2 - hydraulic binders(1)

8/13/2019 2 - Hydraulic Binders(1)

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Hydraulic BindersPROPERTIES OFMATERIALS

CIVE 1002(Y)ENGINEERING MATERIALS

Mrs B K Ramjeeawon


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Brief History

Cementing materials have been used sinceancient Egyptian and Roman times. Egyptiansused calcined impure gypsum while Romans and

Greeks used calcined limestone Later sand, crushed stone or brick and broken

tiles were added to this lime and water mixtureproducing the first concrete in history

Because of limited use of lime under water,Romans have ground together lime and a volcanicash or finely ground burnt clay tiles


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Manufacture of Portland Cement

Portland cement was developed in 1824 andderives its name from Portland limestone inDorset because of its close resemblance to this

rock after hydration has taken place.

The basic raw materials used in the manufactureof Portland cements are calcium carbonate,

found in calcareous rocks such as limestone orchalk, and silica, alumina and iron oxide found inargillaceous rocks such as clay or shale.


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Manufacture of CementWet Process

Intimate grinding and mixing of the rawmaterials in certain proportions

Burning of this mixture at very high

temperature to produce clinker

Mixing may be in dry or wet state; wet

process is used, in general, for softermaterials such as chalk or clay. Water

added to produce slurry which is

eventually led off to a kiln

Clinker formed by diffusion between

solid particles, therefore intimate mixing

essential for producing uniform cement

Slurry fed at the upper end of the kiln and clinker is

discharged at the lower end where fuel is injected; with

temperature increasing progressively, slurry undergoesa number of changes as it travels down the kiln


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Manufacture of Cement contd

Burning of this mixture at very high temperature

to produce clinker

Grinding it into powder form

CEMENT

Done on a steel cylinder, with a

refractory lining, slightly inclined to thehorizontal and rotates continuously

about its own axis 3.5m dia and

150m long

At 100oC, water evaporates; at about 850oC

CO2is given off and at about 1400o

C incipientfusion takes place in the firing zone where

calcium silicates and calcium aluminates are

formed in the resulting clinker

Clinker allowed to cool and then ground,

with 1 to 5 percent of gypsum (calcium

sulphate) to required fineness


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Manufacturing Process


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Manufacture of Cement

Different types of Portland cements are obtained by varyingthe proportions of the raw materials, the temperature ofburning and the fineness of grinding. And, in some cases, byintergrinding the clinker with other recognised materials suchas PFA (pulverished-fuel ash) or granulated blastfurnace slag.

Gypsum is added to regulate the setting of concrete whenmixed with water, which would otherwise set much tooquickly for general use.

Certain additives may also be introduced for producing specialcements, e.g., calcium chloride is added in the manufacture ofextra-rapid-hardening-cement.


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Manufacture of Cement

Dry or semi-dry process is used for the harder rocks such aslimestone and shale. The constituent materials are crushedinto powder form and, with a minimum amount of water,passed into an inclined rotating nodulising pan where nodules

are formed. These are known as raw meal.

This is fed into a kiln and thereafter the manufacturingprocess is similar to the wet process although a much shorterkiln is used.

It should be noted that the dry and semi-dry processes aremore energy efficient than the wet process.


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Manufacture of Portland Cement

The grinding of the clinker produces a cementpowder which is still hot and this hot cement isusually allowed to cool before it leaves the

cement works.

A wide range of cement is produced byincorporating other materials during manufacture

including air-entraining cement andcombinations of Portland Cement with mineraladditions.


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Cement

The different cements used for making concreteare finely ground powders and all have theimportant property that when mixed with water,a chemical reaction (hydration) takes place which,

in time, produces a very hard and strong bindingmedium for the aggregate particles.

The following table shows the different types of

concrete. Of these, Portland Cement is the mostwidely used, the others being used whereconcretes with special properties are required.


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Basic Chemistry of Cement

All Portland Cements contain the same active

compoundsonly the proportion of each is

different:

Tricalcium silicate, C3S3CaO.SiO2

Dicalcium silicate, C2S2CaO.SiO2

Tricalcium aluminate, C3A3CaO.Al2O3

Tetracalcium aluminoferrite, C4AF4CaO.Al2O3.Fe2O3


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Chemical Composition The calculation of the potential composition

of Portland cement is based on the work of RH Bogue and others.

The Bogue Composition:

C3S = 4.07(CaO)7.60(SiO2)6.72(Al2O3)1.43(Fe2O3)2.85(SO3)

C2S = 2.87 (SiO2)0.754(3CaO.SiO2)

C3

A = 2.65(Al2

O3

)1.69(Fe2

O3

)

C4AF = 3.04(Fe2O3)

Terms in bracket represent the percentage ofthe given oxide in the total weight of cement


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Chemical Composition It is to be noted that the Bogue composition

underestimates the C3S content (andoverestimates the C2S) because other oxidesreplace some of the CaO in C3S

Three minor compounds of cementamountingto not more than a few % of the weight of cement:

MgO, TiO2, Mn2O3, K2O and Na2O

The oxides of potassium and sodium, known as the

alkalis, have been found to react with someaggregates, the products causing disintegration ofconcrete; they also affect the rate of gain ofstrength of cement


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Role in Cement

However, C3A is beneficial in the manufactureof cement in that it facilitates the combinationof lime and silica.

C4AF is also present in small quantities, andcompared with the other components, it doesnot affect the behaviour significantly.

However, it reacts with gypsum to formcalcium sulphoferrite and its presence mayaccelerate the hydration of the silicates


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Main Compounds in Portland Cement


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Hydration of Cement

The two silicates, C3S and C2S, which are the moststable of these compounds, together form 70 to80 percent of the constituents in the cement andcontribute most to the physical properties of

concrete. When cement comes into contact with water, C3S

begins to hydrate rapidly, generating aconsiderable amount of heat and making a

significant contribution to the development ofthe early strength, particularly during the first 14days.


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Hydration of Cement

In contrast C2S, which hydrates slowly and is

mainly responsible for the development in

strength after about 7 days, may be active for a

considerable period of time. It is generally believed that cements rich in C2S

result in a greater resistance to chemical attack

and a smaller drying shrinkage than do otherPortland cements.

The contents of C3S and C2S are interdependent.


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Hydration of Cement

The hydration of C3A is extremely exothermicand takes place very quickly, producing littleincrease in strength after about 24 hours.

Of the four principal compounds, tricalcium

aluminate, C3A, is the least stable andcements containing more than 10% of thiscompound produce concretes which areparticularly susceptible to sulphate attack.

Tetracalcium aluminoferrite, C4AF, is of lessimportance than the other three compoundswhen considering the properties of hardenedcement mortars or concrete.


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Hydration of Cement

There are two ways in which compoundspresent in cement may react with water:

True reaction of hydration: direct addition of

water molecules onto the compounds Hydrolysis: breaking down of water molecules

into hydrogen and hydroxyl ions and reaction ofthese ions with the compounds of cement

It is convenient, however, to apply the termhydration to all reactions of cement withwater


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Hydration of Cement

Le Chatelier observed that the products ofhydration of cement are chemically the same asthe products of hydration of the individualcompounds under similar conditions - later

confirmed by Steinour (1952), and Bogue andLerch (1934).

The products of hydration of cement have a lowsolubility in waterindicated by the stability of

hardened cement paste in contact with water The hydrated cement bonds firmly to the

unreacted cement


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Hydration of Cement Main hydrates are:

calcium silicate hydrates denoted as C-S-H(previously referred to as tobermorite gel), and

tricalcium aluminate hydrate. C4AF believed tohydrate to tricalcium aluminate hydrate and an

amorphous phase Hydration of silicates (by hydrolysis):

2C3S + 6H C3S2H3+ 3Ca(OH)2

2C2S + 4H C3S2H3+ Ca(OH)2Product of hydration is themicrocrystalline hydrate

C3S2H3and some lime separating out as crystallineCa(OH)2


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Hydration of Cement

The hydration of C3S does not proceed at aconstant rate. There is an initial rapid release if calcium hydroxide

into the solution leaving an outer layer of calciumsilicate hydrate.

This is followed by a dormant period during whichlittle hydration takes place (C-S-H forms a coating onC3S)

Eventually the coating ruptures because of thepressure of the products of hydration, and hydration

speeds up again. Further slowing down: diffusion becomes the

controlling factor


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Hydration of Cement The hydration of C3A with water is very violent

and leads to immediate stiffening of the paste(flash set). Even with addition of gypsum, therate of reaction is quicker than that of calcium

silicates Gypsum (CaSO4.2H2O) reacts with C3A to form

insoluble calcium sulphoaluminate, and eventuallya tricalcium aluminate hydrate is formed

There is some evidence that the hydration of C3Ais retarded by Ca(OH)2liberated by the hydrolysisof C3S (product forms a protective coating on thesurface of unhydrated grains of C3A)


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Addition of Gypsum The amount of gypsum added to the clinker is

crucial and needs careful control as an excessleads to expansion and consequent disruptionof the set cement paste.

Amount depends on: C3A content and indirectly on the fineness of

cement

Alkali content of cement

Optimum gypsum content is determined onthe basis of the heat of hydration, so that adesirable rate of early reaction occurs andlittle C3A is available for reaction after all thegypsum has combined


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Addition of Gypsum Amount of gypsum expressed as weight of SO3

present - limited by BS12:1978 to 2.5% when C3A content is not more than 5%

3% when C3A content exceeds 5%

The presence of dehydrated products of gypsum

in cement any lead tofalse set: When gypsum is interground with too hot clinker

formation of hemihydrate (CaSO4. H2O) oranhydrite (CaSO4) are formed

When cement containing these dehydrated productsare mixed with water, these hydrate to form gypsum.

Thus plaster set takes place with stiffening of thepaste


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Heat of Hydration

The hydration of cement is exothermic (like manychemical reactions)

Heat of hydration defined as the quantity of heat(J) per gram of unhydrated cement, evolved upon

complete hydration at a given temperature For usual range of Portland cements:

of total heat liberated between 1 and 3 days

in 7 days, and

90% in 6 months

The heat of hydration depends on the chemicalcomposition of the cement


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Heat of Hydration Total heat of hydration sum of heats of hydration of

individual pure compounds, when their respectiveproportions by mass are hydrated separately. Typical values

are:

Compound Heat of Hydration (J/g)

Tricalcium silicate 502

Dicalcium silicate 260

Tricalcium aluminate 867

Tetracalcium aluminoferrite 419

Therefore reducing the proportions of C3A and C3S will

result in a decrease in the heat of hydration


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Heat of Hydration

Points to be noted:

There is no relation between the heat of hydrationand the cementing properties of the individual

compounds. The fineness of cement affects the rate of heat

development but not the total amount of heatliberated.

The total amount of heat liberated can becontrolled in a concrete mix by varying thequantity of cement added (the richness)


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Strength Development

There are two classical theories of hardening orgain of strength of cement:

Le Chatelier (1882) states that the products ofhydration of cement have a lower solubility than theoriginal compounds, so that the hydrates precipitatesfrom a supersaturated solution. The precipitate is inthe form of interlaced elongated crystals with highcohesive and adhesive properties

Colloidal theory by Michaelis (1893) states that thecrystalline aluminate, sulpho-aluminate and hydroxideof calcium give the initial strength.


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(Michaelis contd) The lime-saturated water thenattacks the silicates and forms a hydrated calciumsilicate which, being almost insoluble, forms agelatinous mass. This hardens gradually as a result of

water loss (either by external drying or hydration ofinner unhydrated cement grains).

Modern knowledge has shown that both theoriesare plausible and the colloidal behaviour is a

function of the size of the surface area ofparticles rather than the non-regularity of theirinternal structure


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In the case of Portland cement:

When mixed with large amount of water: cementproduces within a few hours a solution

supersaturated with calcium hydroxide andcontaining concentrations of C-S-H in metastablecondition. This precipitates in agreement with LeChateliers theory

The subsequent hardening may be due to thewithdrawal of water from the hydrated material aspostulated by Michaelis


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Strength Development of Pure Compounds

0

10

20

30

40

50

60

70

80

0 60 120 180 240 300 360

Age (days)

CompressiveStre

ngth(MPa)

C3S

C2S

C3A

C4AF


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Convenient rule to strength development:

C3S contributes most to strength development

during first 28 days

C2S influences later gain in strength

After about 1 year, both compounds contribute

approximately equally to strength


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Setting of Concrete Term used to describe the stiffening of the

cement paste In strict terms, setting refers to a change in

state from fluid to rigid

During setting, there is some strength gainbut, for practical purposes it is convenient todistinguish between setting and hardening

Hardening refers to the gain of strength of a

set cement paste Two stages are used to describe the state

reached by cement on hydration: initial setand final set


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Setting

Caused by the selective hydration of cementcompounds

First two compounds to react are C3A and C3S. With the addition of gypsum, C3S sets first when

mixed with water and exhibits an initial set C2S stiffens in a more gradual manner The setting process is accompanied by

temperature changes in the cement paste

Initial set corresponds to a rapid rise intemperature Final set corresponds to the peak temperature

i


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Setting False set: abnormal premature stiffening of

cement within minutes of mixing with water No appreciable heat evolved and remixing of the

cement paste without the addition of waterrestores plasticity of the paste without a loss of

strength Potential causes:

Hydration of dehydrated products of gypsum (asdetailed previously)

Presence of alkalis in cement: these may carbonateduring storage and then on hydration, the alkalicarbonates react with Ca(OH)2liberated by hydrolysisof C3S to form CaCO3. This precipitates and induces arigidity of the paste


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Setting

Activation of C3S by aeration at moderately highhumidities: water is adsorbed on the grains of cementand these freshly activated surfaces can combine veryrapidly with more water during mixing producing false

set Flash set: takes place in cement with insufficient

gypsum to control the rapid reaction of C3A withwater.

Can only be overcome by adding more water and re-agitating the mix.

The addition of water causes a reduction in strength


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Tests on Cement

Tests undertaken to ensure that the cementproduced is of desired quality and conforms tonational or international standards:

Chemical composition (beyond scope of lecture) Fineness

Consistence of standard paste

Setting time

Soundness

Strength of cement


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Fineness of cement

The rate of hydration depends on the fineness ofcement particles and for rapid development ofstrength, a high fineness is necessary

However, increased fineness involved increased

costs: For grinding

Increased gypsum requirement

Effect on other properties, such as workability of fresh

concrete and long-term behaviour Fineness measured by the determination of the

specific surface (in m2/kg)


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Fineness

Direct approach: measure particle sizedistribution by sedimentation or elutriation

The method is based on Stokes law giving theterminal velocity of fall under gravity of a

spherical particle in a fluid medium Two methods: Wagner turbidimeter: concentration of particles

under suspension at a given level in kerosene isdetermined under a beam of light

Air permeability (Lea and Nurse) method:measurement of the pressure drop when dry air flowsat a constant velocity through a bed of cement ofknown porosity and thickness


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Consistence of standard paste

Neat cement of standard paste consistence has tobe used in setting time and soundness tests

Therefore it is necessary to determine the watercontent, for any given cement, which will produce

a paste of standard consistence Consistence is measured by the Vicat apparatus

Standard consistence is the point at which aplunger penetrates to a point 5 1mm from thebottom of the mould.

Water content measured are typically in therange of 26 to 33%

S tti ti


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Setting time

The Vicat apparatus is also used for this test

Initial set is the time since mixing with waterwhen the paste has stiffened sufficiently forthe needle to penetrate to a point 5 1mmfrom the bottom of the mould

Minimum time of 45 minutes prescribed forordinary and rapid-hardening Portland cement

Final set is the time, since mixing with water,at which a specific needle makes animpression on the surface of the cement pastein a specified manner

This is required by BS not to exceed 10 hours

for ordinary, rapid hardening Portland cement


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Setting time

Approximate relationship between initial andfinal setting times:

Final time (min) = 90 + 1.2 x initial time (min)

Since setting is affected by temperature andhumidity, certain values are prescribed by BS ascontrol namely:

Mixing at room temperature of 20 2oand min.

relative humidity of 65% Curing or storing at same temperature and max.

relative humidity of 90%


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Soundness

Cement paste is required to have volume stability,i.e. it should not undergo large volume changeonce it has set

Disruptive expansion of cement may occur due to

reactions with free lime, magnesia and calciumsulphatecements exhibiting these types ofexpansion are classified as unsound

Different tests prescribed for detecting different

deleterious materials present; Le Chateliers accelerated test (BS): detecting

unsoundness due to free lime only


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Soundness

Autoclave test (ASTM): sensitive to both magnesia

and free lime

No test available for detection of calcium sulphate

but its content can be easily determined bychemical analysis


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Strength

Strength tests are not made on neat cement

paste because of difficulties in obtaining good

specimens and in testing with a consequent

large variability of test results

Tests are done on cement-sand mortar or

concrete of prescribed proportions

Compressive strength tests are more common


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Types of Portland cement The different types exhibit different properties

when mixed with water Ordinary Portland Cement (OPC):

Most common cement

Used where there is no exposure to sulphates inthe soil or groundwater

Typical fineness of about 275m2/kg

Rapid-hardening Portland Cement

Similar to OPC Develops strength rapidly; has a higher C3S

content (up to 70%) and higher fineness(minimum of 325m2/kg)


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Types of Portland Cement

Used principally when formwork need to be removedearly

It should not be used in mass concrete construction orin large structural sections

Same setting time as OPC

Marginally higher cost than OPC

Special rapid-hardening PC Highly rapid-hardening

Fineness ranging from 700 to 900m2

/kg Higher gypsum content

Used for early prestressing and urgent repairs


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Low-heat Portland cement: It has a low heat of hydration: 250J/g at age of 7 days

and 290J/g at 28 days

Developed in the US for use in large gravity dams

Slower development of strength than OPC due tolower content of C3S and C3A

Portlandpozzolan cement of low-heat variety

Sulphate-resisting cement:

Low C3A content (limited to 3.5%) so as to avoidsulphate attack from outside the concrete

Minimum fineness of 250m2/kg


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Types of Portland Cement Has the advantage of being relatively low heat

(not much higher than low-heat cement) Higher cost due to special composition of raw

materials, therefore not for general use

Portland blast-furnace cement

Produced when intergrinding or blending PCclinker with ground granulated blastfurnace slag

Hydration is initiated when lime liberated in the

hydration of PC provides the correct alkalinity Used in various proportions (refer to table earlier)

Similar to OPC wrt fineness, setting times andsoundness


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However, lower early strength than OPC with similar

later strength

Typical uses in mass concrete due to lower heat of

hydration and in sea-water construction due to better

sulphate resistance (lower C3A content) than OPC

White or coloured PC

Use for architectural purposes

Made with china clay and requires special precautionsduring grinding

Therefore it is costlytwice as much as OPC


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Portland-pozzolan cement Use of PFA or fly asha pozzolan (a siliceous or

siliceous/aluminous material, which when in finelydivided form and in the presence of moisture, reactwith lime at ordinary temperature to form compounds

with cementitious properties) Slow gain in strength and require curing over a

comparatively long period High long-term strength Use of PFA improves sulphate resistance

Pozzolans may often be cheaper than PC Main advantage lies in slow hydration and therefore

low rate of heat development

2 - hydraulic binders(1)

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