CMT Reviewer (Cement)

8/11/2019 CMT Reviewer (Cement)

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1 | Chapter 2 - CEMENTP a g e

Portland Cement

The basic ingredient of concrete, is a closely controlled chemical combination of calcium

silicon, aluminum, iron and small amounts of other ingredients to which gypsum is added in the

final grinding process. Lime and silica make up about 85% of the mass.

Joseph Aspdin, a British bricklayer, in 1824 was granted a patent for a process of making

cement which he called Portland cement; its name is derived from a type of building stone thatwas quarried on the Isle of Portland in Dorset, England.

Many people have claimed to have made the first Portland cement, but it is generally

accepted that it was first manufactured by William Aspdin.

Properties of Cement

1. Fineness

Fineness tests indirectly measures the surface area of the cement particles per unit mass :

Wagner turbidimeter test (ASTM C 115)

Blaine air-permeability test (ASTM C 204)

Sieving using No. 325 (45 m) sieve (ASTM C 430)

Fineness is expressed in terms of specific surface of the cement (cm2/gr). For OPC specific

surface is 2600-3000 cm2/gr.

As hydration takes place at the surface of the cement particles, it is the surface area o

cement particles which provide the material available for hydration. The rate of hydration is

controlled by fineness of cement. For a rapid rate of hydration a higher fineness is necessary.

However,

Higher fineness requires higher grinding cost

Finer cements deteriorate faster upon exposure to atmosphere.


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Finer cements are very sensitive to alkali-aggregate reaction.

Finer cements require more gypsum for proper hydration.

Finer cements require more water.

2. Soundness

Soundness is the ability of a hardened paste to retain its volume after setting.

Soundness is also defined as the volume stability of cement paste.

A cement is said to be unsound (i.e. having lack of soundness) if it is subjected to delayed

destructive expansion.

Unsoundness of cement is determined by the following tests:

Le-Chatelier accelerated test (BS 4550: Part 3)

Autoclave-expansion test (ASTM C 151)

3. Consistency

Consistency refers to the relative mobility of a freshly mixed cement paste or mortar or its

ability to flow.

Normal or Standard consistency of cement is determined using the Vicats Apparatus. It isdefined as that percentage of water added to form the paste which allows a penetration of 10

1 mm of the Vicat plunger.

Normal consistency of O.P.C. Ranges from 20-30% by weight of cement.

Consistency can be determined by:

Vicat Plunger Consistency Test

Consistency Test for mortar using the flow table

4. Setting Time

Setting refers to a change from liquid state to solid state. Although, during setting cementpaste acquires some strength, setting is different from hardening.

The water content has a marked effect on the time of setting. In acceptance tests fo

cement, the water content is regulated by bringing the paste to a standard condition of

wetness. This is called normal consistency.

This is the term used to describe the stiffening of the cement paste.


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Setting time is used to determine if cement sets according to the time limits specified in

ASTM C 150.

Setting time is determined using either

the Vicat apparatus (ASTM C 191)

Gillmore needle (ASTM C 266)

The factors affecting the setting time are:

Temperature & Humidity

Amount of Water

Chemical Composition of Cement

Fineness of Cement (finer cement, faster setting)Initial setting time is the time from the instant at which water is added to the cement unt

the paste ceases to be fluid and plastic which corresponds to the time at which the Vicatsinitial set needle penetrate to a point 5 mm from the bottom of a special mold.

Final setting time the time required for the paste to acquire certain degree of hardness

This corresponds to the time at which the Vicats final set needle makes an impression on thepaste surface but the cutting edge fails to do so.

In practice, the terms initial set&final set are used to describe arbitrary chosen time of

setting. Initial set indicates the beginning of a noticeable stiffening & final set may be regarded

as the start of hardening (or complete loss of plasticity).

ASTM C 150 prescribes a minimum initial setting time of 60 minutes and a maximum fina

setting time of 10 hours for Portland cements.

Gypsum in the cement regulates setting time. Setting time is also affected by

cement fineness

w/c ratio

admixtures

5. Early Stiffening False Set and Flash Set)

Early stiffening is the early development of stiffening in the working plasticity of cement

paste, mortar or concrete. This includes both false set and flash set.


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The approximate amount of heat generated using ASTM C 186, during the first 7 days

(based on limited data) are as follows:

Type Name Heat of hydration (kj/kg)

I Normal 349

II Moderate 263

III High early strength 370

IV Low heat of hydration 233

V Sulfate resistant 310

8. Loss on Ignition LOI)

Loss on Ignition (L.O.I.) is the loss in weight of cement after being heated to 1000C. It

indicates the prehydration or carbonation due to prolonged or improper storage of cement &

clinker.

If cement is exposed to air, water & CO2 are absorbed & by heating the cement upto

1000C loose these two substances.

The test for loss on ignition is performed in accordance with ASTM C 114.

A high weight loss on ignition of a cement sample (between 900 to 1000C) is an indication

of pre-hydration and carbonation, which may be caused by:

Improper and prolonged storage

Adulteration during transport and transfer

Loss on ignition values range between 0 to 3 .

9. Density and Specific Gravity

Density is the mass of a unit volume of the solids or particles, excluding air between

particles. The particle density of Portland cement ranges from 3.10 to 3.25 Mg/m3, averaging

3.15 Mg/ m3.

For mixture proportioning, it may be more useful to express the density as relative density

(specific gravity). On an average the specific gravity of cement is 3.15.


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Storage of Cement

Cement is moisture-sensitive material; if kept dry it will retain its quality indefinitely.

When exposed to moisture, cement will set more slowly and will have less strength

compared to cement that kept dray.

At the time of use cement should be free-flowing and free of lumps.

Raw Materials of Portland Cement

1) Calcareous Rocks (CaCO3> 75%)

Limestone

Marl

Chalk Marine shell deposits

2) Argillocalcareous Rocks (40%


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Kiln Pyro-processing device used to raise materials to a high temperature in a continuous

process.

Clinkerthe solid material produced by the cement kiln stage that has sintered into lumps or

nodules, typically of diameter 3-25 mm.

Portland Cement Production Steps

1) Raw materials are crushed, screened & stockpiled.

2) Raw materials are mixed with definite proportions to obtain raw mix. They are mixedeither dry (dry mixing) or by water (wet mixing).

3) Prepared raw mix is fed into the rotary kiln.

4)

As the materials pass through the kiln their temperature is rised upto 1300-1600 CThe process of heating is named as burning. The output is known as clinker which is0.15-5 cm in diameter.

5) Clinker is cooled & stored.

6) Clinker is ground with gypsum (3-6%) to adjust setting time.

7) Packing & marketting.

Reactions in the Kiln 100Cfree water evaporates. 150-350Cloosely bound water is lost from clay. 350-650Cdecomposition of claySiO2&Al2O3 600Cdecomposition of MgCO3MgO&CO2(evaporates) 900Cdecomposition of CaCO3CaO&CO2(evaporates) 1250-1280Cliquid formation & start of compound formation. 1280Cclinkering begins. 1400-1500Cclinkering

100Cclinker leaves the kiln & falls into a cooler.Sometimes the burning process of raw materials is performed in two stages: preheating

upto 900C & rotary kiln.


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Chemical Composition of Portland Cement

Portland cement is composed of four major oxides (CaO, SiO2, Al2O3, Fe2O390%) & someminor oxides. Minor refers to the quantity not important.

Oxide Common Name Abbreviation Approx. Amount ( )

CaO Lime C 60-67

SiO2 Silica S 17-25

Al2O3 Alumina A 3-8

Fe2O3 Iron-oxide F 0.5-6

MgO Magnesia M 0.1-4

Na2O Soda N

0.2-1.3

K2O Potassa K

SO3 Sulfuric Anhydride 1-3

CaOlimestone SiO2-Al2O3Clay Fe2O3Impurity in Clays SO3from gypsumnot from the clinker

The amount of oxides in a P.C. Depend on the proportioning of the raw materials and how

well the burning is done in the kiln. The chemical composition is found by chemical analysis.

CaO (C), SiO2(S), Al2O3(A) & Fe2O3are the major oxides that interact in the kiln & form the

major compounds.

Portland Cement alone sets quickly so some gypsum is ground with clinker to retard thesetting time. However, if too much gypsum is included it leads to distruptive expansions of the

hardened paste or concrete.

Alkalies (Na2O & K2O) may cause some dificulties if the cement is used with certain types of

reactive aggregates in making concrete. The alkalies in the form of alkaline hydroxides can react


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3) The grade of cooling of clinker may also be effective on the internal structure of majo

compounds.

There are also some minor compounds which constitute few %, so they are usually

negligible. Moreover, portland cement compounds are rarely pure.

For example in C3S, MgO & Al2O3replaces CaO randomly.

C3SALITE & C2SBELITE

Ferrite Phase: C4AF is not a true compound. The ferrite phase ranges from C 2AF to C6AF. *C4AF

represents an average.

Methods of Determining Compound Composition

Each grain of cement consists of an intimate mixture of these compounds. They can be

determined by:

1) Microscopy

2) X-Ray Diffraction

However, due to the variabilities involved the compound composition is usually calculated

using the oxide proportions.

3) Calculations (Bouges Equations)

ssumptions in Bouges Equations

1) The chemical reactions in the kiln proceeded to equilibrium.

2) Compounds are in pure form such as C3S & C2S

3) Presence of minor compounds are ignored.

4) Ferrite phase can be calculated as C4AF

5)

All oxides in the kiln have taken part in forming the compounds.


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Capillary Pores: 12.5 m diameter, with varying sizes, shapes & randomly distributed in thepaste.

Volume of capillary pores decreases as hydration takes place. Water in capillary pores is

mobile, can not be lost by evaporation or water can get into the pores. They are mainly

responsible for permeability.

C2S & C3S: 70-80% of cement is composed of these two compounds & most of the strength

giving properties of cement is controlled by these compounds.

Calcium-Silicate-Hydrate (C-S-H gel) is similar to a mineral called TOBERMORITE. As aresult it is named as TOBERMORITE GEL

Upon hydration C3S & C2S, CH also forms which becomes an integral part of hydration

products. CH does not contribute very much to the strength of portland cement.

C3S having a faster rate of reaction accompanied by greater heat generation developes

early strength of the paste. On the other hand, C2S hydrates & hardens slowly so results in lessheat generation & developes most of the ultimate strength.

Higher C3Shigher early strength-higher heat generation (roads, cold environments) Higher C2Slower early strength-lower heat generation (dams)

C3A: is characteristically fast reacting with water & may lead to a rapid stiffening of the paste

with a large amount of the heat generation (Flash-Set)-(Quick-Set). In order to prevent this rapid

reaction gypsum is added to the clinker. Gypsum, C3A&water react to form relatively insoluble

Calcium-Sulfo-Aluminates.

When there is enough gypsum ettringite forms with great expansion If there is no gypsumflash-set more gypsumettringite formation increases which will cause cracking

Also Calcium-Sulfo Aluminates are prone (less resistant) to sulfate attack & does not

contribute much for strength. The cement to be used in making concretes that are going to be

exposed to soils or waters that contain sulfates should not contain more than 5% C3A.

C4AF: The hydration of ferrite phase is not well understand. Ferrite phase has lesser role in

development of strength. The hydration products are similar to C3A. Alumina & iron oxide occu

interchangebly in the hydration products.


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Heat of Hydration

Hydration process of cement is accompanied by heat generation (exothermic).

As concrete is a fair insulator, the generated heat in mass concrete may result in expansion

& cracking. This could be overcome by using suitable cement type.

The heat of hydration of OPC is on the order of 85-100 cal/gr.

About 50% of this heat is liberated within 1-3 days & 75% within 7 days.

By limiting C3S & C3A content,heat of hydration can be reduced.

Heat of Hydration of Pure Compounds

eat of Hydration (cal/gr)

C

3

120

C

2

62

C

3

207

C

4

F 100

Strength of Cement

1. Direct Tension (Tensile Strength)

2. Flexural Strength (Tensile Strength in Bending)

3. Compressive Strength

Types of Portland CementCements of different chemical composition & physical characteristics may exhibit different

properties when hydrated. It should thus be possible to select mixtures of raw materials for the

production of cements with various properties.

In fact several cement types are available and most of them have been developed to ensure

durability and strength properties to concrete.


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It should also be mentioned that obtaining some special properties of cement may lead to

undesirable properties in another respect. For this reason a balance of requirements may be

necessary and economic aspects should be considered.

Standard Types: these cements comply with the definition of P.C., and are produced by

adjusting the proportions of four major compounds.

Special Types: these do not necessarily couply with the definiton of P.C. & are

produced by using additional raw materials.

Standard Cements (ASTM)

Type I: Ordinary Portland Cement

This is the general concrete construction cement when the special properties of the othe

types are not required. It is commonly used where concrete will not be exposed to severeweathering conditions. Its uses include pavements and sidewalks, reinforced concrete buildings

water pipes, culverts and masonry units.

Suitable to be used in general concrete construction when special properties are not

required.

Type II: Modified Portland Cement

This is used where resistance to moderate sulfate attack is important. This produces less

heat of hydration than Type I. Their use is commonly on piers, abutments and retaining wallsThey are used in warm weather concreting because of their lower rise in temperature.

Suitable to be used in general concrete construction. Main difference between Type I&II is

the moderate sulfate resistance of Type II cement due to relatively low C3A content (%8)Since C3A is limited rate of reactions is slower and as a result heat of hydration at early ages is

less. It is suitable to be used in small scale mass concrete like retaining walls.

Type III: High Early Strength P.C.

This cement is used where an early strength gain is important and heat generation is not acritical factor. This cement supplies the strength required in a shorter period of time.

Strength development is rapid.

It is useful for repair works, cold weather & for early demolding.

Its early strength is due to higher C3S & C3A content.


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Type IV: Low Heat P.C.

Strength development of this type is slower than of type I. It is primarily used in large mass

placements such as gravity dams.

Generates less heat during hydration & therefore gain of strengthis slower.

In standards a maximum value of C3S&C3A& a minimum value for C2S are placed.

It is used in mass-concrete and hot-weather concreting.

Type V: Sulfate Resistant P.C.

This type is primarily used where the soil or groundwater contains high sulfate concentration

and the structure would be exposed to severe sulfate attack.

Used in construction where concrete will be subjected to external sulfate attack chemica

plants, marine & harbor structures. C3A is limited to 5%.

Type IA, IIA, IIIA: Air Entrained Portland Cement

Only difference is adding an air-entraining agent to the cement during manufacturing to

increase freeze-thaw resistance by providing small sized air bubbles in concrete.

Special Cements

Portland Pozzolan Cement P.P.C.)

P.P.C. Produces less heat of hydration & offers higher sulfate resistance so it can be used

in marine structures & mass concrete. However, most pozzolans do not contribute to strength

at early ages.

The early strength of PPC is less.

Portland Blast Furnace Slag Cement P.B.F.S.C.)

This cement is less reactive (rate of gain of strength & early strength is less but ultimate

strength is same)

High sulfate resistance, suitable to use in mass concrete construction and unsuitable fo

cold weather concreting.


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Note: Both P.P.C. & P.B.F.S.C. Are called blended cements. Their heat of hydration &

strength development are low in early days. Because upon adding water C3S compounds start

to produce C-S-Hgels & CH. The Ch & the pozzolanic material react together to produce new C-

S-H gels. Thats why the early strength is low but the ultimate strength is the same whencompared to O.P.C.

White Portland Cement

The selected raw materials of this type have negligible amounts of iron and manganese

oxides, and process of manufacture is controlled to produced white color. Its primary used is

for architectural concrete products, cement paints, tile grouts and decorative concrete

W.P.C. s made from materials containing a little iron oxide & manganese oxide.

To avoid contamination by coal ash, oil is used as fuel.

To avoid contamination by iron during grinding, instead of steel balls nickel-molybdenumalloys are used.

High Alumina Cement

The raw materials for H.A.C. s limestone and Bauxite (Al2O3& Fe2O3)

These raw materials are interground & introduced in the kiln clinkered at 1600C. Then the

obtained material is ground to a fineness of 2500-3000 cm2/gr.

It is basically different from O.P.C. & the concrete made from this cement has very differentproperties.

It has high sulfate resistance.

Very high early strength (emergency repairs)

About 80% of ultimate strength is obtained within 24 hours. But the strength is adversely

affected by temperature. The setting time is not as rapid as gain of strength.

Initial setting time is 4 hrs & final setting time is 5 hrs.

Air-Entraining Portland Cement

This cement contains air entraining materials integrated with the clinker during manufacture

This provides the resulting concrete an improved resistance to scaling and freeze-thaw action

Also concrete produced from this cement contains microscopic air bubbles evenly spaced.


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Masonry Cement

This cement contain Portland cements, air-entraining additives and materials selected fo

their ability to impart workability, plasticity and water retention properties.

Standard Turkish Cements

TS 19 groups them into 3:

P.. 32.5 min. Comp. strength is 32.5 MPa in 28 days. P.. 42.5 min. Comp. strength is 42.5 MPa in 28 days. P.. 52.5 min. Comp. strength is 52.5 MPa in 28 days.

Special Cements are:

TS 20Blast Furnace Slag CementC 32.5Cruflu imento

TS 21White Portland Cement BP 32.5-42.5 TS22Masonry Cement, H 16 (Har imentosu) TS 26Trass Cement, T 32.5 (Trasl imento) TS 640Fly Ash Cement, UK 32.5 (Uucu Kll imento)

TS EN 197-1

CEM cements :Binding property is mainly due to hydration of calcium-silicates.

CEM IPortland Cement CEM IIPortland Composite Cement CEM IIIPortland Blast Furnace Slag Cement CEM IVPozzolanic Cement CEM VComposite Cement

Mineral Admixtures :

K : Clinker

D : Silica Fume

P : Natural Pozzolan

Q : Calcined Natural Pozzolan

T : Calcined Shale

W : ClassC Fly Ash


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CMT Reviewer (Cement)