Chapter-7: CEMENTING MATERIALS

CLAY

INTRODUCTION Clay is a natural earthy material that is found almost everywhere on

the earth surface.

Size of clay particles is typically less than 0.002mm

Clay is formed from the rocks containing feldspar mineral over long

periods of time by the gradual chemical decomposition, mechanical

disintegration by various natural agencies such as wind, water, ice,

frost and atmospheric impurities.

Fig.: Feldspar

Properties of ClayPlasticityFineness of grain

CohesionShrinkage under firing and

under air dryingHardness

Classification of ClayA. According to the mode of formation

i. Residual Clay:

• Known as primary clay.

• Found near the place of origin or found overlying the parent rock.

• Eg. Kaolin or china clay

Fig.: Kaolin

ii. Transported Clay:

• Known as secondary clay.

• Transported naturally a relatively great distance from its place of

origin.B. According to dominant characteristicsi. China Clay• Purest type of clay• Contains high amount of mineral kaolinite.• Used for manufacture of crockery and other porcelain ware.

ii. Fire Clay

• Contains silica and alumina in high percentage.

• Can withstand very high temperature.

• Used for making refractory bricks.

iii. Vitrifying Clay

• Rich in iron oxide and carbonates of calcium and magnesium.

• Used in the manufacture of flooring tiles, facing bricks, sewer

pipes, etc.

Silica Alumina

iv. Brick Clay

o Considered as low grade clay.

o Used for the manufacture of the building bricks.

o Rich in silica, alumina, oxides of iron, calcium and magnesium.

LIME

Introduction Lime is the oxide of calcium.

It is produced by heating limestone (Calcium

Carbonate).

Due to burning of lime, the moisture and CO2 gas is

removed, the remaining product is known as lime.

At present, the lime is replaced by cement to a great

extent.

But it is still considered as reliable and economical

cementing material where cement is either costly or

not available.

Fig.: Limestone

Technical Terms

i. Calcination

Process of heating limestone to redness in contact with

air.

CaCo3 CaO + CO2

ii. Slaking of Lime

CaO + H2O Ca(OH)2

Fig: Slaked Lime

iii. Hydraulicity

It is the property of lime by which it sets or hardens in

damp and moist places or underwater.

iv. Setting

It is the hardening of lime when it has been converted

into paste form.

Composition of Limestonei. Clay

Responsible for producing hydraulicity.

It makes lime insoluble in water.

Small quantity results in retardation of slaking.

ii. Soluble Silica

It helps to develop hydraulicity.

Hydraulicity is caused due to the silicates of calcium,

magnesium and aluminium.

iii. Magnesium Carbonate

It increases the setting process but reduces slaking.

iv. Alkalies of metallic oxide

They tend to become soluble silicates at low

temperatures only and thus cause hydraulicity.

Composition of Limestone Contd..

v. Sulphates

The presence of sulphates in small quantities increases

the process of setting and reduces the slaking action.

vi. Iron compunds

The iron compounds, if present in small quantity lower

the temperature of calcination of limestone.

Composition of Limestone Contd..

vii. Carbonaceous matterThe presence of carbonaceous matter is harmful; it produces very poor quality of lime.

Composition of Limestone Contd..

Types of Limei. Fat Lime

Also known as high calcium lime, pure lime, rich lime or

white lime.

It is known as fat lime as it slakes vigorously and it volume

is increased to about 2 to 2.5 times the volume of quick lime.

It is prepared by calcination of comparatively pure lime.

Properties of Fat Lime

It hardens very slowly.

It has high degree of plasticity.

It is pure white in colour.

It slakes vigorously.

It sets very slowly in the presence of air.

Uses

It is used in white washing and plastering of walls.

Lime mortar, made of lime and sand, is used for thin

joints of brick work and stone work.Lime mortar, made of lime and surkhi may be used for

thick masonry walls , foundation etc.

Fig: Expansion JointFig: Construction Joint

Fig: Contraction Joint

Fig,: Lime pointing

ii. Hydraulic LimeAlso known as the water lime as it sets under water.

Contains 5-30 % of clay.

Depending upon the percentage of clay, the hydraulic lime is

classified as:

a) Eminently hydraulic lime

b) Moderately hydraulic lime

c) Feebly hydraulic lime

iii. Poor LimeAlso known as impure or lean lime.It contains more than 30% of clay.It has poor binding properties and its colour is

muddy white.It sets or hardens very slowly.UsesIt can be used for inferior type of work or at place where good lime is not available.

CEMENT

Introduction Cement is a well known binding material which has cohesive

and adhesive properties which make it capable of bonding

different mineral fragments together and form the compacted

assembly.

It is obtained by burning mixture of calcareous and

argillaceous raw materials together at a high temperature of

about 1300-1500 oC

Source: Rajput, 2010

Properties of Cement Provides strength to masonry.

Stiffens or hardens very quickly.

Possesses good plasticity.

Has excellent binding property.

Easily workable

Provides good resistance to moisture.

Uses of CementUsed in cement mortar for masonry work, plastering, pointing,

etc.

Used in concrete for laying floors, roofs and constructing lintels,

beams, columns, stairs, etc.

Used for making joints for pipes, drains, etc.

Used for manufacturing precast pipes, piles, fencing posts, etc.

Used in the construction of important engineering

structures such as bridges, culverts, dams tunnels, etc.

It is used for the construction of wells, water taknks,

lamp posts, roads, etc.

It is used for the preparation of foundations,

footpaths, etc.

Fig.: Precast Pipes Fig.: Pile Foundation

Fig.: Fencing Posts

Fig Slab Culvert

Composition of Cementi. Lime (CaO) Constitutes 63% by weight. It is the major constituent of cement. Lime in right proportion makes the cement sound and

strong. Lime in excess makes the cement unsound and causes

the cement to expand and disintegrate. If the lime is in deficiency, the strength of cement is

reduced.

ii. Silica (SiO2)

Constitutes 22% by weight.

It imparts strength to the cement due to the formation

of di-calcium and tri-calcium silicate.

Excess of silica provides strength to the cement but

increases the setting time.

iii. Alumina (Al2O3)

Constitutes 6% by weight.

It imparts quick setting quality to the cement.

It acts as flux and lowers the clinkering temperatures.

Alumina in excess reduces the strength of cement.

iv. Calcium Sulphate( CaSO4)Present in the form of gypsum.It helps in increasing the initial setting time of cement.

V. Iron Oxide ( Fe2O3)

It provides colour, hardness and strength to cement.

It also helps in fusion of raw materials during

manufacture of cement.

Gypsum Iron oxide

vi. Magnesium OxideIf present in small amount, imparts hardness and colour

to the cement.If present in excess, makes the cement unsound.

vii. Sulphur TrioxideIf present in small quantity makes the cement sound.

vii. Alkalis

They should be present in small quantities.

Alkalis in excess cause efflorescence.

Efflorescence

Harmful Constituents of Cement

i. Alkali Oxides (K2O and Na2O)

If present more than 1%, failure of concrete occurs.

ii. Magnesium Oxide

If present more than 5%, it causes cracks after mortar or

concrete hardness.

Manufacture of Cement

Cement can be manufactured by following two methods:

A. Dry Process ( Modern Technology)

B. Wet Process (Old Technology)

Dry Process

Step 1: Treatment of raw materialsA. Crushing of Raw Materials

The raw materials are crushed into small fragments of

size that vary between 6mm to 25 mm.

The machines called crushers are used for this process.

The crushed materials are dried in drying kiln.

Fig: Crusher

B. Grinding

Grinding is done in two steps.

First one is achieved by using ball mills.

Second is called finest grinding, which is achieved using

tube mills.

Fine powder of raw materials is obtained known as raw

mill.

Fig: Ball Mill Layout Fig: Tube Mill Layout

C. Mixing of Raw Meals

Predetermined proportions of finely ground raw meals

from two individual silos are then mixed thoroughly

and made ready to feed to rotary kiln.

Step 2: Formation of Clinker

Rotary kiln may be 30m to 200m in length, 2.5m to

8m in diameter and have rotation 60 to 180 revolutions

per hour.

It is laid at gradient of about 1 in 25 to 1 in 30.

Rotary Kiln

Finely pulverized coal, fuel oril and natural gas are commonly used as fuels in rotary kilns.

It is fired from the lower end of the rotary kiln.The raw mix is burnt in the kiln at a temperature of

approximately 1300 oc -1900 ocWhen the material sinters and turns into balls, it is known as

clinker.The size of nodular shaped clinkers varies from 3mm to 25mm.The clinker is then dropped to clinker coolers.

Step 3: Grinding of Clinker and Gypsum

Mixture of cool clinker and 3-5% of gypsum by weight is ground to an extremely fine powder by grinding in cement mills.

It is done in two stages: preliminary grinding using ball mills and fine grinding by tube mills.

The resulting product is known as Portland Cement.It is then stored in specially designed storage tanks called

cement silos.

Fig: Cement Silos

Wet Process

Composition of Cement Clinker

i. Tri-calcium Silicate

It provides very good binding quality.

It hydrates more rapidly and develops strength in concrete

for the first 28 days.

It generates heat more rapidly as compared to other

compunds.

It offers less resistance to chemical attacks.

ii. Di-calcium Silicate

It hydrates slowly.

It hardens very slowly. This compound may begin

hardening after 28 days.

It imparts ultimate strength to the cement.

It offers more resistance to chemical attacks.

iii. Tricalcium Aluminate

It reacts very fast with a lot of heat evolution during the

reaction.

This is the first compound to react with water and

causes initial setting of cement.

It does not or contributes little strength to cement.

iv. Tetra calcium aluminoferrite

It is least important compound.

It has poor cementing value.

It reacts slowly during setting and generates small

amount of heat.

Types of CementRefer Class Notes

Reaction between Lime and WaterRefer class notes.

Reaction between cement and waterRefer class notes.

For Listening..Of Course..!!!