Lecture5 Cement 140309165129 Phpapp01

Topic 5: CEMENT

A short series of lectures prepared for the Third year of

Geology, Tanta University

2013- 2014

by

Hassan Z. Harraz

[email protected]

13 March 2014 Prof. Dr. H.Z. Harraz Presentation Cement

http://en.wikipedia.org/wiki/File:06_Contes_cimenterie.jpg

OUTLINE OF TOPIC 5: CEMENT

TYPES OF CEMENTS

PORTLAND CEMENT

TYPES OF PORTLAND CEMENT

GENERAL FEATURES OF THE MAIN TYPES OF PORTLAND CEMENT

ORDINARY PORTLAND CEMENT (OPC)

RAPID HARDENING PORTLAND CEMENT

SPECIAL TYPES OF RAPID HARDENING PORTLAND CEMENT

MANUFACTURE OF PORTLAND CEMENT:-

1) Raw Materials

2) Crushing & Grinding of Raw Materials

3)Type of cement processes:

a) Wet Process

b) B) Dry process

4) Burning Process

5) Grinding

6) storage, packing, dispatch

CEMENT CHEMISTRY

Chemical Compositions

Bogue’s Equations

Fineness of cement

13 March 2014 Prof. Dr. H.Z. Harraz Presentation Cement 2

CEMENT

DEFINATION:

Cement is the mixture of calcareous, siliceous, argillaceous and other substances.

Cement is a hydraulic binder and is defined as a finely ground inorganic material which, when mixed with water, forms a paste which sets and hardens by means of hydration reactions and processes which, after hardening retains it's strength and stability even under water.


Cement used in construction is characterized as hydraulic or non-hydraulic.

1) Hydraulic cements (e.g., Portland cement) harden because of hydration, chemical reactions that occur independently of the mixture's water content; they can harden even underwater or when constantly exposed to wet weather. The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water-soluble.

2) Non-hydraulic cements (e.g. Gypsum plaster) must be kept dry in order to retain their strength.

TYPES OF CEMENTS:

1)Portland cements

2)Natural cements

3)Expansive cements

4)High-alumina cements

http://en.wikipedia.org/wiki/Mineral_hydration

http://en.wikipedia.org/wiki/Anhydrous

PORTLAND CEMENT Portland Cement

is a hydraulic cement that hardens in water to form a water-resistant compound.

The hydration products act as binder to hold the aggregates together to form concrete.

A hydraulic cement made by finely pulverizing the clinker produced by calcining to incipient fusion a mixture of argillaceous and calcareous materials

Portland cement is the fine gray powder that is the active ingredient in concrete

Limestone + Shale/Clay + Heat = Clinker +CKD + Exit Gas

Material Temperatures Exceed 2700 oF

Pulverized Clinker + Gypsum = Portland Cement

Cement is powder so fine that one pound contains 150 billion grains

The name Portland Cement comes from the fact that the colour and quality of the resulting concrete are similar to Portland stone, a kind of limestone found in England.


TYPES OF PORTLAND CEMENT

According to the ASTM standard, there are five basic types of Portland Cement:.

Regular cement, general use, called Ordinary Portland cement (OPC) – Type Ι

Moderate sulphate resistance, moderate heat of hydration, C3A < 7% , Modified cement -

Type ΙΙ

Rapid-hardening Portland cement , With increased amount of C3S, High early strength – Type

ΙΙΙ

Low heat Portland cement – Type ΙV

High Sulfate-resisting Portland cement – Type V

It is possible to add some additive to Portland cement to produce the following types:

Portland blastfurnace cement – Type ΙS

Pozzolanic cement - Type ΙP

Air-entrained cement - Type ΙA

White Portland Cement (WPC)

Colored Portland Cement

Note:

sulphates can react with C4ASH18 to from an expansive product. By reducing the C3A content, there will be less C4ASH18 formed in the hardened paste.

Physically and chemically, these cement types differ primarily in their content of C3A and in their fineness.

In terms of performance, they differ primarily in the rate of early hydration and in their ability to resist sulfate attack.

• The general characteristics of these types are listed in Table 2.

• The oxide and mineral compositions of a typical Type I Portland cement were given in Tables. 13 March 2014 Prof. Dr. H.Z. Harraz Presentation Cement 5

GENERAL FEATURES OF THE MAIN TYPES OF PORTLAND CEMENT

ASTM Type Classification Characteristics Applications

Type I General purpose Fairly high C3S content for good early strength development

General construction (most buildings, bridges, pavements, precast units, etc)

Type II Moderate sulfate resistance (Modified cement)

Low C3A content (<8%) Structures exposed to soil or water containing sulfate ions

Type III High early strength (Rapid-hardening)

Ground more finely, may have slightly more C3S

Rapid construction, cold weather concreting

Type IV Low heat of hydration (slow reacting)

Low content of C3S (<50%) and C3A

Massive structures such as dams. Now rare.

Type V High sulfate resistance Very low C3A content (<5%) Structures exposed to high

levels of sulfate ions

White White color No C4AF, low MgO Decorative (otherwise has properties similar to Type I)


Chemical composition of

the main types of

Portland cement

ORDINARY PORTLAND CEMENT (OPC)

This type of cement use in constructions when there is no exposure to sulfates in the soil or groundwater.

The chemical composition requirements are listed as shown below:

Lime Saturation Factor (L.S.F.) =

{CaO-0.7(SO3)}/ {2.8(SiO2)+1.2(Al2 O3)+0.65(Fe2O3)}

L.S.F. is limited between 0.66-1.02

Where each term in brackets denotes the percentage by mass of cement composition.

This factor is limited – to assure that the lime in the raw materials, used in the cement manufacturing is not so high, so as it cause the presence of free lime after the occurrence of chemical equilibrium. While too low a L.S.F. would make the burning in the kiln difficult and the proportion of C3S in the clinker would be too low.

Free lime – cause the cement to be unsound.

Percentage of (Al2O3/Fe2O3) : is not less than 0.66

Insoluble residue: not more than 1.5%

Percentage of SO3 : limited by 2.5% when C3A≤7%, and not more than 3% when C3A>7%

Loss of ignition L.O.I. : 4% (max.)

Percentage of MgO : 5% (max.)

Fineness : not less than 2250 cm2/gm

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Prof. Dr. H.Z. Harraz Presentation Cement 7

Typical compound composition of Ordinary Portland Cement (OPC)

Chemical Name Oxide Formula Cement

Notation

Mineral

Name

Weight

(%)

Tricalcium Silicate 3CaO.SiO2 C3S Alite 50

Dicalcium Silicate 2CaO.SiO2 C2S Belite 25

Tricalcium Aluminate 3CaO.Al2O3 C3A Aluminate 12

Tetracalcium Aluminoferrite 4CaO.Al2O3.Fe2O3 C4AF Ferrite 8

Calcium sulfate dihydrate CaO.SO3.2H2O CSH2 Gypsum 3.5

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The differences between these cement types are rather subtle. All five types contain about 75 wt% calcium silicate minerals, and the properties of mature concretes made with all five are quite similar. Thus these five types are often described by the term “Ordinary Portland Cement”, or OPC.

Types II and V OPC are designed to be resistant to sulfate attack. Sulfate attack is an important phenomenon that can cause severe damage to concrete structures. It is a chemical reaction between the hydration products of C3A and sulfate ions that enter the concrete from the outside environment. The products generated by this reaction have a larger volume than the reactants, and this creates stresses which force the concrete to expand and crack. Although hydration products of C4AF are similar to those of C3A, they are less vulnerable to expansion, so the designations for Type II and Type V cement focus on keeping the C3A content low. There is actually little difference between a Type I and Type II cement, and it is common to see cements meeting both designations labeled as “Type I/II”. The phenomenon of sulfate attack will be discussed in much more detail in Sections 5.3 and 12.3, but it should be noted here that the most effective way to prevent sulfate attack is to keep the sulfate ions from entering the concrete in the first place. This can be done by using mix designs that give a low permeability (mainly by keeping the w/c ratio low) and, if practical, by putting physical barriers such as sheets of plastic between the concrete and the soil.

Type III cement is designed to develop early strength more quickly than a Type I cement. This is useful for maintaining a rapid pace of construction, since it allows cast-in-place concrete to bear loads sooner and it reduces the time that precast concrete elements must remain in their forms. These advantages are particularly important in cold weather, which significantly reduces the rate of hydration (and thus strength gain) of all Portland cements. The downsides of rapid-reacting cements are a shorter period of workability, greater heat of hydration, and a slightly lower ultimate strength.

Type IV cement is designed to release heat more slowly than a Type I cement, meaning of course that it also gains strength more slowly. A slower rate of heat release limits the increase in the core temperature of a concrete element. The maximum temperature scales with the size of the structure, and Type III concrete was developed because of the problem of excessive temperature rise in the interior of very large concrete structures such as dams. Type IV cement is rarely used today, because similar properties can be obtained by using a blended cement.

White Portland cement (WPC) is made with raw ingredients that are low in iron and magnesium, the elements that give cement its grey color. These elements contribute essentially nothing to the properties of cement paste, so WPC actually has quite good properties. It tends to be significantly more expensive than OPC, however, so it is typically confined to architectural applications. WPC is sometimes used for basic cements research because the lack of iron improves the resolution of nuclear magnetic resonance (NMR) measurements.

RAPID HARDENING PORTLAND CEMENT

This type develops strength more rapidly than ordinary Portland cement. The initial strength is higher , but they equalize at 2-3 months

Setting time for this type is similar for that of ordinary Portland cement

The rate of strength gain occur due to increase of C3S compound, and due to finer grinding of the cement clinker ( the min. fineness is 3250 cm2/gm (according to IQS 5)

Rate of heat evolution is higher than in ordinary Portland cement due to the increase in C3S and C3A, and due to its higher fineness

Chemical composition and soundness requirements are similar to that of ordinary Portland cement

Uses a)The uses of this cement is indicated where a rapid strength development is desired (to develop

high early strength, i.e. its 3 days strength equal that of 7 days ordinary Portland cement), for example:

i) When formwork is to be removed for re-use

ii) Where sufficient strength for further construction is wanted as quickly as practicable, such as concrete blocks manufacturing, sidewalks and the places that can not be closed for a long time, and repair works needed to construct quickly.

b) For construction at low temperatures, to prevent the frost damage of the capillary water.

c) This type of cement does not use at mass concrete constructions.

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SPECIAL TYPES OF RAPID HARDENING PORTLAND CEMENT

A) Ultra High Early Strength Cement The rapid strength development of this type of cement is achieved by grinding the cement to a very high

fineness: 7000 to 9000 cm2/g. Because of this, the gypsum content has to be higher (4 percent expressed as SO3). Because of its high fineness, it has a low bulk density. High fineness leads to rapid hydration, and therefore to a high rate of heat generation at early ages and to a rapid strength development ( 7 days strength of rapid hardening Portland cement can be reached at 24 hours when using this type of cement). There is little gain in strength beyond 28 days.

It is used in structures where early prestressing or putting in service is of importance.

This type of cement contains no integral admixtures.

B) Extra Rapid Hardening Portland Cement This type prepare by grinding CaCl2 with rapid hardening Portland cement. The percentage of CaCl2 should not

be more than 2% by weight of the rapid hardening Portland cement.

By using CaCl2:

The rate of setting and hardening increase (the mixture is preferred to be casted within 20 minutes).

The rate of heat evolution increase in comparison with rapid hardening Portland cement, so it is more convenient to be use at cold weather.

The early strength is higher than for rapid hardening Portland cement, but their strength is equal at 90 days.

Because CaCl2 is a material that takes the moisture from the atmosphere, care should be taken to store this cement at dry place and for a storage period not more than one month so as it does not deteriorate.


MANUFACTURING OF CEMENT

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1) Quarry

2) Raw Material

3) Mixing and crushing of raw materials:

a) Dry process b) Wet process

4) Burning

5) Grinding

6) Storage

7) Packing

8) Dispatch

Step in the Manufacture of Portland Cement

1. BLASTING : The raw materials that are used to manufacture cement (mainly limestone and clay) are blasted from the quarry.

Quarry face

1. BLASTING 2. TRANSPORT

quarry

3. CRUSHING & TRANSPORTATION

2. TRANSPORT : The raw materials are loaded into a dumper.

crushing

conveyor

dumper

storage at

the plant

loader

Typical Quarry operation:

Typically shale provides the argillaceous components: Silica (SiO2, Aluminum (Al2O3) & Iron (Fe2O3)

Limestone provides the calcareous component: Calcium Carbonate (CaCO3 )

Raw materials may vary in both composition and morphology.

THE CEMENT MANUFACTURING PROCESS

1. RAW GRINDING : The raw materials are very finely ground in order to produce the raw mix.

1. RAW GRINDING

Raw grinding and burning

2. BURNING

2. BURNING : The raw mix is preheated before it goes into the kiln, which is heated by a flame that can

be as hot as 2000 °C. The raw mix burns at 1500 °C producing clinker which, when it leaves the kiln, is

rapidly cooled with air fans. So, the raw mix is burnt to produce clinker : the basic material needed to

make cement.

conveyor Raw mix

kiln

cooling

preheating

clinker

storage at

the plant

Raw mill

THE CEMENT MANUFACTURING PROCESS

1.GRINDING : The clinker and the gypsum are very finely ground giving a “pure cement”. Other secondary

additives and cementitious materials can also be added to make a blended cement.

1. GRINDING

Grinding, storage, packing, dispatch

2. STORAGE, PACKING, DISPATCH

2. STORAGE, PACKING, DISPATCH :The cement is stored in silos before being dispatched either in

bulk or in bags to its final destination.

clinker

storage

Gypsum and the secondary additives are added

to the clinker.

silos

dispatch

bags

Finish grinding

MANUFACTURE OF ORDINARY PORTLAND CEMENT Ordinary Portland Cement (OPC) is one of several types of cement being manufactured throughout the

world. OPC consists mainly of lime (CaO), silica (SiO2) , alumina (Al2O3) , iron (Fe2O3) and sulphur trioxide

(SO3). Magnesium (MgO) and other Oxide elements are present in small quantities as an impurity associated with raw materials.

When cement raw materials containing the proper proportions of the essential oxides are ground to a suitable fineness and then burnt to incipient fusion in a kiln, chemical combination takes place, largely in the solid state resulting in a product named clinker.

This clinker, when ground to a suitable fineness, together with a small quantity of gypsum (SO3) is Portland Cement. SO3 is added at the grinding stage to retard the setting time of the finished cement.

Basic Chemical Components of Portland Cement:

Calcium (Ca) Silicon (Si) Aluminum (Al) Iron (Fe)

Typical Raw Materials:

Limestone (CaCO3) Sand (SiO2) Shale, Clay (SiO2, Al2O3, Fe2O3) Iron Ore/Mill Scale (Fe2O3)

2/3 calcareous materials (lime bearing) - limestone 1/3 argillaceous materials (silica, alumina, iron)- clay

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2) Raw Materials for Cement Manufacture The first step in the manufacture of Portland Cement is to combine a variety of raw ingredients so that

the resulting cement will have the desired chemical composition. These ingredients are ground into small particles to make them more reactive, blended together, and

then the resulting raw mix is fed into a cement kiln which heats them to extremely high temperatures. Since the final composition and properties of Portland Cement are specified within rather strict bounds,

it might be supposed that the requirements for the raw mix would be similarly strict. As it turns out, this is not the case. While it is important to have the correct proportions of calcium, silicon, aluminum, and iron, the overall chemical composition and structure of the individual raw ingredients can vary considerably. The reason for this is that at the very high temperatures in the kiln, many chemical components in the raw ingredients are burned off and replaced with oxygen from the air.

Table 1 lists just some of the many possible raw ingredients that can be used to provide each of the main cement elements.

Table 1: Examples of raw materials for Portland Cement manufacture

Calcium Silicon Aluminum Iron Limestone Clay Clay Clay

Marl Marl Shale Iron ore Calcite Sand Fly ash Mill scale

Gypsum Shale Aluminum ore refuse

Shale

Marly limestone Fly ash Phyllite Blast furnace dust

Sea Shells Rice hull ash

slate slag

Cement kiln dust Silica Chalk Sand

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Typical Composition of Raw Materials

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The ingredients listed above include both naturally occurring materials such as limestone and clay, and industrial byproduct materials such as slag and fly ash. From Table 1 it may seem as if just about any material that contains one of the main cement elements can be tossed into the kiln, but this is not quite true.

The raw materials used in the manufacture of cement are limestone, shale, sand and iron ore, typical chemical compositions of which are given in the table below.

Limestone makes up approximately 80% of the raw material requirements, composes of mainly calcium carbonate with small intrusions of magnesium carbonate. Quarrying operations are geared to minimizing the intrusions. MgO in the cement, if present in sufficient quantities will cause expansion upon hydration thus resulting in unsoundness in the concrete.

Materials that contain more than minor (or in some cases trace) amounts of metallic elements such as magnesium, sodium, potassium, strontium, and various heavy metals cannot be used, as these will not burn off in the kiln and will negatively affect the cement.

Another consideration is the reactivity, which is a function of both the chemical structure and the fineness. Clays are ideal because they are made of fine particles already and thus need little processing prior to use, and are the most common source of silica and alumina. Calcium is most often obtained from quarried rock, particularly limestone (calcium carbonate) which must be crushed and ground before entering the kiln. The most readily abundant source of silica is quartz, but pure quartz is very unreactive even at the maximum kiln temperature and cannot be used.

2) Raw Materials for Cement Manufacture (Cont.)

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Raw Material Proportioning

The raw materials are extracted from the hoppers via weigh-feeders. The materials are conveyed to the grinding mill and are ground to a suitable fineness, called raw meal at this stage. This is then stored in a blending silo and blended to ensure homogeneity.

The proportions of the 4 components are controlled by the continuous sampling and testing of this raw meal.

The raw meal chemical composition is determined by the use of an x-ray fluorescence analyzer. This is linked to the computer which will automatically adjust the weigh-feeders, so that the resultant raw meal stored in the blending silo meets the preset parameters. After blending this material is then discharged into the storage silos ready for the next phase of production.

The parameters used in the control of the raw meal are lime saturation factor, silica modulus and iron modulus. These are actually proportions of the various chemical components which are desired in the resultant clinker.

As coal is used as a fuel the coal ash, a combustion product of the coal, has to be treated as an individual raw material component and the appropriate corrections made at the weigh-feeder stage.

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2) Crushing & Grinding of Raw Materials

Due to the variable nature of these components, they are pre-blended prior to their use. It is crushed and stored in a pre-blending hall, utilizing the chevron pile stacking method. In this method, stacking takes place at one end of the pile. At the other end of the pile the material is reclaimed and then stored in a feeding hopper which is ready for use.

The limestone is crushed to less than 25mm in size. Grinding and blending prior to entering the kiln can be performed with the raw

ingredients in the form of a slurry (the wet process) or in dry form (the dry process). The addition of water facilitates grinding. However, the water must then be removed by evaporation as the first step in the burning process, which requires additional energy. The wet process, which was once standard, has now been rendered obsolete by the development of efficient dry grinding equipment, and all modern cement plants use the dry process. When it is ready to enter the kiln, the dry raw mix has 85% of the particles less than 90 µm in size

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The raw materials used for manufacturing Portland Cement are limestone, clay and Iron ore.

a) Limestone (CaCO3) is mainly providing calcium in the form of calcium oxide (CaO)

CaCO3 (1000oC) → CaO + CO2 b) Clay is mainly providing silicates (SiO2) together with small amounts of Al2O3 +

Fe2O3 Clay (1450oC) → SiO2 + Al2O3 + Fe2O3 + H2O

c) Iron ore and Bauxite are providing additional aluminum and iron oxide (Fe2O3) which help the formation of calcium silicates at low temperature. They are incorporated into the raw mix.

d) The clinker is pulverized to small sizes (< 75 μm). 3-5% of gypsum (calcium sulphate) is added to control setting and hardening. The majority particle size of cement is from 2 to 50 μm. (Note: “Blaine” refers to a test to measure particle size in terms of surface area/mass)

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a) Dry process: In this process calcareous material such

as lime stone (calcium carbonate) and

argillaceous material such as clay are

ground separately to fine powder in the

absence of water and then are mixed

together in the desired proportions.

Water is then added to it for getting

thick paste and then its cakes are

formed, dried and burnt in kilns. This

process is usually used when raw

materials are very strong and hard.

In this process, the raw materials are

changed to powdered form in the

absence of water.

Dehydration zone requires a somewhat

shorter distance than wet process.

74% of cement produced.

kilns less fuel requirements

b) Wet process:

In this process, the raw materials are changed to powdered

form in the presence of water.

In this process, raw materials are pulverized by using a

Ball mill, which is a rotary steel cylinder with hardened

steel balls. When the mill rotates, steel balls pulverize the

raw materials which form slurry (liquid mixture). The

slurry is then passed into storage tanks, where correct

proportioning is done. Proper composition of raw

materials can be ensured by using wet process than dry

process. Corrected slurry is then fed into rotary kiln for

burning.

This process is generally used when raw materials are soft

because complete mixing is not possible unless water is

added.

Actually the purpose of both processes is to change the

raw materials to fine powder.

dehydration zone would require up to half the length of

the kiln

easiest to control chemistry & better for moist raw

materials

high fuel requirements - fuel needed to evaporate 30+%

slurry water

The kiln is a continuous stream process vessel in which

feed and fuel are held in dynamic balance

Type of cement processes

3) Burning in a Kiln – Formation of Cement Clinker The next step in the process is to heat the blended mixture of raw ingredients (the

raw mix) to convert it into a granular material called cement clinker. This requires maximum temperatures that are high enough to partially melt the raw

mix. Because the raw ingredients are not completely melted, the mix must be agitated to ensure that the clinker forms with a uniform composition.

This is accomplished by using a long cylindrical kiln that slopes downward and rotates slowly.

To heat the kiln, a mixture of fuel and air is injected into the kiln and burned at the bottom end. The hot gases travel up the kiln to the top, through a dust collector, and out a smokestack. A variety of fuels can be used, including pulverized coal or coke, natural gas, lignite, and fuel oil. These fuels create varying types and amounts of ash, which tend to have compositions similar to some of the aluminosilicate ingredients in the raw mix. Since the ash combines with the raw mix inside the kiln, this must be taken into account in order to correctly predict the cement compassion. There is also an increasing trend to use waste products as part of the fuel, for example old tires. In the best-case scenario, this saves money on fuel, reduces CO2 emissions, and provides a safe method of disposal.

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Kiln Feed Preparation

Proportioning of feed stock. Size reduction to < 125μ. Control of moisture. Blending to reduce standard deviation. Uniform delivery rate of feed to the Kiln.

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Manufacturing control criteria in the Kiln

Silica Modulus (SM) :

Rate of reactions

SiO2

SM = --------------------

Al2O3 + Fe2O3

2.3 to 3.5 (desired at least 3.0),

slow reaction if SM is high

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Alumina Modulus (AM):

controls melt temperature

Al2O3

AM = --------------

Fe2O3

AM ~2

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Lime Saturation factor (LSF): Designed to insure against equilibrium free lime

CaO LSF = -----------------------------

SiO2 + Al2O3 + Fe2O3 LSF : 0.92-0.96

Or

CaO LSF = ---------------------

Al2O3 + Fe2O3 C/(A+F) should be equal to 2

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CEMENT KILNS

And the Hottest

High temperature Long residence time Natural alkaline environment CKD is only by-product of the process Thermal stability

the Largest Moving Equipment in any Manufacturing Operation

Kiln Process Thermochemical Reactions

Kiln Process

Process Temperature (oC) Reactions Chemical Transformation

Drying 20 - <100 Escape of free water (i.e., Free

Water evaporates)

Pre-heat

100 - 300 Escape of adsorbed water (i.e.,

Crystallization water driven out)

400 - 750 Chemical water driven out,

Decomposition of shale., with

formation of metakaolinite

Al4Si4O10(OH)8

2(Al2O3.2SiO2) + 4H2O

600 - 900 Decomposition of metakaolinite and

other compounds, with formation of

reactive oxide mixture

Al2O3.2SiO2 Al2O3.+ 2SiO2

Calcining 600 - 1000 Decomposition of limestone, CO2

Driven out, Formation of Free lime ,

with formation of CS (CaO.SiO2) and

CA (CaO.Al2O3)

3CaCO3 3CaO + 3CO2

3CaO + 2SiO2 + Al2O3

2(CaO.SiO2) + CaO.Al2O3

Sintering

Clinkering

800 – 1550 (1350

exothermic)

Uptake of lime by CS and CA,

Formation of Liquid Phase,

Formation of: Belite (C2S),

Aluminates (C3A) and Ferrites (C4AF)

CS + C C2S

2C + S C2S

CA + 2C C3A

CA + 3C + F C4AF

Cooling

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Clinker Burning For the production of cement clinker, the raw meal which is known as kiln feed at this stage

has to be heated to a temperature of about 1550 oC in the long cylindrical rotating kiln. The kiln feed enters the system at the top of the pre-heater and fall until the lower end of

the kiln. The heat exchange occurs during this process when the hot gases from the kiln end rise up

to the top of the pre-heater. The clinker formation process is divided into four parts: drying, calcining, sintering and

cooling. As the kiln feed moves towards the lower end of the kiln it undergoes successive reactions

as shown in the table:

CLINKER

• Clinker is what comes out of the kiln • 3 to 25 mm in diameter • 20 – 25 % Molten

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Clinker Microstructure

C3S crystals magnified 3000 times Dark, Rounded – C2S

Light, Angular – C3S

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Burning Process This description refers to a standard dry-process kiln. Such a kiln is typically about 180 m

long and 6 m in diameter, has a downward slope of 3-4%, and rotates at 1-2 revolutions per minute.

The raw mix enters at the upper end of the kiln and slowly works its way downward to the hottest area at the bottom over a period of 60-90 minutes, undergoing several different reactions as the temperature increases. It is important that the mix move slowly enough to allow each reaction to be completed at the appropriate temperature. Because the initial reactions are endothermic (energy absorbing), it is difficult to heat the mix up to a higher temperature until a given reaction is complete.

The general reaction zones are as follows: 1) Dehydration zone (up to ~ 450˚C) 2) Calcination zone (>450˚C – 900˚C) 3) Solid-state reaction zone (>900˚ - 1300˚C) 4) Clinkering zone (>1300˚C – 1550˚C) 5) Cooling zone

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Reaction Zone Temperature (oC) Characteristics

Dehydration up to ~ 450

This is simply the evaporation and removal of the free water

Even in the “dry process” there is some adsorbed moisture in the raw mix.

Although the temperatures required to do this are not high, this requires significant time and

energy.

In the wet process, the dehydration zone would require up to half the length of the kiln, while

the dry process requires a somewhat shorter distance.

Calcination 450˚C – 900

The term calcination refers to the process of decomposing a solid material so that one of its

constituents is driven off as a gas.

At about 600˚C the bound water is driven out of the clays,

and by 900˚C the calcium carbonate is decomposed, releasing carbon dioxide.

By the end of the calcination zone, the mix consists of oxides of the four main elements which

are ready to undergo further reaction into cement minerals.

Because calcination does not involve melting, the mix is still a free-flowing powder at this

point.

Solid-state

reaction >900˚ - 1300

This zone slightly overlaps, and is sometimes included with, the calcination zone.

As the temperature continues to increase above ~ 900˚C there is still no melting, but solid-

state reactions begin to occur.

CaO and reactive silica combine to form small crystals of C2S (dicalcium silicate; Belite), one of

the four main cement minerals.

In addition, intermediate calcium aluminates (C3A) and calcium ferrite (C4AF) compounds form.

These play an important role in the clinkering process as fluxing agents, in that they melt at a

relatively low temperature of ~1300˚C, allowing a significant increase in the rate of reaction.

Without these fluxing agents, the formation of the calcium silicate cement minerals would be

slow and difficult.

In fact, the formation of fluxing agents is the primary reason that Portland (calcium silicate)

Cements contain aluminum and iron at all.

The final aluminum- and iron-containing cement minerals (C3A and C4AF) in a Portland Cement

contribute little to the final properties.

As the mix passes through solid-state reaction zone it becomes “sticky” due to the tendency

for adjacent particles to fuse together.

Burning Process (Cont.)

Reaction Zone Temperature (oC) Characteristics

Clinkering >1300 – 1550

This is the hottest zone where the formation of the most important cement mineral, Alite

(C3S), occurs.

The zone begins as soon as the intermediate calcium aluminate (C3A) and ferrite (C4AF) phases

melt.

The presence of the melt phase causes the mix to agglomerate into relatively large nodules

about the size of marbles consisting of many small solid particles bound together by a thin

layer of liquid.

Inside the liquid phase, Alite (C3S) forms by reaction between Belite (C2S) crystals and CaO.

(C2S + C C3S)

Crystals of solid Alite (C3S) grow within the liquid, while crystals of Belite (C2S) formed earlier

decrease in number but grow in size.

The clinkering process is complete when all of silica is in the C3S and C2S crystals and the

amount of free lime (CaO) is reduced to a minimal level (<1%).

Cooling

As the clinker moves past the bottom of the kiln the temperature drops rapidly and the liquid

phase solidifies, forming the other two cement minerals C3A (aluminate) and C4AF (ferrite).

In addition, alkalis (primarily K) and sulfate dissolved in the liquid combine to form K2SO4 and

Na2SO4.

The nodules formed in the clinkering zone are now hard, and the resulting product is called

cement clinker.

The rate of cooling from the maximum temperature down to about 1100˚C is important, with

rapid cooling giving a more reactive cement.

This occurs because in this temperature range the C3S can decompose back into C2S and CaO,

among other reasons.

It is thus typical to blow air or spray water onto the clinker to cool it more rapidly as it exits the

kiln.

Rapid cooling of the clinker is essential as this hampers the formation of crystals, causing part of the liquid phase to solidify as glass.

The faster the clinker cooling the smaller the crystals will be when emerging from the liquid phase.

Dual Line Preheater Planetary Cooler

i) Generalized Diagram of a Long Dry Process Kiln

Reaction

Material Temperature

Gas Temperature

The kiln exit gas temperature will depend on the process

Zone

Exhaust Gases Raw

Feed

Clinker Out

Suspension Preheaters and Calciners

The chemical reactions that occur in the dehydration and calcination zones are endothermic, meaning that a continuous input of energy to each of the particles of the raw mix is required to complete the reaction. When the raw mix is piled up inside a standard rotary kiln, the rate of reaction is limited by the rate at which heat can be transferred into a large mass of particles. To make this process more efficient, suspension preheaters are used in modern cement plants to replace the cooler upper end of the rotary kiln. Raw mix is fed in at the top, while hot gas from the kiln heater enters at the bottom. As the hot gas moves upward it creates circulating “cyclones” that separate the mix particles as they settle down from above. This greatly increases the rate of heating, allowing individual particles of raw mix to be dehydrated and partially calcined within a period of less than a minute.

Alternatively, some of the fuel can be burned directly within the preheater to provide even more heating to the suspended particles. The area of the preheater where fuel is burned is called a precalciner. With a precalciner, the particles are nearly completely calcined as they enter the rotary kiln. Preheaters and precalciners save on fuel and increase the rate at which the mix can be moved through the rotary kiln.

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ii) General Diagram of Preheater/Precalciner Reactions

Dry Process Preheater/Precalciner System

Preheater Precalciner Kiln

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Kiln Process Control

Critical Parameters: Fuel, Feed, Kiln Speed, Gas Flow

Kiln Temperatures - Burning Zone

Kiln Stability

Chemistry

Instrumentation

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4) Grinding and the Addition of Gypsum

Now the final process is applied which is grinding of clinker, it is first cooled down to atmospheric temperature.

Grinding of clinker is done in large tube mills. After proper grinding gypsum (Calcium sulphate CaSO4) in the ratio of 01-04 % is added for

controlling the setting time of cement. Finally, fine ground cement is stored in storage tanks from where it is drawn for packing. Once the nodules of cement clinker have cooled, they are ground back into a fine powder in a

large grinding mill. At the same time, a small amount of calcium sulfate such as gypsum (calcium sulfate) is blended into the cement. The calcium sulfate is added to control the rate of early reaction of the cement

Cement is produced by grinding clinker with gypsum (calcium sulfate) in the finish-grinding mill to a required fineness.

A small quantity of gypsum, about 3 to 5 %, is needed to control the setting time of cement produced.

The amount of gypsum being used is determined by the Sulphuric anhydride (SO3) contents in cement.

Cement Grinding

• Clinker, gypsum, and optional additives are weighed to proper proportions and ground in the cement mills.

• Additives may include: Fly-ash, Limestone…..

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Schematic of a Grinding Mill


Cement Storage & Distribution

• At this point the manufacturing process is complete and the cement is ready to be bagged or transported in bulk away from the plant After the grinding process, cement is pumped into the storage silos.

• This silo is preventing the moisture to react with cement. • When needed cement from the silos is packed into bags or

loaded into road tankers and rail wagons for dispatch. • However, the cement is normally stored in large silos at the

cement plant for a while so that various batches of cement can be blended together to even out small variations in composition that occur over time.

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CEMENT CHEMISTRY

CHEMICAL COMPOSITIONS

Oxide Notation

CaO C

SiO2 S

Al2O3 A

Fe2O3 F

SO3

H2O H

MgO M

Na2O N

S

The properties of cement during hydration vary according to:

Chemical composition

Degree of fineness

It is possible to manufacture different types of cement by changing the percentages of their raw materials.

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1) Cement chemistry notation based on oxides

Chemical Name Chemical Formula

Oxide Formula Cement Notation

Mineral Name

Tricalcium Silicate Ca3SiO5 3CaO.SiO2 C3S(40-60%) Alite

Dicalcium Silicate Ca2SiO4 2CaO.SiO2 C2S(16-30%) Belite

Tricalcium Aluminate Ca3Al2O6 3CaO.Al2O3 C3A(7-15%) Aluminate

Tetracalcium Aluminoferrite

Ca2AlFeO5 4CaO.Al2O3.Fe2O3 C4AF(7-12%) Ferrite

Calcium hydroxide Ca(OH)2 CaO.H2O CH Portlandite

Calcium sulfate dihydrate

CaSO4.2H2O CaO.SO3.2H2O CSH2 Gypsum

Calcium oxide CaO CaO C Lime

Compound Composition of Clinker / Cement

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Phase Diagram

Tricalcium Silicate Tricalcium Aluminate

SiO2

CaO

CaO CaO

CaO

Al2O3

CaO CaO

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Bogue’s Equations – Compound composition

To calculate the amounts of C3S, C2S, C3A, and C4AF in clinker (or the cement) from its chemical analysis (from the mill certificate)

Assumptions in calculations: Chemical equilibrium established at the clinkering temperature Components maintained unchanged through the rapid cooling

period Compounds are “pure”

• A simple estimate of the phase composition of a Portland Cement can be obtained from the oxide composition if one assumes that the four main cement minerals occur in their pure form.

• With this assumption, all of the Fe2O3 is assigned to C4AF and the remaining Al2O3 is assigned to C3A.

• This leaves a set of two linear equations to be solved for the amounts of C2S and C3S.

• This method is named after the cement chemist R.H. Bogue. A standardized version of this simple method is given in ASTM C 150. There are two sets of equations, based on the ratio of A/F in the cement (both inputs and outputs are in weight percent):

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Case 1 : If A/F >=0.64

C3S = 4.071C - 7.6S - 6.718A - 1.43F - 2.852S

C2S = 2.867S - 0.7544C3S

C3A = 2.65A - 1.692F

C4AF = 3.043F

Case 2 : If A/F < 0.64

C3S = 4.071C - 7.6S - 4.479A – 2.859F - 2.852S

C2S = 2.867S - 0.7544C3S

C3A = 0

C4AF = 2.10A + 1.702F

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Fineness of cement • Grinding is the last step in

processing • Measures of fineness

Specific surface Particle size

distribution • Blaine’s fineness

Measure of air permeability

• Typical surface areas 350 m2 / kg (Normal

cements) ~ 500 m2 / kg (High early

strength cements)

Significance of fineness Finer cement = Faster reaction Finer cement = Higher heat of

hydration Large particles do not react

with water completely Higher fineness

Higher shrinkage Reduced bleeding Reduced durability More gypsum needed

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Lecture5 Cement 140309165129 Phpapp01

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