lecture 1 - cement (2013)

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LECTURE 1CEMENT

Presented by: Mr. Milton McIntyreSep. 2015

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• Introduction• Brief History• Manufacture of Portland cement

• Chemical Composition of Portland cement• Hydration of cement• Physical properties of Portland cement• Types of Portland cement

• Test on properties of cement• Questions

Aug.13, 2010Prepared by: Mrs. Barbara A. Cooke

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Cement is a fine, powdery substancethat has adhesive and cohesiveproperties when mixed with water andis capable of bonding mineralparticles together.

The term “cement” is used to describethe bonding material used with stones,

sand, bricks, building blocks, etc.

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Cement is classified into two categories: hydraulic and non-hydraulic,based on the way in which they set and harden.

• Hydraulic cement - have the property of setting and hardeningunder water by virtue of a chemical reaction.

• Non-hydraulic cement – do not have the property of setting and

hardening under water but require air to harden(Lime – CaCO 3 CaO + CO 2).

Cement used in the making ofconcrete are considered to beHydraulic cements.

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Pantheon in Rome

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Hydraulic lime : made from calcination of siliceous orargillaceous (clay-rich) limestone. (used as stucco and plaster)

Natural cements (Roman cement): made from calcination ofnaturally occurring argillaceous limestone and clay or shalebelow sintering temp.

Type N – used with Portland cement in general concreteconstruction

Type NA – air-entrained cement used like type N.

Inferior to Portland cement (not recommended for use wherestrength is important.

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Portland cement : most used cement and most importanthydraulic cement. Made by the mixing clinker with gypsum.(2900°F).

Blended cements : Mix of Portland cement with one or moreSupplementary Cementitious Material (SCM) like pozzolan orslag additives.

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Expansive cements : Contains a constituent that duringhydration, setting, or hardening, undergoes expansion butremains sound and eventually develops strength. Used where

shrinkage cannot be tolerated.

Aluminous cements (high-alumina): Limestone and bauxiteare the main raw materials. Used for refractory applications(such as cementing furnace bricks) and certain applicationswhere rapid hardening is required (high-early strength - setsand harden within 48hrs). (2900 ° F). Sensitive to high temp.(reduced strength when moist)

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Special Portland cementsWhite Portland cementColored cements – decorative purposesOil-well cements – slow setting used to seal deep wells

Regulated cements – rapid setting or hardening used in themanufacture of blocks, pipes, pre-stressed and precastconcrete also patch works.Waterproofed cement – made with water repellant toreduce permeability.Hydrophobic cements – made with water-repellant materialsused to prolong the life of cement during storage (shipment)

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Ancient Egyptians used calcined impure gypsum to make cementmaterials.

Greeks and Romans used calcined limestone and later addedlime and water, sand, crushed stone and brick or broken tiles(lime mortar).For underwater construction they mixed lime and volcanic ash orfinely grounded burnt clay tiles (known as Pozzolanic cement).Named after the village of Pozzuoli, Mt. Vesuvius, where the

volcanic ash was found.The term „ Pozzolanic cement‟ is still used today to describecements obtained by the grinding of natural materials atnormal temperature.


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It was not until the eighteen century that John Smeaton, the firstto understand the chemistry of hydraulic lime (obtained byburning a mixture of lime and clay), discovered that the bestmortar was produced when pozzolana was mixed with limestone

containing considerable proportions of clayey matter.James Parker developed what was referred to as the „Romancement‟. This was another type of hydraulic cement obtained bycalcining argillaceous limestone, patented as Portland cement by

Joseph Aspdin in 1824.This cement was prepared by heating a mixture of finely-dividedclay and hard limestone in a furnace until CO 2 was driven off.(no clinkering)


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Modern cement prototype was first made in 1845 Isaac Johnson.He burnt a mixture of clay and chalk at higher temperatures untilclinkering, allowing the reactions necessary for the formation ofstrongly cementitious compounds to take place.

The name „Portland cement‟ was so called due to theresemblance of the colour and quality of the hardened cement toPortland stone - a limestone quarried in Dorset, England.

The term „Portland cement‟ is still used world wide today todescribe cement obtained by intimately mixing togethercalcareous and argillaceous , or other silica, alumina and ironbearing materials and burning them at clinkering temp andgrinding the resulting clinker. Aug.13, 2010Prepared by: Mrs. Barbara A. Cooke

Brief History of Portland Cement11

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Modern Portland cement is madefrom a mixture of naturalargillaceous and calcareous

materials.


Essential ingredients are:Lime (CaO)Silicon Dioxide (SiO2)

Aluminum Oxide (Al2O3)Iron (Fe2O3)Magnesium (MgO)Gypsum (Ca(SO 4)•2(H 2O))

Found in nearly every country

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The manufacture of cement consists essentially of grinding the rawmaterials, mixing them intimately in certain proportions and burningin a large rotary kiln at temperatures between 1400 and 1600 ˚C,where the raw material sinters and partially fuses into balls known

as clinker.The clinker is cooled and ground to fine powder, with some gypsumadded, and the resulting product is the commercial Portland cementso widely used throughout the world.

The mixing and grinding of the raw materials are done mainly byone of two processes, „wet‟ or „dry‟. The method of manufacturedepends largely on the hardness of the raw materials used and ontheir moisture content.


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http://www.youtube.com/watch?v=n-Pr1KTVSXo&feature=player_detailpage 15

http://www.youtube.com/watch?v=n-Pr1KTVSXo&feature=player_detailpagehttp://www.youtube.com/watch?v=n-Pr1KTVSXo&feature=player_detailpagehttp://www.youtube.com/watch?v=n-Pr1KTVSXo&feature=player_detailpagehttp://www.youtube.com/watch?v=n-Pr1KTVSXo&feature=player_detailpage

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Abbreviation notation: C = CaO, S = SiO 2, A = Al2O3, F = Fe 2O3.


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Minor compounds are :

Magnesium Dioxide (MgO)Titanium Dioxide (TiO2)

Manganese (III) Oxide (Mn2O3)Potassium Oxide (K2O)Sodium Oxide (Na 2O)Sulphur Oxide (SO 3)

K2O and Na 2O are the main ones of concern and are calledalkali‟s, which cause disintegration and affect rate of strengthdevelopment in concrete.

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The presence of alkalis in Portland cement has these main effects;It increases the early strength development and reduces thelong-term strength development of a cement paste.

It also increases the alkalinity (pH level) of a fresh cementpaste, which may cause dermatitis or burns in the human skin.It also reacts with some alkalis reactive aggregate, which cancause concrete to deteriorate.

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C3S and C 2S are mainly responsible for the strength anddurability properties of the cement and the C 3A and C 4AFcontents need to be strictly controlled as the C 3A contentparticularly affects the ability of the cement to resist sulphate

attack.

C3S hardens rapidly and is responsible for the initial set andearly strength, while C 2S hardens slowly and contributeslargely to strength increase at ages beyond one week.

The ratio of C 3S to C2S helps to determine how fast the cementwill set, with faster setting occurring with higher C 3S contents.

Prepared by: Mrs. Barbara A. Cooke

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C3A releases large amounts of heat during the first few daysof hardening. Cements with a lower C 3A content promoteresistance to sulphates.

C4AF acts as a flux in the burning of clinker by reducing theclinkering temperature. It hydrates rapidly but contributes verylittle to strength.

The ferrite phase causes the brownish gray colour in cementsand is often used for aesthetic purposes.


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Varying the proportions of the main compounds in cement canresult in different chemical properties suitable for variousapplications.


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Type I - no special characteristic requirements and is used ingeneral construction applications.

Type II - has moderate sulphate resistance and heat ofhydration. This type of cement is used mainly for structures builtin hot environments, or in soil or water high in sulphate, such asdrainage systems, floor slabs and foundations.

Type III - cement has applications requiring strength at an earlystage, as it develops strength soon after pouring.

Type IV - is useful in limiting heat caused by hydration and istherefore used in massive concrete structures, such as dams.

Type V - has a high sulphate resistance, which is best used toresist that chemical radical in soil or water.

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Blended hydraulic cement consists of a mixture of either OPC andPozzolan or OPC and ggbs, combined during or after the grindingprocess of manufacture.

The use of blended cements in concrete:reduces mixing water requirementsImprove strength performance in some casesreduce bleedingreduce energy costimproves finishability and workabilityenhances sulphate resistanceinhibits alkali-aggregate reactionreduces heat evolution during hydration, thus moderating theoccurrences for thermal cracking.


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Aug.13, 2010Prepared by: Mrs. Barbara A. Cooke27

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There are two main types of blended cements; they are as follows:Type 1S – Portland-Blast Furnace Slag cementType 1P – Portland Pozzolan cement

Type 1S cement is an intimate blend of Portland cement andfinely ggbs.

Slag is obtained as a waste product from the manufacture of pig

iron.

It varies greatly in composition and physical structure dependingon the steel making processes used and the method of cooling ofthe slag.

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Type 1OPC

Type 1P

Type 1SSlag

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Hydration is the term given to the chemical reaction that takeplace when cement comes in contact with H 2O. Either by truehydration or hydrolysis.

In the presence of water the silicates and aluminates in cementform products of hydration which in time produce a firm hardmass.

The products of hydration of cement have a very low solubilityin water. The hydrated cement bonds firmly to the un-hydratedcement.


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Hydration of cement is a continuous process and can take yearsto complete.

The main products of hydration are classified as „calciumsilicate hydrate‟ (C 3S2H3) and tricalcium aluminate hydrate

(C3AH6)The C3S (alite) and C 2S (belite), hydrates to form C 3S2H3described as C-S-H

2C3S + 6H – C3S2H3 + 3Ca(OH) 2

2C2S + 4H – C3S2H3 + Ca(OH) 2The C3A compound hydrates to form C3AH6 described asC-A-H

C3A + 6H – C3AH6

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The progress of hydration of cement can be determined bydifferent means, such as the measurement of:

1. The amount Ca(OH) 2 2. The heat evolved by hydration3. The specific gravity of the paste4. The amount of chemically combined water5. The amount of un-hydrated cement present (using X-ray

quantitative analysis)6. The strength of the hydrated paste

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Setting is the term used to describe the stiffening of cementpaste after it has been mixed with water. This is often confusedwith 'hardening' but setting and hardening are two differentthings.

Hardening is the process of strength growth and may continuefor weeks, months, or even years after the concrete has beenmixed and placed.

Hardening is due largely to the formation of calcium silicate

hydrates as the cement paste hydrates over time.Setting is due to early-stage calcium silicate hydrate (C 3S2H3)formation .


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The terms 'initial set' and 'final set' are arbitrary definitions ofstages of setting (early and later set) of cement. The initialsetting of cement is as a result of the rapid hydration of C 3S andthe final set results from the slow rate of hydration of the C 2S

compound.

Even though C3A reacts readily with H 2O to form calciumaluminate hydrate crystals, the addition of gypsum retards thisformation allowing C 3S to set first.

If C3A was allowed to set first, the calcium aluminate hydratecrystals formed would be very porous and that would adverselyaffect the strength characteristics of the cement paste.


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The setting of cement paste is also affected by temperature.At low temperatures setting is retarded (set slower).

In temperatures higher than 30 ° C (85 ° F), the rate of the settingof cement may increase with time; therefore care must betaken to control the temperature of concrete for maximumstrength development.


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False Set

False set is the term used to describe the abnormal premature(early) stiffening of cement within a few minutes of mixing withwater.During this occurrence no considerable heat is evolved.The plasticity of cement paste that experience false setting canbe easily restored without affecting the strength, by remixingwithout the addition of water until normal setting has occurred.False setting maybe as a result of dehydrated gypsum beingmixed with clinker that is too hot. It could also be caused bythe association with the alkalis present in cement.

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Flash set

Flash set is the term used to describe the violent reaction ofpure Tricalcium Aluminate (C3A) with water, which occurs wheninsufficient gypsum is added to Portland cement.This reaction causes a rapid (immediate) hardening of thecement paste usually with the production of considerableamount of heat.Unlike false setting, the plasticity of flash set cement pastecannot be restored by further mixing, without the addition ofwater.

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Setting time

This test determines the initial & final settingtimes of a cement paste, by observing thepenetration of a needle (Vicat) into a cementpaste of standard consistence until it reachesa specified value (5 ± 1mm).


Standard consistencyThis determines the water content for anygiven cement required to produce a neatpaste of a standard consistency (specifiedresistance to penetration by a standardplunger) [6 ± 1mm).

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Fineness is the term use to describe the size of the cement grains.A change in the fineness can cause variations on the strength ofcement and by extension in concrete.

The fineness affects the heat release and the rate of hydration.The rate of hydration of cement starts at the surface of thecement particles and it is this surface area that represents thecement availability for hydration.

Greater cement fineness or surface area, increases the rate atwhich cement hydrates and accelerates strength developmentover time.


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Finer cements also leads to a stronger reaction with alkali-reactive aggregates (higher shrinkage and cracking).The quantity of gypsum required to be added in cementincreases for cements having finer particles to ensure adequate

retardation of C 3A hydration.An increase in the fineness of cement particles also has an effecton the water requirement for a cement paste to reach standardconsistency. The water required for consistency decreases as thefineness of cement increases up to approximately 4000 cm 2/g.

The reverse occurs for particle sizes finer than approx.4750cm 2/g (ASTM, 1970).The minimum surface area for a typical Portland cement grainwhen measured using the Blaine method is given as 280 m 2/kg(2800 cm 2/g).

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Fineness (gran size)

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Blaine apparatus Wagner turbidmeter

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Soundness is the ability of hardened cement paste to retain itsvolume after setting. This test determines the extent ofexpansion of any given hardened cement paste. The termexpansion refers to a change in volume of paste after it hasset.

Such expansion may take place due to the delayed or slowhydration, or other reaction of free lime, magnesia (excesspericlase – dead-burnt crystalline MgO) and calcium sulfate

(excess gypsum) present in hardened cement.

Cement which undergoes expansion is considered as unsound.Le Chatelier or Autoclave apparatus is used to conduct this test.

Soundness of cement45

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Autoclave Expansion46

Le Chatelier mould set

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This test determines themechanical strength ofhardened cement-sand mortaror concrete of specificproportions at a given age(usually 3, 7 and 28 days)under strictly controlledconditions.

There are different forms ofstrength tests: tension, directcompression and flexure.

Strength of cement 47

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Heat of Hydration (HOH)

This refers to the amount of heat generated when cementreacts with water. The heat of hydration is the result of anexothermic chemical reaction between cement and water.HOH is measured as the quantity of heats of solution of un-hydrated and hydrated cement, J/kg , in a mixture of nitric andhydrofluoric acids at a given temperature: the differencerepresents the heat of hydration.HOH is influenced by the chemical composition of the cement(C3S &C3A), cement content, water/cement ratio, fineness,admixtures and curing temperature. Any variation in one ofthese factors may cause an increase in the heat of hydration.

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Loss on Ignition (HOH)

The loss on ignition of PC is a test usedto show the extent of carbonizationand hydration of free lime and freemagnesium due to the exposure of

cement to the atmosphere.It is determined by heating a cementsample of known mass toapproximately 1000°C until a constant

mass is obtained and the loss of masscalculated.A high loss on ignition may be cause byimproper and prolonged storage

during transport and transfer.

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Insoluble Residue

IR‟s are non -cementing materials that are present in Portlandcement.It is measured to show the extent of adulteration of cementwhich is largely as a result of impurities in gypsum.It is determined by treating a sample of cement withhydrochloric acid.The presence of insoluble residues in the excess of 7% by masscan reduce the compressive strength of cement mortar by asmuch as 11.5% during the early ages.

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Reading Assignment52

Read-up on Concrete Aggregates fornext week.

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lecture 1 - cement (2013)

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