Knowledge About Cement And Its Production Procedure Biology Essay

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After the appearance of cement in 19th century, it took the place of other construction materials like lime and gypsum (1). Although cement is widely used nowadays in almost all of the buildings and constructions, still most of the people do not have knowledge about the cement and its production procedure.

Cement is a soft powder type substance normally in grey or white color which hardens in presence of water and widely used to hold other substances like concrete together. The most common form of the cement produced in industries is Portland cement.

Portland cement production has 4 main steps of quarrying, raw material preparation, clinkering and cement milling.

Introduction:

The main material that is used to keep the concrete together is cement. Cement starts hardening and strengthening when it is combined with water after some processInvalid source specified.. There are many different types of cement; the most common type of cement which is used in construction is Portland cement. Portland cement can be categorised in eight different types according to its use, but generally it can be divided in two main groups of natural and artificial (Portland) cement. Indeed, natural cement is the precursor of Portland cement. Portland cement was first produced in 1824 by Aspdin (Cement Company). Since it was harder than natural cement, it took the place of natural cement. The eight types of Portland cement are shown in the table below: (Popovics, 1992, p. 14)

Type

Name

Purpose

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I

Normal

General cement for most purposes

IA

Normal-air entering

Air entering modification of type I

II

Moderate sulphate resistance

Used to prevent moderate sulphate attack. Generates less heat than type I

IIA

Moderate sulphate resistance-air entering

Air entering modification of type II

III

High early strength

Used when high early strength is needed.

IIIA

High early strength-air entering

Air entering modification of type III

IV

Low heat of hydration

Used when hydration heat must be minimized in large volumes.

V

High sulphate resistance

Used to prevent severe sulphate action where soils and ground water have high sulphate content.

Table

One type of Portland cement is missing in the above table which is white Portland cement; the major difference is the colour (Cement Company, p. 1). The colour difference is due to the raw material that is used in the production and in commonly used for decoration and where cement can be seen.

Alternatively, natural cement can be divided in two main groups as:

Rapid natural cement(RNC): it consist of low clay content marl and is produced in temperature between 1000-1200 Ëš C for 12-20 hr. Also, it set so fast in less than 30 minutes. (1)

Slow natural cement(SNC): it consits of high clay content marl, it is produced in 8-12 hr at 800-1000 ËšC. But it sets from 30 minutes to 12 hr.(1)

2 Composition:

Oxide composition:

There are four major compounds used to manufacture Portland cement; lime (CaO), silica (SiO2), alumina (Al2O3) and iron oxide (Fe2O3) in which made up to 95% of the Portland cement. however, these four main compounds can made 4 different chemical forms which is shown in table below:

Compound

Abbreviation

Chemical formula

Typical concentration%

Tricalcium silicate

C3S

3CaOâ-SiO2

60-70

Dicalcium silicate

C2S

2CaOâ-SiO2

10-20

Tricalcium aluminate

C3A

3CaOâ-AL2O3

5-10

Tetracalcium alumino-ferrate

C4AF

4CaOâ-AL2O3â-Fe2O3

3-8

Table-1- (2)

But the amount of these materials varies due to the type of Portland cement. (Popovics, 1992, p. 14). But as said, these material made up to 95% of the portland cement and the other 5% is made from impuritis such as magnesia, sodium, potassium oxide, titania, phosphorus and manganese oxide. (Popovics, 1992, p. 14).

2.2 major phase of composition:

Composition of Portland cements consist of major phases that contains Tricalcium silicate (3CaO.SiO2), dicalcum silicate (2CaO.SiO2), and tricalcium aluminate (3CaO.Al2O3). in the ferrite phase, the average composition of 4CaO.Al2O3.Fe2O3 has existed. (V.S.Ramachandram, 1995, p. 2) The typical composition of different types of cement can be found in Table below.

Cement

Type of data

Compound composition (%)

Number of Samples

C3S

C2S

C3A

C4AF

CaSO4

Free

CaO

MgO

Ignition Loss

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Type I

Max.

Min.

Mean.

67

42

49

31

8

25

14

5

12

12

6

8

3.4

2.6

2.9

1.5

0.0

0.8

3.8

0.7

2.4

2.3

0.6

1.2

21

Type II

Max.

Min.

Mean.

55

37

46

39

19

29

8

4

6

16

6

12

3.4

2.1

2.8

1.8

0.1

0.6

4.4

1.5

3.0

2.0

0.5

1.0

28

Type III

Max.

Min.

Mean.

70

34

56

38

0

15

17

7

12

10

6

8

4.6

2.2

3.9

4.2

0.1

1.3

4.8

1.0

2.6

2.7

1.1

1.9

5

Type IV

Max.

Min.

Mean.

44

21

30

57

34

46

7

3

5

18

6

13

3.5

2.6

2.9

0.9

0.0

0.3

4.1

1.0

2.7

1.9

0.6

1.0

16

Type V

Max.

Min.

Mean.

54

35

43

49

24

36

5

1

4

15

6

12

3.9

2.4

2.7

0.6

0.1

0.4

2.3

0.7

1.6

1.2

0.8

1.0

22

Table (Popovics, 1992, p. 28)

3 manufacturing:

The way cement is produced can vary, therefore, cement produced by solidification/stabilization technology can be called Portland cement. This process can be done in organic or inorganic way, except inorganic wat is used for producing Portland cement. (Abdel M.O. Mohamed & Hogan E. Antia, 1998, p. 529). The production of portalnd cement can be divided in to four mani steps that is shown rafly in the figure below:

Figure (Deborah N. Huntzinger, 2009, p. 669)

3.1 quaryring

In the first step the raw materials for production of cement, which can be found in natutral rock mixture or limestone is used that make 80% of the cement, also the other 20% is made from clay or shale. The presence of lime and silica results in strength of cement and availability of iron reduces the reaction temperature and also gives grey color to the product (2) (Nzic).

3.2 preparations of raw materials:

After quarrying, second step is to prepare the raw material mixture or kiln feed for the pyro-processing operation. (Epa, pp. 11.6-4) . This process split in to five dissimilar methods: 1. Wet process, 2.dry process, 3.semidry process, dry process with preheater, and dry process with preheater/precalciner. (Epa, pp. 11.6-4) .

The two most common processes are explained in details below:

3.2.1 Dry process:

In this process clay and lime stone are crushed separately in to small size sample ant then the sample is controlled in the laboratory for mineral analysis and to ensure the presence of raw materials. After all the mixture of rocks are fed into the mill to ground the rock until 85% of rocks are less than 90µm in diameter. (Nzic, p. 4).

3.2.2. Wet process:

In wet process, clay is mixed to paste in tank and pulverised in the presence of water and is known as wash mill. In the next step the crushed limestone is added to this mixture. Then both materials with big size are grounded together. In the end, the slurry is checked in laboratory by technicians for its composition.(2) (Nzic, p. 5) .

3.3 clinkering:

Clinkering is the most significant process in the production of cement according to pyro-processing which happens in clinkering. In this step, all the materials is dried, heated, and cooled. Since the product from dry process does not have excess amount of moisture, it can be dried in pre-heater tower. The moisture in the cement will be heated from 70 to 800 ËšC for around 30 seconds and the moisture will evaporate up to 20% of decarbonation and initial reaction appear. Following this, the powder is ready to be fed into the kiln. (Nzic, p. 5) . all the above explanation is governed when the material is prepared with dry process method.

However, if the preparation was done in wet process the scenario is different. This can took place in two ways. Either the slurry from wet process can be fed directly in the kiln and converts into dry balls by the heat and rotation of the kiln. or else, the slurry from wet process can be preheated for drying. After drying the slurry are fed to the kiln. In general the kiln that is used for the wet process is longer than the kiln that is used for the dry process. (Nzic, p. 5).

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Moreover, the mixture of raw materials are sent to the kiln, kiln is machine that has rotating cylinder with furnaces, and material moved along the rotary cylinder and heated slowly with the furnaces. After the heating in high temperature, the material changed to either cementitious or hydraulic. Kilns can have ratio of length to diameter according to the type of process; we can have these ratios from 15:1 to 40:1. As explained before, in wet process the kiln can have length more than 210 meters long, but kiln from dry process is shorter. (Epa, pp. 11.6-4) .

After heating the kiln, big coolers that uses air are supplied to decrease the temperature of the mixture from approximately 1000ËšC to 150ËšC. (Nzic, p. 6) . Therefore, the new mixture is made and is called clinker by means of all the Portland cement characteristics.

3.3.1 reactions in kiln

When kiln is forming chemical reactions are taking place. These chemical reactions vary according to the temperature of the process. According to the temperature these reactions can be divided in to four major groups.

These reactions are listed below and are categorized in simple zones:

Zone 1: 0-35 min, 800-1100 OC

Decarbonation and melting of fluxing compounds AL2O3 and Fe2O3 happens at temperature above 900OC.

CaCO3 + heat CaO + CO2 (2)

Zone 2: 35-40 min, 1100-1300 OC

Formation of secondary silicate phase:

2CaO + SiO2 2CaO â-SiO2 + heat (2)

Zone 3: 40-50 min, 1300-1450-1300 OC

Sintering reaction

2CaO â-SiO2 + CaO 3CaO â-SiO2 + heat + time (2)

3CaO â-AL2 O3 +CaO + Fe2O3 4CaO â-AL2 O3 â- Fe2O3 (2)

Zone 4: 50-60 min, 1300-1000 OC

3.4 cement milling:

After all, milling is the last stage of the production of the Portland cement. In this part clinker is sent to the rotating large tube mill and it would be mixed with gypsum, also up to 5% of natural anhydrites are added to the mixture during the grinding to control the setting time of finished cement (Nzic, p. 6). This works as set retarder and ground for 30 minutes in the tube mill. The amount of grinding has direct relationship with volume of the cement, thus more volume results in coarser the grind will be. Clinker enters the mill from one side and Portland cement goes out from another side simultaneously. As the process takes place the temperature of mill and materials increases, so water is supplied to cool down both the materials and the mill.

4 technical challenges:

Production of cement has wide range of use in construction society. But production of this compound involves emission of unwanted products that should be controlled very carefully. Clinkerization is very complex part in the chemical processes, XRD and IR techniques are used to understand the thermal analysis of clinkerization.

The primary emissions in manufacture of Portland cement are particulate matter (PM), nitrogen oxide (NOx), sulfur dioxide (SO2), carbon monoxide (CO) and carbon dioxide (CO2) (4). Rates of reactions are influenced by different factors, such as temperature. Above 1000C, water starts to evaporate, while at > 5000C, combined water from calcium hydroxide is ejected. After that, C2S, CA and C2F start to form by the consumption of calcium carbonate and magnesium carbonate at > 8000C. C12A7 is formed between 800 and 9000C, and at 900-10000C, C3A and C4AF start to appear and calcium carbonate decomposes completely. Above this temperature until 14500C, all the materials are formed. (V.S.Ramachandran, 2002, p. 78). The largest source of emission of PM in cement manufacturing plant is pyroprocessing system which includes kiln, clinker and cooler exhaust stacks (4).

Nitrogen oxides are produced during the combustion of fuel. By increasing the temperature, the (NOx) emission increases (4).

Sulfur dioxide is produced from sulfur compounds in raw material and also from the sulfur in the fuel. Depending on location and plant, the sulfur content of both raw materials and fuel varies (4).

5. Product evaluation:

After the production of cement the produced cement must be tested before using it in the construction. There are different types of tests that can be used for cement. These tests can be divided into six main types. in addition, these test must be carried out to ASTM standards. Table below explains the test and the method of the tests:

Test

Definition of Test

Method of Test

Fineness

Fineness test finds the particle size of powders because the size of powders affects the availability of the area for water in the hydration process

1. By Turbidimeter

Soundness

Tests cement to find out its volume expansion after hardness. The sample must be put under the pressure of 2.03 MPa for 3 hours and then measured the length due to the time

1. Autoclave Expansion

Setting Time

Used to find how cement paste sets must be down. Initial set and final set must be calculated

1.Vicant Needles

2. Gillmore Needles

Strength

Tests compressive strength: is carried out on 2 inch sample and compressive load is put on it until failure. It must last between 20 and 80 seconds

1.ASTM C 109

2. ASTM C 349

Tensile strength: direct tension test

Flexural strength: is carried out on the 1.57(inch)* 1.57(inch)* 6.30(inch) sample and put in the centre load by beam

1.ASTM C 348

Specific Gravity

Normally used for mixture calculation

1.ASTM C 188

Heat of Hydration

Measures the heat generated when water and cement react and is influenced by cement-water ratio, fineness, and curing temperature

1.ASTM C 186

Chemical composition

Achieving composition of C3S+C2S >66.7% ; C/S>2.0 ; MgO<5.0% for clinker (3 p. 81).

BS EN 196

Taylor(1994)

Setting Time

The times after completion of mixing that cement paste shows resistances to the penetration of a needle by apparatus (3 p. 81)(figure 1 in appendix).

Vicant Needles(1828)

compressive Strength

The most important test of cement to determine the compressive strength that it produces in mortar or concrete. it depends on material used, the mix proportions, mixing procedure and mixing efficiency (3 p. 82).

ISO-RILEM R679

Mortar strength test

workability

A measure of the ease that a mortar or concrete can be place and compacted (3 p. 85).

ASTM C 143 (5)

Soundness

Cement is considered to be unsound if the hydration of paste is eventually with excessive expansion, causing cracking and strength reduction (3 p. 87).

Le Chatelier apparatus

(figure 2 in appendix)

Heat of hydration

Important test when cement is being supplied for use in large concrete structures (3 p. 89).

ASTM C186 - 05 (5)

Table (training.ce.washington, p. 1)

Furthermore, the quality of cement depends directly on factors that influence its rate of hydration and hydraulicity. So, as said before the cement must be tested before using it.

The standard test procedure of the UK is British standard BS 12 :1996. Table below shows requirements of BS 12 which specifies chemical, physical and performance requirements for Portland cement of British standard 12 (3 p. 80).

Property

Strength class

Requirement: % m/m cement *

Deviation limit â-

Loss of ignition

All

<3

+0.1

Insoluble residue

All

≤1.5

+0.1

Sulfate as SO3

All

≤3.5

+0.1

Chloride

All

≤0.1

+0.01

Initial set

Soundness

32.5N,R

42.5N,R

52.5N,R, 62.5N,R

all

≥60 min

≥45 min

≤10 mm

-15 min

+1mm

Table-3 -Requirement of BS 12:1996 (3 p. 80)

Class

Early strength

Standard strength

Deviation limit â-

2days

7days

28days

All classes

32.5N

32.5R

-

≥10

≥16

-

Lower

Upper

Lower

Upper

≥32.5

≤52.5

2(7)d 28d

-2.0-2.5

Not specified

42.5N

42.5R

≥10

≥20

-

-

≥42.5

≤62.5

52.5N

62.5R

≥20

≥20

-

-

≥52.5

≥62.5

≤72.5

-

Table-4 -Compressive strength/Nmm-2 (3 p. 80)

* Cement defined as clinker+ gypsum+ grinding aid (If any)

â- Maximum deviation permitted for individual result

R: high early strength sub class

6. Conclusion

To conclude, Portland cement is the main material used in construction projects. That is the main reason it has become one of the most important industries in recent decades. On the other hand, this process should be considered one of the main producers of carbon dioxide and is therefore one of the leading factors of global warming.

This process can be classified as a high investment process but with high pay back in the long term and is advantageous enough to become a main choice of investment for companies.

It is necessary to make this process as environmentally-friendly as possible in the future. The transition from fossil fuels to other fuels that can be used as energy sources for this process can make it more environmentally-friendly. This is one of the main concerns of this process because it is highly pollutant.