Cement

 Cement

19.1 Introduction                                               

Ø  one of the most important building materials at present

Ø  second most used resource in the world after petroleum

Ø  Binder, a substance used for construction that sets, hardens and adheres to other materials to bind them together.

Ø  Generally used in building and civil engineering.

Ø  Never used alone but with sand and gravel called concrete and with sand, gravel and iron or steel rods called RCC- reinforced cement concrete.

Ø  Finely ground soft powder, when mixed with water, set to a hard mass.

Ø  Setting and hydration result from hydration, which is a chemical combination of the cement compounds with water that yield sub-microscopic crystals or gel-like material.

Ø  Because of their hydrating properties, constructional cement, which will set and harden under water is often called hydraulic cement.

Ø  Most important is Portland cement, named after a famous strong rock of England, where the cement was discovered around 1820.

Ø  Not a single composition or definite composition but a mixture of different compounds.

Ø  Essentially contains- CaO, SiO₂, Al₂O₃, Fe₂O₃, SO₃ etc. does not have specific formula

Ø  The approximate composition of cement is

CaO = 50-60%

SiO₂ = 20-25%

Al₂O₃ = 5-10%

MgO = 1-2%

Fe₂O₃ = 1-2%

SO₃ = 1-2 %

 

19.2 Raw materials for cement production

Ø  Calcined lime and special clay are the major ingredients.

1.       Calcareous materials: compounds of calcium and magnesium, limestone(CaCO₃) being the main.

2.       Argillaceous materials: mainly provide silica, alumina and iron oxide: commonly clay or shale

Ø  Limestone provides lime, chalk, marble and alkali, metal waste can also be used.

Ø  The raw materials are obtained by blasting rock by boring the rock and setting off explosives or simply by digging with an excavator.

Ø  Coal is also required for production mainly as a fuel for heating

Ø  Gypsum is another ingredient of cement added later in a small amount (2-3%).

Ø  Other supplementary materials include sand and fly ash to achieve the desired composition.

The most important raw materials of cement are extracted from natural sources but cement is not some naturally occurring material; rather it is manufactured through chemical processes from many different ingredients in the production process.

These ingredients are generally extracted from limestone, chalk, shells, shale or calcareous rock

19.3 Main steps in cement production

There are four main stages in the manufacture of Portland cement:

i)                    crushing and grinding the raw materials

ii)                  blending the materials in the correct proportions

iii)               burning the prepared mixture in a kiln,

iv)                grinding the burnt product, known as 'clinker' together with some 5% of gypsum

The first step involves the proper selection of raw materials.

Then the raw materials are properly treated.

Crushing and grinding of raw materials

Ø  following two methods is employed for this purpose

a. wet process and b. dry process

Ø  The choice between the two depends upon the nature of raw materials and the climate conditions.

In the wet process,

·         The clay is washed with water in wash mills to remove foreign materials like flint.

·         The powdered limestone is then mixed with the clay paste in the proper proportions (limestone 75%, clay 25%)

·         The mixture is finely ground and made homogeneous employing a compressed air mixing arrangement.

·         The resulting paste is known as slurry. It contains about 40% water.

In the dry process,

·         The raw materials are dried and mixed in the proper proportions and then powdered.

·         The mixture is then homogenized into a raw meal.

 

Strong heating - Calcination

Ø  Most important step

Ø  Slurry from the wet process or raw meal from the dry process is introduced with the help of a screw conveyer in a rotary kiln which is made from steel and lined inside with firebricks.

Ø  Consists of a tall cylinder of 6-10 feet in diameter and 100-250 ft. in height rotating one or half turn per minute.

Ø  The upper part is attached to the dust chamber

Ø  Here the contents are gradually heated to a temperature of 1400⁰C - 1600⁰C.

Final grinding

Ø  The clinker is finally ground with gypsum into a very fine powder called cement.

Ø  Gypsum regulates the setting time and gives compressive strength to the cement.

19.4 Types of cement- OPC and PPC

Ordinary Portland Cement (OPC) & Portland Pozzolana cement (PPC)

Portland cement is most common due to its abundance and low cost of production.

PPC is a variation of OPC,

PPC has low initial setting strength compared to OPC but hardens over a period of time with proper curing.

PPC	OPC
Pozzolana materials like fly ash, volcanic ash are added.	Pozzolana materials like fly ash, volcanic ash are not added.
Cheaper	Expensive
Low setting strength initially as compared to OPC but hardens over a period of time	Fast setting property
Generally used to make walls of houses	Generally used for fine plasters, dhalan and pillars

19.5 Portland cement manufacturing process

For the production of Portland cement, first the raw materials, limestone and clay are crushed separately in a suitable machine.

Clay is washed with water in a wash mill to remove the foreign material and then powdered lime is added

The two ingredients are finely ground and homogenized.

This is called charge and slowly moves due to the rotation of the kiln.

The charge takes a long time to travel from one end to the other.

Coal dust as fuel mixed with hot gases is blown from the other side with the help of a blower.

During this passage from one end to the other, the charge first loses all the water and CO₂.

By the time it reaches the other end of the kiln, the temperature is between 1400-1600⁰C.

A series of reactions take place between lime obtained from limestone and two main constituents of clay i.e. alumina and silica

2 CaO + SiO₂ → 2 CaO. SiO₂ (Dicalcium silicate)

2 CaO + SiO₂ → 3 CaO.SiO₂ (Tricalcium silicate)

3 CaO + Al₂O₃ → 3 CaO.Al₂O₃ (Tricalcium aluminate)

Similarly, magnesium oxide is changed into magnesium silicate.

These reactions are exothermic

Therefore temperature has to be controlled

When it comes out into the cooler where it is cooled by air, it appears as grey-coloured balls. The resulting product is known as cement clinker.

Clinker is mixed with 2- 3% of its weight of gypsum (CaSO₄.2H₂O) and ground into extremely fine powder in grinding machines.

The role of gypsum is to decrease the setting time of the cement.

Finally, the ready cement is packed in air-tight bags to avoid moisture.

19.6 Cement Industry in Nepal

The use of cement started in the 1950s in Nepal.

Only in 1967, was the first cement industry, Himal Cement Company Limited established

Its operation started in 1975 and was accounting for 18% of the total demand at that time.

It was shut down in 2002 after a large opposition by the local people concerning dust and air pollution.

Hetauda cement industry (established 1976) and Udayapur cement factory (1987) are two other cement industries currently run by the government of Nepal.

Udayapur cement factory was the largest cement factory earlier.  Sells cement under the brand name 'gaida'

After the economic liberalization policy since the 1990s, many private-sector cement industries have been established.

Cement industries are one of the leading industries of Nepal according to the economic survey 2019/20.

Main raw material- limestones and clay are locally mined and gypsum, coal and iron ores are imported

Domestic cement production has been growing steadily over the past decade and demand has also been rising.

Currently, around 55 cement factories are operating which produce more than 160 million sacs of cement annually.

Earlier most of the clinker used to be imported from India, but recently there has been a rapid expansion of domestic clinker production.

The Nepal government is imposing duties on clinker imports and cement producers have been encouraged to set up clinker production.

Currently, Hongsi- Shivm cement factory, Nawalparasi, is the largest cement factory under operation, which is a joint venture company with 30% local investment (Shivm holdings) and 70% Hong Kong red lion cement no. 3 a subsidiary of Chinese giant Hongsi cement company.

1.       Differentiate between OPC and PPC cement.                                                        2

2.       What is Portland cement? Name the major components of Portland cement. 1+2

3.       Define the term clinker.                                                                                              1

4.       Why is gypsum used in clinker during the cement production process?           1

5.       Give two instruments used for the quality control of cement.                             1

Answer:

1.       Compression testing machine- tests compressive strength of concrete

2.       Consistometer- tests thickening time

3.       X-ray fluorescence spectrometer

4.       Flame photometer

 

6.       Write down the importance of the cement industry in Nepal.                             3

7.       Cement is an important material for the construction of roads, buildings, bridges etc.

I)                    Give a difference between hydraulic and non-hydraulic cement.

II)                  List the compositions of Portland cement.

III)               What is the purpose of adding gypsum during cement production?

IV)               Name any two instruments to check the quality of cement.

V)                 Which is the first established state-owned cement industry in Nepal? 2+1+1+1+1

 

1.       Cement does not contain

a.       Calcium       c. Silicon                              d. Iron                   b. Sodium

2.       Which of the following is not an important constituent of cement?

a.       CaO                               c. NaO                  d. MgO                 b. Al₂O₃

3.       What effect does calcium sulphate have on cement?

a.       Retards setting action                             c. Acts as flux

b.       Imparts colour                           d. Reduces strength

 

*** This is not a complete note. It is just to guide you. It is recommended to study prescribed textbooks along with this material. ***