theraw material has moisture content of

15% or more.

1) This process is preferred when the raw

materials are soft.

2) It is a comparatively cheaper and fast

process.

3) Fuel consumption is high as a longer

kiln is needed to drive off the excess of

water.

4) Cost of production is high since the

fuel consumption is more for a longer

kiln.

5) The quality of cement produced is

superior.

6) This process can be adopted even in


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wet conditions.

2. Burning the mixture in a rotary kiln:

The rotary kiln is an inclined steel cylinder 150-200 feet long and 10 feet in diameter

and it is lined inside with fire bricks. The kiln can be rotated at a desired speed, (usually

0.5 to 2 rotations per minute) as it is mounted on rollers. As the kiln rotates, the mixtureof raw materials stored from the above two process, passes slowly from the upper to the

lower end. In other words the slurry of the raw materials enters from the upper end of the

rotary kiln while the burning fuel (pulverized coal, oil, or natural gas) and air are induced

from the lower end of the kiln. As the mixture or slurry gradually descends, the

temperature rises and infact this creates different zones in the rotary kiln, with increasing

temperature. They are

a) The drying zone: This is present in the upper part of the kiln, where the temperature is

around 400oC. In this zone most of the water in the slurry gets evaporated because of the

hot gases. The clay is broken as Al2O3, SiO2 and Fe2O3.

Al2O3 . 2SiO2 . Fe2O3 . 2H2O Al2O3 + 2SiO2 + Fe2O3 + 2H2O

b)Calcination zone or decarbonating zone: This zone is located in themiddle portion of

the kiln where the temperature is of the order of 1000oC. In this zone the lime stone is

completely decomposed into CaO (quick lime) which exists in the form of small lumps

called nodules and carbondioxide which escapes out.

CaCO3CaO +CO2

lime stone quick lime

c) Burning zone or clinkering zone: This zone is at the bottom and is considered to be the

hottest portion of the kiln. The temperatures over here ranges around 1400-1500oC. In the

clinkering zone, lime and clay react with each other forming aluminates and silicates.

2CaO + SiO2Ca2SiO4Dicalcium silicate (C2S)

3CaO + SiO2Ca3SiO5Tricalcium silicate (C3S)

3CaO +Al2O3Ca3Al2O6Tricalcium aluminate (C3A)

4CaO + Al2O3 +Fe2O3

Ca4Al2Fe2O10Tetracalcium alumino ferrite (C4AF)


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These compounds then fuse to form greyish pellets about the size of peas and have a

rough texture, which are called as clinkers. The clinker formation is an exothermic

reaction. So, these clinkers are very hot. The rotary kiln at the base is provided with

another small rotary kiln. In this, hot clinkers fall and cool air is admitted from opposite

direction. Air counter-blast cools the clinkers. Hot air so-produced is used for burning

powdered coal or oil. The cooled clinkers are collected in small trolleys.

3. Grinding or mixing of cement clinkers with gypsum:

The clinkers are cooled and then ground to requisite fineness in ball mills. The finely

ground clinkers set quite rapidly, by absorption of moisture from the atmosphere.

Therefore in order to reduce the rate of setting it is mixed with 2 to 3% gypsum (CaSO4 .

2H2O).

After the initial setting, Al2O3 which is a fast setting constituent of clinker reacts with

gypsum to form the crystals of tricalcium sulphoaluminate, which is insoluble.

3CaO . Al2O3 + x CaSO4 . 7 H2O 3CaO . Al2O3 . x CaSO4 . 7 H2OTricalcium sulphoaluminate

The formation of insoluble tricalcium sulphoaluminate prevents too early reactions of

setting and hardening. This mixture of clinkers and gypsum powder is known as Portland

cement.

4. Packing:

The ground cement is stored in silos from which it is bagged or loaded for shipment.

Further it is noted that Portland cement will retain its cementing quality until it comes in

contact with moisture. Hence it should be stored in a dry and air tight location.

Properties of cement

1) Setting and hardening of cement:

Portland cement on mixing with water, changes to a plastic mass called cement paste,

which slowly loses its plasticity and becomes a stiff and ultimately a rocky mass is

obtained. This process is known as setting. After hydration, anhydrated compounds

become hydrated, which have less solubility. Hence they are precipitated as insoluble gels

or crystals. These have the ability to surround sand, crushed stones, other inert materials

and bind them very strongly. So, hardening is development of strength due to

crystallization.


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The physical changes occurring in the setting and hardening of cement may be

summarized in a flow chart as follows:

Hardening of cement can be explained on the basis of two theories:

a) Crystalline theory (given by Le-Chatlier): According to this theory, constitutional

compounds after hydration form crystalline products. These crystalline products

undergo interlocking which is responsible for hardening of cement.

b) Colloidal theory (given by Michaelis): According to this theory, during hydration

silicate gels are formed which undergo hardening and are responsible for the

hardening of cement.Thus, it can be concluded that setting and hardening of cement is due to the

formation of interlocking crystals reinforced by the rigid gels formed by the hydration

and hydrolysis of the constitutional compounds. Most Portland cements exhibit initial

set in about 3 hours and final set in about 7 hours. Setting can be tested by a standard

needle (vicat needle). If the needle does not penetrate into the paste beyond a certain

limit, then it has reached the initial setting stage. Further if the needle does not

penetrate at all, the cement is said to have reached final setting stage.

Reactions involved in setting and hardening of cement

The basic chemical compounds in the Portland cement are:

Name Chemical formula Abbreviation

Dicalcium silicate

Tricalcium silicate

Tricalcium aluminate

Tetracalcium alumino ferrite

2CaO.SiO2

3CaO.SiO2

3CaO.Al2O3

4CaO.Al2O3.Fe2O3

C2S

C3S

C3A

C4AF


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The behaviour of the cement can be altered by modifying the relative percentages

of these compounds.

When cement is mixed with water, the paste becomes quite rigid within a short

time which is known as initial set or flash set. This is due to C3A which hydrates

rapidly as follows:

3CaO.Al2O3 + 6H2O 3CaO.Al2O3.6H2O(Crystals)

These crystals prevent the hydration reactions of other constitutional compounds

forming barrier over them. In order to retard this flash set, gypsum is added during the

pulverization of cement clinkers. Gypsum retards the dissolution of C3A by interacting

with it forming insoluble complex of sulphoaluminate which does not have quick

hydrating property.3CaO.Al2O3 + xH2O + yCaSO4.2H2O 3CaO.Al2O3.yCaSO4.zH2O

The Tetracalcium alumino ferrite (C4AF) then reacts with water forming both gels

and crystalline compounds as follows:

4CaO.Al2O3.Fe2O3 + 7H2O 3CaO.Al2O3.6H2O + CaO.Fe2O3.H2OCrystals gels

These gels shrink with passage of time and leave some capillaries for the water to

come in contact with C3S and C2S to undergo further hydration and hydrolysis

reactions enabling the development of greater strength over a length of time.

Final setting and hardening of cement paste is due to the formation of tobermonite

gel plus crystallization of calcium hydroxide and hydrated tricalcium aluminate.

2CaO.SiO2 + xH2O 2CaO.SiO2.xH2OGels

2(3CaO.SiO2) + 6H2O 3CaO.SiO2.3H2O + 3Ca(OH)2Tobermonite gel

2) Heat of hydration:

When water is mixed with Portland cement, some amount of heat is liberated due to

hydration and hydrolysis reactions leading to setting and hardening of cement. On an

average, the quantity of heat evolved during complete hydration of cement is of the order

of 500 KJ/Kg. The heats of hydration of the different constitutional compounds are in the

following order:

C3A > C3S > C4FA > C2S

878 502 418 251 KJ/Kg


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Therefore where ever large masses of concrete are used (i.e., construction of dams), it

is essential to dissipate the heat generated during hydration as quickly as possible, to

prevent shrinkage cracks on setting and hardening.

3) Soundness:

If cement on hydration produces only very small volume changes and that such

volume changes are well within tolerance limits laid down in the specifications, the

cement is said to be "sound. Presence of excessive quantities of crystalline magnesia

contributes to delayed expansion or unsoundness. The soundness is determined by Le

Chatliers test in which the expansion of a test piece in boiling water for 3 to 5 hours is

measured. Recently this test is replaced by Autoclave test.

Decay of cement

The constituents of cement are susceptible to attack by salty water and other acidic

solutions. The presence of CO2 in acetic water results in leaching out of free lime.

Ca(OH)2 + CO2CaCO3 + H2O

CaCO3 + H2O + CO2Ca(HCO3)2

Ca(OH)2 + Ca(HCO3)2 2CaCO3 + 2H2O

Till all the CO2 is consumed the above cycle of reactions takes place. Also hydrolysisof silicates and aluminates will lead to decay of cement.

Decay of cement can be minimized by coating the surface with epoxy resin paint or

linseed oil (or other drying oils). This coating makes cement impermeable to acidic waters.

Further when alkaline sewage is carried by concrete pipes, the SiO2 component in cement is

attacked. To overcome this affect, the inner surface of the concrete pipes is treated with SiF4,

where in insoluble CaF2 is formed.

Effect of CO2, SO2 and Chlorides on Cement Concrete

Cement concrete is the most versatile and widely used construction material available.

It is obtained by mixing a binding material, inert aggregate and water in suitable proportions

and which can be readily used or moulded into desired shape. It is compact, rigid, strong and

durable.

Cement concrete is an alkaline/basic material. Being porous in nature, it is vulnerable

to both physical and chemical attacks causing expansion and cracking.

Effect of CO2:


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Ifconcrete is exposed to an excessive CO2 atmosphere during the first 24 hours of life,

a soft, chalk-like carbonate surface will result. Diffusion of CO2 into concrete and conversion

of Ca(OH)2 to CaCO3 is called as carbonation.

Ca(OH)2 + CO2 CaCO3 + H2O

Carbonation depends on the porosity of concrete. Good quality of concrete is less

porous and diffusion of CO2 is less. In porous and low quality concrete, CO2 diffuses readily

and carbonation reaches very early. Carbonation is also highly dependent on the relative

humidity of the concrete. The highest rates of carbonation occur when the relative humidity

is maintained between 50% and 75%. Below 25% relative humidity, the degree of

carbonation that takes place is considered insignificant.

Effect of SO2:

In the environments around industrial agglomerations, SO2 cause deterioration of

concrete. The process of concrete deterioration by SO2 is called sulphation. SO2 is markedly

more corrosive than CO2, but owing to its low concentration in normal atmosphere, the

manifestation of sulphation is less frequent than that of carbonation. SO2 in air combines with

moisture to form first sulphurous acid, which on combination with moisture and oxygen in

air forms sulphuric acid, this sulphuric acid combines with lime in the mortar to form calcium

sulphate. It is a common form of concrete deterioration.Sulphate attack occurs when concrete comes in contact with water containing

sulphates. Concrete is vulnerable to sulphate attack, because the Tricalcium aluminate (C3A)

constituent of Portland cement reacts with sulphate ions to form calcium sulphoaluminate

hydrates (Ettringite) (3CaO . Al2O3 . CaSO4 . 2H2O). The ettringite occupies larger volume

than the original reaction compounds of the hydrated cement, causing cracks due to

expansion and subsequently its disintegration.

Effect of Chlorides:

Chlorides enter concrete through water used for construction and by diffusion from the

environment. Chlorides diffuse through porous and low quality concrete and reach the

reinforcement. In presence of chlorides, the protective oxide layer around iron becomes less

protective and porous that causes electrochemical corrosion of iron.

The rate of electrochemical corrosion-

i.) Decreases with increase in concentration of hydroxide or alkalinity of reinforcement.

ii.) Increases with increase in concentration of chlorides and


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iii.) Increases with increase in the ratio of chlorides and hydroxides.

As the Cl-/OH- ratio increases above 0.65 electrochemical corrosion of reinforcement

starts.2. REFRACTORIES

Introduction

A refractory is a material which does not melt easily, because its fusion temperature is

very high. Therefore refractories are inorganic materials which can withstand high

temperatures as well as abrasion and corrosive action of molten metals, gases, without

suffering a deformation in shape. The main role of a refractory is to confine heat in it.

With increasing use of high temperature processes, the demand for various types of

refractories is constantly growing in mechanical engineering as well as in the metallurgical,

chemical and power industries. Refractories are widely used for providing high temperature

resistant lining for furnaces, kiln, crucibles, etc., in various industries such as ferrous and

non-ferrous, glass, ceramic, power-generation, oil refining and cement. They are also used in

the manufacture of rocket nozzles, launch pads and for domestic heating.

Refractories are available in different shapes and sizes, as bricks, crucibles and tubes,granules and cement.

Classification of Refractories

Refractories are classified on the basis of their chemical properties of the constituent

substances and fusion temperature into the following categories:

On the basis of fusion temperature ranges, they are classified as:

a) Normal refractory: Fusion temperature is 1580 - 1780oC. Eg: Fire clay.

b) High refractory: Fusion temperature is 1780 - 2000oC. Eg: Chromite.

c) Super refractory: Fusion temperature is > 2000oC. Eg: Zircon.

On the basis of chemical composition, they are classified as:

a) Acidic refractories: These refractories consist of acidic materials like alumina

(Al2O3) and silica (SiO2). These are resistant to acid slags and are readily attacked by

basic slags (like CaO, MgO etc.). Hence their contact with these oxide materials should

be avoided.

Eg: Alumina, silica and fire clay refractories.


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b) Basic refractories: These refractories consist of basic materials like CaO, MgO, etc.

and are resistant to basic slags. The presence of acidic materials like silica is harmful to

their high temperature performance. Basic refractories find extensive use in some steel

making open hearth furnaces.

Eg: Magnesite and Dolomite refractories.

c) Neutral refractories: These refractories are made from weakly basic or acidic

materials like carbon, zirconia (ZrO2) and chromite (FeO.CrO2). Neutral refractories show

resistance to the action of acidic and basic materials and also show good chemical

stability.

Eg: Graphite, zirconia and Carborundum (SiC) refractories.

On the basis of the oxide content the refractories are further classified as:

a) Single oxide refractories Eg: Alumina, magnesia and zirconia.

b) Mixed oxide refractories Eg: Zircon, spinel, mullite.

c) Non-oxide refractories Eg: Borides, carbides, silicides and nitrides.

Characteristics of refractory materials

1. A goodrefractory material should have a softening temperature much higher than the

operating temperature.

2. Refractories should be chemically inert under the condition where in they are

employed i.e., they should not react with corrosive agents like acidic or basic molten

slags, hot gases, etc.

3. The refractoriness should be high for a good refractory. Resistance to fusion on

increasing the temperature is called the refractoriness.

4. The refractories should not crack at operating temperatures.

5. They should possess low permeability.

6. They should possess low thermal co-efficient of expansion and should expand and

contract uniformly, with increase and decrease of temperature respectively.

7. They should be able to withstand the overlying load of structure, at operating

temperatures.

8. They should possess good physical, chemical and mechanical properties.

However no single substance can satisfy all of these requirements and therefore a

refractory is usually formulated from a mixture of compounds. If a given refractory material

does not have the above mentioned characteristics, it will fail in service.


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Properties of Refractories

Refractories are characterized by a certain set of properties.

1. Refractoriness:

It is the ability of a material to withstand high temperature without appreciable

deformation or softening under given working conditions. It is usually measured as the

softening temperature of the material. As the temperature increases normally a material

softens and deforms, but a refractory should resist such a tendency. So, higher the softening

temperature, more valuable is the refractory.

Measurement of Refractoriness:

This is determined by pyrometric cone (seger cone) test. The test refractory in the

form of a cone (38mm height and 19mm base) is kept along with similar sized standard cones

and all are heated uniformly at 10oC per minute. Each standard cone is made of a particular

refractory with a definite softening temperature. These standard cones are assigned certain

numbers with increasing softening temperature. When the test cone softens and loses its

shape, one of the standard cones whose softening temperature is close to the test cone will

also soften. The serial number of this standard cone will be the PCE (Pyrometric Cone

Equivalent) of the test cone. If the test cone softens earlier than one standard cone, but latter

than the next cone, the PCE value of the test sample is approximately measured as the

average value of the two.

2. Refractoriness under load (RUL test):

This property gives an idea of the strength of arefractory. The two essential qualities

of refractory are 1) temperature resistance and 2) load bearing capacity. Since very heavy

reactants are charged into the refractory lined furnaces, the refractory should withstand such

heavy loads especially at high temperatures.


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Eg: Fire clay refractories collapse at temperatures well below their fusion temperature,

when appreciable load is applied. On the other hand silica refractories withstand load even at

high temperatures.

Hence RUL test is performed to know the safe upper temperature limit up to which the

refractory can be used.

The test is done in a rectangular container by applying a load of 1.75 Kg/Cm2 on to the

refractory and heating at a constant rate of 10oC per minute. During this process the specimen

will soften and its height will decrease under the load. This decrease in height is measured

and when there is 10% decrease to that of the original height, the temperature is noted. The

RUL is then expressed as the temperature at which this 10% deformation occurs.

3. Chemical inertness:

A refractory material selected for a process should be chemically inert since several

chemicals are used in a furnace, which may react with the refractory material and form

fusible products, which may corrode the furnace and in addition contaminate the furnace

product. Since mostly either acidic or basic environments prevail in the furnace, a simple

guideline in the use of refractories is that, an acidic refractory should not be used in basic

furnace and vice-versa.

Eg: Silica bricks being acidic cannot be used in a basic furnace and Magnesite bricks

being basic cannot be used in acidic furnace.

4. Dimensional stability:

It is defined as the resistance of material to any volume changes which may occur

because of its exposure to high temperatures over a prolonged period of time. These

dimensional changes may be reversible or irreversible. Further the irreversible change may

result in contraction or expansion, due to transformation of one crystalline form into another.

Eg: a) Conversion of magnesite bricks into denser periclase.

b) Conversion of quartz in silica bricks to cristoballite.

5. Thermal expansion and contraction:

Since all solids expand when heated and contract when cooled, allowance has to be

made in the furnace design for thermal expansion. Expansion of a refractory material

decreases the capacity of furnace and also results in developing internal stresses. Further

repeated expansion and contraction contributes to much rapid breakdown of the refractory.


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Therefore lower the thermal expansion and contraction, the better the quality of the

refractory.

6. Thermal conductivity:

In many of the industrial operations, refractory materials of both high thermal

conductivity and low thermal conductivity are required depending upon the type of furnace.

The conductivity of a refractory primarily depends on its chemical composition and porosity.

As porosity increases, the thermal conductivity decreases because the entrapped air in the

pores functions as insulator. In contrast dense refractories have high thermal conductivity.

Most of the furnaces are lined inside with refractory materials of low thermal

conductivity in order to reduce heat losses to the outside environment by radiation.

Refractories which can be used under these circumstances are fireclay and silica, on the other

hand in muffle furnaces, the heat should be efficiently transmitted and therefore carbon and

silicon carbide refractories which are poor insulators can be employed.

7. Porosity:

Refractories usually contain pores which result from their manufacturing methods,

these pores may be open or closed. Porosity is an important property of a refractory as it

affects many physical and chemical characteristics of the refractory. It is the ratio of pore

volume to the bulk volume.

Where, P = porosity

W = saturated weight of specimen

D = dry weight of the specimen

A = saturated weight + moisture content of specimen

Porosity is an important property of refractory bricks since it affects manycharacteristics i.e. chemical stability, strength, abrasion-resistance and thermal conductivity.

Low porosity refractory material will have greater strength, higher heat capacity and thermal

conductivity. On the other hand a highly porous refractory will allow slags, gases etc. to enter

the pores thereby decreasing the strength as well as resistance to abrasion and corrosion. All

these factors ultimately reduce the life of the refractory. Therefore in general a good

refractory should have low porosity.

8. Electrical conductivity:


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Refractories used for lining electric furnaces should have low electrical conductivity. In

general refractories are poor conductors of electricity except graphite, which is a good

conductor. However electrical conductivity of refractories increases with increasing

temperature.

9. Heat capacity:

The heat capacity of a furnace depends on three factors:

i. Thermal conductivity

ii. Specific heat and

iii. Specific gravity of the refractory

Heavy and denserefractory bricks have high heat capacity and hence find use in glass

furnaces and blast furnaces. Whereas light weight refractory bricks because of their low heat

capacity find use in occasionally operated furnaces to achieve the working temperature in a

short time with lesser consumption of fuel.

10.Permeability:

It is a measure of rate of diffusion of gases, liquids and molten solids through a

refractory and depends on the size and number of connected pores. With rise in temperature

the permeability increases since the viscosity of molten metals decreases with an increase of

temperature. Refractories of low bulk density have high porosity and hence high permeability

and are used as insulating materials. A high bulk density improves several properties such as

mechanical strength, heat capacity and resistance to spalling.

11. Thermal spalling:

Thermal spalling is peeling, cracking, fracturing and breaking of the refractories due

to rapid fluctuations in temperature causing uneven stresses and strains in the body of

refractory. A good refractory must show resistance to thermal spalling. The spalling

resistance order for some of the refractories is

Silicon carbide > fireclay bricks > magnesite > silica bricks

Further spalling can be minimized by:

a) avoiding sudden fluctuations in temperature

b) proper selection of refractory materials with high thermal conductivity, uniformity and

high porosity within permissible limits and low co-efficient for thermal expansion.

c) by over firing the refractory materials during manufacture so as to make the materialless susceptible to uneven expansion or contraction and


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d) improved furnace design to minimize stresses and strains during operation.

12. Texture:

Coarse or light textured bricks, because of their large porosity are light in weight and

hence they are more resistant to sudden changes in temperatures. Hence they are more

susceptible to the action of abrasion and corrosion. On the other hand fine or dense textured

bricks possess low porosity and are heavier in weight. These bricks are not resistant to

sudden changes in temperature. However such bricks are less susceptible to the action of

abrasion and corrosion.

Conditions leading to failure of refractory material

1. Using a refractory of less refractoriness than that of the operating temperatures.2. Using refractories which cannot withstand the load of raw materials and products.

3. Rapid changes in temperature of the furnace.

4. Using bricks of high thermal expansion.

5. Using refractory bricks which are not properly fired.

6. Using bricks which undergo considerable volume changes during their use at high

temperatures.

7. Using heavy weight refractory bricks.

8. Using acidic/basic refractory in a furnace in which basic/acidic reactants and/or

products are being processed.

Applications of Refractories

Depending on the area of application such as boilers, furnaces, kilns, ovens etc,

temperatures and atmospheres encountered different types of refractories are used.

Fire Clay Refractories

1. Steel industries are the largest consumers of fire clay refractories.

2. They are used for lining of blast furnaces, open hearths, stoves, ovens, crucible

furnaces, etc.

3. They are also used in lime kilns, pottery kilns, glass furnaces, boiler settings,

metallurgical kilns, etc.

Silica Bricks

1. These are used for arches in large furnaces because of their physical strength.

High alumina refractories


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1. These bricks are employed in cement industry, paper industry, oil-fire furnaces, high

pressure oil stills, in the roofs of lead softening furnaces, etc.

Magnesite refractories

1. These refractories are preferred when basic materials in molten state are to be heated

at high temperatures.

2. These refractories are used in open-hearth and electric furnace walls in the roofs of

non-ferrous reverberatory furnaces.

3. These are also used for lining of basic converters in steel industry, hot mixer linings,

refining surfaces for Ag, Au and Pt.

Dolomite refractories

1. These are quite cheap and used in granular form to pitch the bottom of open-hearth

furnace and for repairing works.

2. Stabilized dolomite bricks are used in Bessemer convertors, ladle linings and for basic

electric furnace lining.

Carbon refractories

1. These refractories are used for making electrodes and for linings in chemically

reactive equipments.

Graphite refractories1. These are used for construction of electrodes, atomic reactors, electric furnaces and in

non-ferrous metal smelting furnaces.

Silicon carbide or carborundum refractories

1. Mainly used in muffles.

2. Their ability to absorb and release heat rapidly and their resistance to spalling under

repeated temperature fluctuations make them an ideal choice for recuperators.

3. Owing to their high electrical conductivity, they are used as heating elements in

furnaces in the form of rods and bars are known as globars.

4. Silicon carbide bonded with tar is excellent for making high conductivity crucibles.

Non-oxide refractories

1. These refractories are expensive and are mostly used at present in nuclear and space

research programmes.


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3. A flux or a glassy portion: This is generally provided by feldspar which helps in

bonding and cementing the ingredients together.

Small quantities of other materials such as dolomite, magnesite, talc, some organic

additives are also added to improve workability.

Classification of Ceramics

There are various classification systems of ceramic materials, which may be attributed

to one of the two principal categories: application based system or composition based system.

Classification based on composition of ceramics

1. Silicate ceramics: Presence of glassy phase in a porous structure.

Eg: Clay ceramics and Silica ceramics

2. Oxide ceramics: Dominant crystalline phase, with small glassy phase.

Eg: Single oxide (Al2O3), Modified oxide (Zirconia toughened alumina), Mixed oxide

(Mullite)

3. Non-oxide ceramics:

Eg: carbon, SiC, BN, TiB2, etc.

4. Glass ceramics: Partially crystallized glass.

Eg: SiO2-Li2O, LAS, etc.

Classification based on applications of ceramics

1. Glasses: Glasses are familiar group of ceramicsbased on SiO2, with additions to

reduce melting point or give special properties.

Eg: containers, households, optical glasses.

2. High performance advanced ceramics: These are special ceramics having improved

toughness, wear resistance, electrical properties, etc.

Eg: Cutting tools, grinding, bearings, sensors, lasers, super conductors, etc.

3. Traditional vitreous ceramics: These are clay based products.

Eg: Porcelain, sanitary ware, tiles, bricks, refractories, etc.

4. Cement and concrete: These are complex ceramics with many phases.

Eg: Structural materials, composites, etc.

5. Natural ceramics: This class includes rocks and minerals including ice, bones, etc.

Glazed and unglazed ceramicsGlazed ceramics


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A glaze is a fine powder, consisting of a mixture of glass-forming materials of proper

composition, Eg: lead silicates, borosilicates, etc. The glazing mixture free from iron and

other colouring pigments, form colourless glaze, while for colour glazing, coloured metal

oxides (or pigments) are mixed in proper proportions.

Eg: i.) Iron oxide for red and brown colours.

ii.) Iron oxide and lime for cream and yellowish tints.

iii.) Copper oxide for green colour.

iv.) Cobalt blue for blue colour.

Purpose of glazing

1. To produce decorative effect.

2. To make the surface impervious to liquids, water, etc.

3. To improve appearance of the article/material.

4. To increase durability of ceramic materials.

5. To provide a smooth and glossy surface to treated material.

6. To protect the surfaces from environment/atmospheric conditions.

Methods of glazing

Glazing is accomplished by any of the following two methods:

1. Salt glazing employs common salt or sodium chloride for getting glossyfilms overthe earthen wares. The process consists in throwing common salt into the furnace, in

which the articles are in red-ho condition. Intensive heat causes sodium chloride to

volatilize and then react with silica of articles to form glossy and impervious film of

sodium silicate.

2. Liquid glazing is much superior method, in which fine powder of glaze mixture plus

requisite quantity of colouring pigments are mixed with water to form a colloidal solution

called glaze-slip. The articles to be glazed are then burnt at a low temperature in kilns.

They are then taken out and dipped momentarily in the glaze-slip, thereby the glaze

material enters and fills up the pores in the articles. Then, the articles are fired in the kilns

at a higher temperature so that the glaze material fuses and forms thin glossy films over

them. During firing every care should be taken to see that articles do not come in direct

contact with fire: otherwise dust and soot are likely to discolor them.

Unglazed ceramics


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Unglazed ceramics are similar to glazed ones except that their surface is uncoated.

These types of ceramics are compact, hard and dense. Its homogeneous composition is

inherited from the texture and colour of the material used for manufacturing. The method of

cooking the clay is another factor for its appearance. The top and bottom colour and texture

of unglazed ceramics are the same. They are very suitable for commercial and industrial

purposes.

Properties and applications of ceramics

1. White wares: These are the materials which after firing give white or pale cream in

colour products having a fine texture. White wares are made by mixing china clay,

feldspar and flint. The proportions of these ingredients are adjusted according to the

properties in the finished product. White ware products consist of a refractory body and a

glassy coating called glaze.

Properties: They possess good strength, translucency, when fired at higher temperatures,

partial vitrification takes place and the white wares become porous.

Uses: White wares find applications as spark plugs, electrical insulators, laboratory

equipments, crucibles, dishes, high-class potteries, etc.

2. Porcelain: The term porcelain is used to indicate fine earthen ware which is white,

thin and semi-transparent. Since the colour of porcelain is white, it is also referred to as

the white ware. It is prepared from clay, feldspar, quartz and minerals. The constituents

are finely ground and then they are thoroughly mixed in liquid state. The mixture is given

the desired shape and it is burnt at high temperature.

Properties: Porcelain is hard, brittle and non-porous.

Uses: Porcelains are employed for various uses such as sanitary wares, electric insulators,

storage vessels, reactor chambers and crucibles, etc.

Low voltage porcelains are used for making switch block, insulating tubes and lamp

sockets, etc.

High voltage porcelains are used for making electrodes and in the construction of

atomic reactor rockets, as lining material for electric furnace, vacuum tubes, as electric

insulator for high intensity electric current, etc.

3. Earthen wares: The term earthen ware is used to indicate wares or articles prepared

from clay which is burnt at low temperature and cooled down slowly, the clay is mixed


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with required quantity of sand, crushed pottery, etc. The addition of such materials

prevents the shrinkage during drying and burning.

Properties: Earthen wares are usually soft and porous. Glazed earthen wares are

impervious to water, not affected by acids or atmospheric agencies.

Uses: Earthen ware is used for making ordinary drain pipes, electrical cable conduits and

partition blocks, etc.

Terra-cotta is a kind of earthen ware. Terra means earth and cotta means baked.

Hence terra-cotta means baked earth. It is thus a type of earthen ware or porous pottery

made from local clays and glazed with glazes containing galena.

Terra-cottas are more durable, lighter, stronger, harder and cheaper than most

building stones. It is also non-affected by acids and atmosphere, easily moulded into

intricate designs and easy to clean. It is fire-proof and most suited for RCC, but unlike

stone, it cannot be fixed during progress of the work. It is twisted due to unequal

shrinkage in drying and burning.

4. Stone wares: The term stone ware is used to indicate the wares or articles prepared

from refractory clays which are mixed with stone and crushed pottery. Such a mixture is

then burnt at a high temperature and cooled down slowly. The stone ware is more

compact and dense than earthen ware.Properties: The stone wares are strong, impervious, durable and resistant to corrosive

fluids and they resemble fire-bricks.

Uses: The stone wares can be keptclean easily and hence they have become very popular

as the sanitary articles such as wash basins, sewer pipes, glazed tiles, water closets, etc.

They are also used as jars to store chemicals.

Engineering applications of Ceramics

1. Ceramics are used as refractories in furnaces and as durable building materials.

2. They are used as electrical and thermal insulators in manufacture of spark plugs,

telephone poles, electronic devices, nose cones of space-craft, etc.

3. Ceramic composites are used in tennis rackets, bicycles, automobiles and for making

break-resistant cookware.

4. Some ceramics like cubic boron nitride are good conductors of heat; ruthenium oxide

is good electrical conductor and SiC is used as a semi-conductor.


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5. Some ceramics such as barium ferrite or Nickel zinc ferrites are magnetic materials

that provide stronger magnetic fields, weigh less and cost less than metal magnets and are

used in computers and microwave devices.

6. Ceramics are important in medicine. They are used to fabricate artificial bones and to

crown damaged teeth. They are also useful as biosensors.

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