chapter 2 cem(selfdec09)

8/14/2019 Chapter 2 Cem(Selfdec09)

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CEMENT

- Cement which is an important building material was first of all

introduced in 1824 by Joseph Aspedin, a brick layer of Leeds, England.

- He called it Portland cement because, after hardening it

resembled in colour the stone quarried near Portland in England.

- Cement manufacturing was first introduced in Germany.

Definition :

- Portland cement is obtained by mixing a definite proportion of

lime, stone and clay, thereafter burning it at a high temp (to drive off

carbonic acid gas).

- The colour of Portland cement is Greenish Grey.


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CEMENT

Definition :

- Cement in a general sense are adhesive and cohesive materialswhich are capable of bonding together particles of solid matter into a

compact durable mass.

-For civil engg works , the y are restricted to calcareous cements

containing compounds of lime as their chief constituent, its primary

function being to bind the fine (sand) and coarse ( grits) aggregate

particles together.

-HYDRAULIC: SETS AND HARDEN IN WATER AND GIVE APRODUCT WHICH IS STABLE

-NON-HYDRAULIC: DOES NOT SET AND HARDEN IN WATER

SUCH AS NON HYDRAULIC LIME OR WHICH ARE UNSTABLE

IN WATER e.g. PLASTER OF PARIS


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S.No Chemical Composition Percentage

1. Lime (CaO) 60-65

2. Silica (SiO2) 17-25

3. Alumina (Al2O3) 3-8

4. Iron Oxide (Fe2O3) 0.5-6

5. Magnesium Oxide (MgO) 0.5-4

6. Sulphur trioxide (SO3) 1-2

7. Insoluble residue 0.7-1.9Total 100.0 %

Chemical composition of Cement


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Functions of Cement Ingredients

Function of raw materials needed for the cement manufacture are as

follows:

(1) Lime (CaO) : It is the chief material for cement manufacture.

It sets and harden on hydration. If present in excess, makes the cement

unsound.Its defi reduces strength & setting time.

(2) Silica (SiO2) : It forms di and tricalcium silicates of calcium,

which imparts strength to cement.Excess causes slow setting.

(3) Alumina (Al2O3) : It forms tricalcium aluminate which has the

property of imparting quick setting quality to cement. If present in

excess, causes greater volume change after shrinking and weakens the

cement.It lowers the strength.

(4) Magnesia (MgO) : It helps in giving colour to cement. When inexcess uantit it causes unsoundness to cement causes cracks.


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(5) SO3: Makes cement sound.

(6)Iron Oxide (SiO2) : It forms the mineral tricalcium alumino-ferrite

which has the property of imparting strength and colour to

cement.Helps in fusion of different elements.

(7)Residues: Na2o + K2O , TiO2 , P2O5 :These residues ifin excess cause efflorescence and cracking.


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Cement Hydration

The chemical reaction between cement and water is called hydration

of cement. The reaction takes place between the components of cement

and water. During hydration new compounds of hydrated calcium

aluminate, hydrated calcium silicate and calcium hydroxide are formed.

C3S + H2O Hydrated calcium silicate + Ca(OH)2

C2S + H2O Hydrated calcium silicate + Ca(OH)2

C3A + H2O Hydrated calcium aluminate + Ca(OH)2


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Manufacturing of cement

Wet Process Dry Process

Wet Process :

The wet process is mostly used for manufacturing as the quality

of cement obtained in this process is much betterand the various

ingredients are thoroughly mixed.


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Manufacturing of Portland Cement

calcareous mtrl (cement rock, lime stone , chalk ,marine shell)

argillaceous mtrl ( silicates of alumina in the form of clay , shale and blast furnace

slag)

Flow diagram of Cement Manufacture

Calcareous Material Argillaceous Material Water

Crushing Elevators Washmills

Clay Storage

Proportioning

Wet grinding

Ball and Tube mill

Raw slurry

Slurry Blending Soil [Lime slurry, Clay slurry]


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Slurry Blending Soil

Corrected slurry storage

Rotary kiln Powdered coal

Clinker

Clinker storage

Clinker Grinding Gypsum(Ball and Tube mill)

Elevator, Cement silos

Weighing, packing and Dispatch


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Wet Process

(i) Collection of raw material

From the nearby quarry, calcareous material is quarried by blasting.

Generally lime stone, marl, chalk, or shells are used as calcareous

materials.

Clayey materials include clay, shale and blasting furnace slag.

These materials are transported to the site by means of dumping trucks

or wire rope ways.


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Wet Process

(ii) Crushing grinding and mixing

In case the raw materials are not found in a finely divided form these are

crushed to pebble size separately.

Crushed calcareous material is stored in large bins, by means of elevators.

Clayey material is taken to wash mills, which are thick steel cylinders, nearly

3 meters in diameter and 10 meters in height.

Clay is mixed with 30 to 50% of water in mills, where it disintegrates to form a

thin fluid known as Clay Slurry. By means of pump, this slurry is stored in

vessels known as Clay slurry storage basin.

Lime stone, broken to the size of pebbles, is mixed with clay slurry

according to the proportion required, in the proportioning equipment.


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After proportioning wet mixture containing calcareous and argillaceous

material, it is passed through Ball and Tube mills respectively, to form thin paste

of uniform consistency.

This is known as cement slurry and is stored in big tanks known as silos orbasins.

In these basins slurry is tested by an experienced chemist, and if needed, more

calcareous or argillaceous material is mixed in the form of slurry to correct its

chemical composition.

These basin are also known as correcting Silos, and from these silos

corrected cement slurry is taken to storage basins, where it is constantly stirred.


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(iii) Burning

From the storage basins slurry is pumped into the upper end of rotary kiln in a

regulated stream through a kiln feeder.

The rotary kiln consists of a cylinder made from steel plates 2 to 4 cms thick.

It is 60 to 90 metres in length, 3 to 4 metres in diameter and is given an inclination of

1 in 20 along its length.

Inner surface of the kiln is lined with fire bricks and it is mounted on roller bearings

which are supported by masonry, concrete or steel columns.

The kiln is rotated at a speed of about one revolution per minute.

Through the upper end of the kiln slurry is fed at a uniform rate and through the lower

end powdered coal is injected by the blast of air.

Due to the burning of coal, temperature near the lower end is higher while near the

upper end it is lower.

As the slurry reaches inside the kiln, where temperature is neary 2500C, moisture gets

evaporated and slurry changes into flakes.


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Due to gravity these flakes reach next zone known as calcination

zone. Here temperature is about 700-12000C and carbondioxide from

the calcareous material gets eliminated.

As the particles pass down further and reach nearer to lower end

where temperature is nearly 16000C, calcareous and argillaceous

materials combine together to form silicates, aluminates and ferrites

of calcium.

In this zone, known as clinkering zone, these compounds come out

in the form of vitrified glassy nodules, called clinker.

To cool the so formed clinker it is passed through small rotatingcylinders known as cooler.

Cooled clinker is stored in storage bins by means of elevators.


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(i ) G i di f Cli k


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(iv) Grinding of Clinker

The final step in the production of portland cement is to grind the

clinder, to a very fine powder in Ball and Tube mills.

Measured quantity of gypsum (CaSO4), nearly 3 per cent is also added

during the grinding of clinker, to regulate the setting of the finished

product.

In case of air-entraining portland cement, a small amount of a suitable

air-entraining agents (resins) are also added at this stage of the process.

Clinker is grinded in Ball mills first which consist of a steel cylinder,containing iron balls 5 to 10 cm in diameter.

Cylinder is kept I horizontal position and is rotated at the rate of 60 to

80 revolution per minute.


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Grinded material, obtained from ball mills is further grinded in tube

mill to a very fine powder.

Tube mill consists of a horizontal rotating cylinder 2 to 2.5 metre in

diameter and 8 to 10 metre in length, containing smaller steel balls 2

to 3 cm in size.

Fine product so obtained is cement and is stored in cylindrical bins

known as cement silos.

From these silos cement is taken out and automatically weighed and

packed in bags.

Each bag contains 50 kgs of cement and is dispatched. Portland

cement is shipped in bulk also.


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MANUFACTURING OF CEMENT BY DRY PROCESS

In dry process the raw mtrl (calcareous, argillaceous) are crushed

and are then dried in the rotary driers.

The dried raw materials are again regrinded to a fine powder in

the ball and tube mills.

The final grinded mtrl is separated by means of air separators andis stored in Silos, where its chemical composition is checked and if

needed, correction is done by adding calcareous or argillaceous material

in the required proportion.

The materials from the silos is fed into the rotary kilns through

the rotary feeder.

The remaining procedure carried out for burning on the rotary

kiln and grinding is the same as the wet process.


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Testing Portland Cement

To check the suitability of ordinary portland cement which is

commonly used, is tested. As per IS : 269-1967 the following tests are

performed:

1. Fineness Test

2. Consistency Test

3. Soundness Test

4. Compression Strength Test

5. Test for Tensile Strength

6. Test for Setting Time


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To check the suitability of ordinary portland cement which is commonly used, is

tested. As per IS : 269-1967 the following tests are performed:

1. Fineness Test : The object of this test is to check the proper grinding of cement.

If grinding is not uniform and fine, large quantity of water will be needed for mixing

and the chemical reactions set up during setting, will be delayed. Moreover, bleeding

will occur even before concrete begins to set, as water will come to the top due to

settlement of solids. Finer the cement, more surface area of the aggregates will be

covered by it.

(a) Sieve test : 100 gms of cement is continuously sifted for a period of 15

minutes on I.S. test sieve No. 9 and the residue by weight retained on sieve shall not

be greater than 10%.


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(b) Surface Area test : It is also known as Specific Surface

test and is generally adopted.

Specific surface is defined as the total surface area of all

the particles in one gram of cement, the particles being

considered as spheres.

It can be determined by Air Permeability method or by

Wagners Turbidimeter.

Specific surface of a cement should not be less than

2250sq cm/gm of cement if found by Air permeability method or 1600

sq cm/gm of cement as per Turbidimeter method.


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2. Consistency Test : The object of this test is to determine the percentage of

water required for obtaining the cement paste of normal consistency, used in other

tests. Following procedure is adopted for determining the same. About 300 gm of

cement is weighed and nearly 30% of water (90 gm) is added to it and mixed

thoroughly on a non-porous surface.


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Cement paste is filled in the mould of vicat apparatus. Time of gauging

(i.e., the interval from the moment of adding water to dry cement, till the

mould is filled with paste) should obtain less than 3 minutes and not greater

than 5 minutes. Plunger (needle 10 mm in diameter) is brought to the surfaceof the paste and released. If the settlement of plunger is between 33 to 35 mm

from the top surface of the mould water added is correct. If not, the process is

repeated by making cement pastes with different percentages of water till the

settlement is 33 to 35 mm from the top surface of the mould.

Let W = weight of cement taken.

w = weight of water added when the plunger has a penetration

of 33 to 35 mm from the top of the mould.

Percentage of water Pw. = w/W x 100

Percentage of water for normal consistency Pn = Pw.

So the percentage of water required for the cement paste of normal

consistency (C.P.N.C.) should be such as to give a paste which will permit the

settlement of vicat needle (10mm in dia.) by 33 to 35 mm.


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3. Soundness test : The object of this test is to determine the presence of

uncombined lime. It uncombined lime is present in cement, during the setting

action it might slake causing increase in volume and will cause the

development of cracks. In structure these cracks are objectionable as they

deteriorate the structure.

Cement which undergoes large changes in volume is known to be

unsound, soundness of a cement is judged by Lachateliers apparatus. It

consist of a small split cylinder of brass 0.5 mm thick, 30 mm internal diameter

and 30 mm in height. On either side of the split are attached two indicatorswith pointed ends. Lengths of these indicators upto the centre of the cylinder is

165 mm. A cement paste made by adding 0.78 Pn percentage of water is

prepared with 100 gm of cement. Lechateliers apparatus is placed on a glass

plate and the cement paste is filled upto the top. It is covered with another

glass plate placed at top and the whole combination is immediately immersed

in water for 24 hours, to avoid shrinkage during setting. It is then taken out

and the distance between the tips is measured. The cylinder is immersed in

water,water is brought to boiling point in 30 minutes and kept boiling for 1

hour.


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Cylinder is removed and allowed to cool. Again the distance

between the tips, is measured, which should not exceed 10 mm.

For cements in which the slaking of free lime takes place by

aerating the cement for 7 days at a humidity of 50 to 80 percent

the expansion should not be greater than 5 mm.


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4. Setting Time :

The object of test is to distinguish between quick setting and

normal setting cements and to check the initial setting time

which should be sufficient for the various operations.

For example, mixing transporting, placing and compaction of

cement concrete. Cement is to be checked for Initial and

final setting time, the test is performed by means of VicatApparatus.


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To perform the test, 300 gm of cement are taken and clean water

0.85 times Pn is added to form a paste.

The paste is filled in the mould of vicat apparatus placed on a glass

plate. Needle of 1 mm x 1 mm in cross section is attached to the rod of

vicat apparatus is released gently so as to pierce into the paste.

Needle is taken out and same process is repeated after every minute,

till it ceases to pierce fully, but remains 5 mm above the glass plate. At

this stage note the time, known as initial setting time. Initial setting

time should not be less than 30 minutes.

i l i i f i f d b h i h


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Final setting time of cement is found by changing the

needle. It has a sharp point projecting in the centre with an

annular attachment.

This needle is released gently on the paste as prepared

before.

A stage will reach when the needle makes an impression,

but not the attachment. Time is noted, which gives the final

setting time. Final setting time of ordinary cement should

not be greater than 10 hours.


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4 Compression Test :


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4. Compression Test :

This test is performed to determine suitability of cement for

developing the required compressive strength of concrete and mortar.

Cement and standard sand in the ratio of 1:3 are used.

Six cubes of sides 70.6mm are prepared for this test. The

materials are mixed each time and cubes made separately.

Moulds containing cement mortar are placed in damp place for24hrs. Cubes are then taken out and kept immersed in water.

Cubes are then tested with 03 cubes at the end of 3 days and 03

cubes at the end of 7 days.

Rate of loading during test should be at the rate of 350 kg/cm2.

As per IS specifications, Compressive strength should not be less

than 115 kg/cm2 and 175 kg/cm2 after 3 and 7 days respectively.

5 T t f T il St th


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5. Test for Tensile Strength :

This test is performed by mixing water to the mixture of cement

and sand in the ratio of 1:3, to form a paste.

Six moulds are made in the form of standard briquettes form this

paste.

When mould is wet, it is beaten with standard spatula till water

just appears on the surface. This is done to avoid air voids.

These six moulds are kept in moist atmosphere (80-90%

humidity) at a temp of 27+/- 20 C for 24 hrs.

These are then immersed in water at a temp of 27+/- 40C till

testing is done by Briquette Tensile Testing machine.

Average strength of briquettes at the end of 3 and 7 days should

not be less than 20 k /cm2 and 25 k /cm2 res ectivel .


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6. Setting Time :

The object of this test is to distinguish between quick setting and

normal setting cements and to check the initial setting time which should

be sufficient for the various operations viz mixing, transporting etc.

Cement is to be checked for Initial and final setting time, the test

is performed by means of Vicat Apparatus.

To perform the test, 300 gm of cement are taken and clean water

0.85 times Pn is added to form a paste.

The paste is filled in the mould of vicat apparatus placed on a

glass plate. Needle of 1 mm x 1 mm cross section attached to the rod of

vicat apparatus is released gently, so as to pierce into the paste.

dl i k d i d f


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Needle is taken out and same process is repeated after every

minute, till it ceases to pierce fully, but remains 5 mm above the glass

plate. At this stage note the time. This is known as initial setting time

and should not be less than 30 minutes.

Final setting time of cement is found by changing the needle. It

has a sharp point projecting in the centre with an annular attachment.

This needle is released gently on the paste as prepared before.

A stage will reach when the needle makes an impression, but not

the attachment. Time is noted, which gives the final setting time. Final

setting time of ordinary cement should not be greater than 10 hours

TYPES OF CEMENT


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TYPES OF CEMENT

1. Portland Cement

(a) Ordinary Portland Cement

(b) Rapid Hardening Cement

(c) Low Heat Cement

(d) Blast Furnace Slag Cement

(e) Sulphate Resistant Cement

(f) Air Entraining Cement

(g) White and Coloured Cement

2. Other varieties of Cement

(a) High Alumina Cement

(b) Pozzolanic Cement

chapter 2 cem(selfdec09)

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