cement knowledge

8/10/2019 Cement Knowledge

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UNDERSTANDING CEMENT

Clinker: compositional parameters

Parameters based on the oxide composition are very useful in describing clinkercharacteristics. The following parameters are widely used (chemical formulae representweight percentages):

Lime Saturation Factor

The L! is a ratio of "a# to the other three main oxides. $pplied to clinker% it iscalculated as:

L!&"a#'(.i#* +.$l#,* -./!e#,)

#ften% this is referred to as a percentage and therefore multiplied by +--.

The L! controls the ratio of alite to belite in the clinker. $ clinker with a higher L! willhave a higher proportion of alite to belite than will a clinker with a low L!.

Typical L! values in modern clinkers are -.01-.0% or 02102.

3alues above +.- indicate that free lime is likely to be present in the clinker. This isbecause% in principle% at L!&+.- all the free lime should have combined with belite toform alite. 4f the L! is higher than +.-% the surplus free lime has nothing with which to

combine and will remain as free lime.

4n practice% the mixing of raw materials is never perfect and there are always regionswithin the clinker where the L! is locally a little above% or a little below% the target forthe clinker as a whole. This means that there is almost always some residual free lime%even where the L! is considerably below +.-. 4t also means that to convert virtually allthe belite to alite% an L! slightly above +.- is needed.

The L! calculation can also be applied to Portland cementcontaining clinker andgypsum if (-.5 x #,) is subtracted from the "a# content. (67: This calculation (ie: -.5x #,) does not account for sulfate present as clinker sulfate in the form of potassiumand sodium sulfates) and this will introduce a slight error. 8ore particularly% it does not

account for fine limestone or other material such as slag or fly ash in the cement. 4fthese materials are present% calculation of the original clinker L! becomes morecomplex. Limestone can be 9uantified by measuring the "#content and the formulaadusted accordingly% but if slag or fly ash are present% calculation of the original clinker

L! may not be conveniently practicable.)

Silica Ratio (SR

The silica ratio (also known as the ilica 8odulus) is defined as:


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; & i#'($l#,* !e#,)

$ high silica ratio means that more calcium silicates are present in the clinker and lessaluminate and ferrite. ; is typically between .- and ,.-.

The silica ratio is sometimes called the .

Clinker: reactions in t!e kiln

This page reviews the reactions which take place as the feed passes through the kiln.

The blended% milled raw materials go to a silo and from there to the kiln.

The reactions which take place in the kiln can be considered under three broadheadings:

?ecomposition of raw materials 1 reactions at temperatures up to about +,-- ".

$lite formation and other reactions at +,-- "1+>/- " in the burning @one.

"ooling of the clinker.

Decomposition o" ra# materials $ reactions at temperatures up to a%out&' C

This includes:

i. Aater evaporation in the raw feed% if any.ii. Loss of carbon dioxide from the limestone (ie: calcining).

iii. ?ecomposition of the siliceous and aluminosilicate fractions of the feed.


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iv. !ormation of a sulfate melt phase.

The decomposition products react with lime to form intermediate compounds which in

turn form other compounds as clinkering proceeds.

)ater e*aporation

4n wet1process kilns% and their derivatives% water must first be driven off. 4n a wet1

process kiln% calcining takes place after the water has been driven off% about a third of

the way down the kiln. 4n the more modern pre1calciner kilns% the feed is calcined prior

to entering the kiln.

Calcinin+

4n isolation% decarbonation of calcium carbonate at + atmosphere takes place at 0> ".

This temperature is reduced to /-- "1-- " if the reaction takes place in contact with

9uart@ or the decomposition products of clay minerals% which react with the calcium

oxide as it forms.

4n a wet1process or preheater system without a pre1calciner% most of the calcination

takes place in the rotary kiln within a moving mass of feed. This situation is not ideal

for calcination because heat transfer has to take place through a large mass of material

and "#has to escape outwards as heat moves inwards.

$ pre1calciner calcines the raw material much more efficiently than a wet1process kiln.

;aw meal is dispersed in the hot gas and calcination takes place in seconds% rather than

the half an hour or so inside a kiln at the same temperature.

Formation o" earl, an- interme-iate compoun-s

?uring calcination% the lime produced starts to react with other components of the raw

feed. The initial silicate product is belite. ome calcium aluminate and ferrite phases

also start to form.

$ number of phases are formed in the clinker feed before the burning @one proper is

reached. These intermediate phases dissociate in the burning @one and are not

therefore found in clinker but assist in forming the final clinker minerals.

Sul"ate melt p!ase

$t intermediate temperatures% sulfates combined with calcium and alkalis form a li9uid

phase. This is separate from the aluminate and aluminoferrite1based li9uid formed in

the burning @one 1 the two li9uids are immiscible.


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$s with the main li9uid phase% the sulfate li9uid phase contributes to ion mobility andpromotes combination.

Alite "ormation an- ot!er reactions at&' C$&./ C in t!e %urnin+ 0one

4n the burning @one% above about +,-- "% reactions take place 9uickly. The clinker is in

the burning @one for perhaps +-1- minutes but in this time a lot happens:

i. The proportion of clinker li9uid increases and nodules form.ii. 4ntermediate phases dissociate to form li9uid and belite.

iii. 7elite reacts with free lime to form alite.

iv. ome volatile phases evaporate.

Clinker li1ui- an- no-ule "ormation

$bove about +,-- " the proportion of li9uid starts to increase 1 by +>/- "% perhaps -1

,-2 of the mix is li9uid. The li9uid forms from melting ferrite and aluminate phases

and some belite. The li9uid content is more than the sum of the aluminate and ferrite

phases in the cooled clinker because of the dissolved lime and silica.

The additional li9uid causes coalescence of clinker particles% leading to the formation of

nodules.

Dissociation o" interme-iate p!ases

The intermediate phases dissociate to form mainly aluminate phase% which then

becomes part of the li9uid% and belite.

Alite "ormation$lite forms by the transition of some of the belite to alite and also directly from freelime and silica to alite. These reactions occur rapidly once the clinker temperature is

above about +>-- ".

E*aporation o" *olatiles3olatile phases in the cement kiln are principally alkali sulfates% with a much smallerproportion of alkali chlorides. $s the part1burned feed approaches the burning @one%these volatile phases are in li9uid form and a proportion volatili@es% the remainderpassing out of the kiln in the clinker as inclusions within the pores.


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The volatili@ed material passes back down the kiln% where it condenses on the relativelycool incoming feed. 4t again becomes part of the sulfate melt phase% promotingreactions% and is once again carried within the clinker towards the burning @one.

This recirculating load of alkali and sulfate can occasionally become excessively high.Large 9uantities of condensing volatiles can then cause blockages in the kiln or in the

preheater as the condensed li9uid sticks feed particles together% forming accretions.

Coolin+ o" t!e clinker

$s the clinker cools% the main li9uid phase crystalli@es to form aluminate phase% ferrite

and a little belite.

!ast cooling of clinker is advantageous 1 it makes for more hydraulically1reactivesilicates and lots of small% intergrown% aluminate and ferrite crystals.

low cooling gives less hydraulically1reactive silicates and produces coarse crystals ofaluminate and ferrite 1 over1large aluminate crystals can lead to erratic cement setting

characteristics. 3ery slow cooling allows alite to decompose to belite and free lime.

Cement kiln re"ractories

;efractories play a critical% if unseen% role in both the rotary kiln lining% and the lining ofthe high volume static e9uipment areas that comprise a modern pre1calciner kilnsystem.

The durability of the rotary kiln brick lining generally determines the duration of a kilnoperating campaign. Ahilst great efforts are made to ensure appropriate refractory

brick materials selection% installation% deployment (@oning) and the optimisation of thekiln light1up% it is primarily the nature and stability of the subse9uent kiln operation that

influences rotary kiln lining life.

$ large number of kilns now use alternative fuels. 8any of them substitute these

alternative fuels through the main kiln burner as well as in the pre1heater ' pre1calcinerB the chemical and heat1release changes caused by inevitable short1term

fluctuations in the fuel(s) mix has a maor influence on the thermal% coating andatmosphere conditions existing through the rotary kiln into the pre1calciner.

Cxpert systems are available that help limit the influence of fuel mix fluctuations on theflame shape% but not many cement kilns are e9uipped with such systems."onse9uently% the main influence of the use of alternative fuels is often on clinkerchemistry due to the different ash compositions of individual alternative fuels.

uch fuel variations may produce any or all of the following :

$lternative fuel fall through ( from the pre1heater' pre1calciner )


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$dverse minor phase re1circulation effects ( gasriser )

Cxcessive build1up or clogging ( kiln inlet or gasriser )

Cxcessive coating in the rotary kiln ( ringformation )

;educing conditions or damaging atmosphere effects( rotary kiln )

Thermal wash1outs ( rotary kiln )

!luctuations in flame length ( rotary kiln )

?estabilisation and stripping of clinker coating (rotary kiln )

2ortlan- cement clinker $

o*er*ie#

$lite or tricalcium silicate or ",

7elite% or dicalcium silicate or "

Tricalcium aluminate (or the Daluminate phaseD) or ",$

"alcium alumino1ferrite (or the


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microscope image). 8ost of the nodule is alite (light grey) 1 some clusters ofbelite are visible (arrowed). $luminate and ferrite are present but not visible atthis relatively low magnification.

6odules range in si@e from +mm to /mm or more and are composed mainly of

calcium silicates% typically 5-21-2. The strength of concrete is mainly due tothe reaction of these calcium silicates with water.

Portland cement clinker contains four main minerals:

Alite:approximately tricalcium silicate (typically about /2 of the total)

3elite:approximately dicalcium silicate (typically about +/2 of the total)

Aluminate:very approximately tricalcium aluminate (typically about 52 of the

total)

Ferrite:very approximately tetracalcium aluminoferrite (typically about 2 ofthe total)

The balance is made up of alkali sulfates and minor impurities. The typical

mineral contents shown are subect to wide variation.

#ptical

microscope image (polished section) of nodule. 7rown crystals are alite% bluecrystals are belite% bright interstitial material is mainly ferrite% with small darkinclusions of aluminate. Frey material is the epoxy resin used to make thespecimen. 67: $lite is not actually brown and belite is not actually blue 1 theyappear brown and blue here because the polished section has been etched to

show the crystals more clearly.


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$ clinker chemical analysis is normally given in oxide form 1 a typical examplemight be:

E4ample o" a t,pical clinker anal,sis (o4i-e #ei+!t56

Si78 Al87' Fe87' Ca7 M+7 987 Na87 S7' L7I IR Total

+./ /. . . +.- -. -. +.- +./ -./ 0.0

!ree lime & +.-2 "a#

7alance is typically due to small amounts of oxides of titanium% manganese%phosphorus and chromium.

!rom the chemical analysis% the 9uantity of each of the four main minerals canbe calculated using the D7ogueD calculation (click on the link below for more

information.)

Cement !,-ration

7y the process of hydration (reaction with water) Portland cement mixed with sandgravel and water produces the synthetic rock we call concrete. "oncrete is as essentiala part of the modern world as are electricity or computers.

#ther pages on this web site describe how P" is made and what is in it. Gere% we willdiscuss what happens when it is mixed with water.

This page is a short introduction. The Enderstanding "ement e1book contains a moredetailed description of the hydration process and the hydration products.

"linker is anhydrous (without water) having come from a hot kiln. "ement powder isalso anhydrous if we ignore the small amount of water in any gypsum added at theclinker grinding stage.

The reaction with water is termed HhydrationH. This involves many different reactions%often occurring at the same time. $s the reactions proceed% the products of thehydration process gradually bond together the individual sand and gravel particles% andother components of the concrete% to form a solid mass.

T!e !,-ration process: reactions

4n the anhydrous state% four main types of minerals are normally present: alite% belite%

aluminate (",$) and a ferrite phase (">$!). !or more information on the composition of

clinker% see the clinker pages. $lso present are small amounts of clinker sulfate (sulfates


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of sodium% potassium and calcium) and also gypsum% which was added when the clinker

was ground up to produce the familiar grey powder.

Ahen water is added% the reactions which occur are mostly exothermic% that is% thereactions generate heat. Ae can get an indication of the rate at which the minerals are

reacting by monitoring the rate at which heat is evolved using a techni9ue calledconduction calorimetry. $n illustrative example of the heat evolution curve produced isshown below.

Three principal reactions occur:

$lmost immediately on adding water some of the clinker sulphates and gypsum dissolve

producing an alkaline% sulfate1rich% solution.

oon after mixing% the (",$) phase (the most reactive of the four main clinker minerals)

reacts with the water to form an aluminate1rich gel (tage 4 on the heat evolution curveabove). The gel reacts with sulfate in solution to form small rod1like crystals of

ettringite. (",$) reaction is with water is strongly exothermic but does not last long%typically only a few minutes% and is followed by a period of a few hours of relatively low

heat evolution. This is called the dormant% or induction period (tage 44).


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The first part of the dormant period% up to perhaps half1way through% corresponds towhen concrete can be placed. $s the dormant period progresses% the paste becomestoo stiff to be workable.

$t the end of the dormant period% the alite and belite in the cement start to react% withthe formation of calcium silicate hydrate and calcium hydroxide. This corresponds to the

main period of hydration (tage 444)% during which time concrete strengths increase.The individual grains react from the surface inwards% and the anhydrous particlesbecome smaller. (",$) hydration also continues% as fresh crystals become accessible towater.

The period of maximum heat evolution occurs typically between about +- and - hoursafter mixing and then gradually tails off. 4n a mix containing P" only% most of thestrength gain has occurred within about a month. Ahere P" has been partly1replacedby other materials% such as fly ash% strength growth may occur more slowly andcontinue for several months or even a year.

!errite reaction also starts 9uickly as water is added% but then slows down% probablybecause a layer of iron hydroxide gel forms% coating the ferrite and acting as a barrier%preventing further reaction.

,-ration pro-ucts

The products of the reaction between cement and water are termed Hhydration

products.H 4n concrete (or mortar or other cementitious materials) there are typically

four main types:

Calcium silicate !,-rate:this is the main reaction product and is the main source ofconcrete strength. 4t is often abbreviated% using cement chemistsD notation% to H"11G%H

the dashes indicating that no strict ratio of i#to "a# is inferred. The i'"a ratio issomewhat variable but typically approximately -.>/1-./- in hydrated Portland cement

but up to perhaps about -. if slag or fly ash or microsilica is present% depending on theproportions.

Calcium !,-ro4i-e1 "a(#G): often abbreviated to D"G.D "G is formed mainly fromalite hydration. $lite has a "a:i ratio of ,:+ and "11G has a "a'i ratio of

approximately :+% so excess lime is available to produce "G.

AFm an- AFt p!ases:these are two groups of minerals that occur in cement% and

elsewhere. #ne of the most common $!m phases in hydrated cement is monosulfate.7y far the most common $!t phase in hydrated cement is ettringite. The general

definitions of these phases are somewhat technical% but for example% ettringite is an $!tphase because it contains three (t1tri) molecules of anhydrite when written as


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",$.,"a#>.,G# and monosulfate is an $!m phase because it contains one (m1mono)molecule of anhydrite when written as ",$."a#>.+G#.

The most common $!t and $!m phases in hydrated cement are:

Ettrin+ite:ettringite is present as rod1like crystals in the early stages of reaction orsometimes as massive growths filling pores or cracks in mature concrete or mortar. Thechemical formula for ettringite is I"a,$l(#G).+G#J.G#J or% mixing notations%",$.,"a#>.,G#.

Monosul"ate:monosulfate tends to occur in the later stages of hydration% a day or twoafter mixing. The chemical formula for monosulfate is ",$."a#>.+G#. 6ote that bothettringite and monosulfate are compounds of ",$% "a#>(anhydrite) and water% indifferent proportions.

Monocar%onate:the presence of fine limestone% whether interground with the cementor present as fine limestone aggregate% is likely to produce monocarbonate(",$."a"#,.++G#) as some of the limestone reacts with the cement pore fluid.

#ther $!m phases that may be present are hemicarbonate% hydroxy1$!m and !riedelDssalt.

ome important points to note about $!m and $!t phases are that:

They contain a lot of water% especially $!t 1 principally ettringite in the context of

cement. $!m contains a higher ratio of aluminium'calcium compared with $!t.

The aluminium can be partly1replaced by iron in both $!m and $!t phases.

The sulfate ion in monosulfate $!m phase can be replaced by other anionsB a

one1for1one substitution if the anion is doubly1charged (eg: carbonate% "#1) or

one1for1two if the substituent anion is singly1charged (eg: hydroxyl% #G1or

chloride% "l

1

).

The sulfate in ettringite can be replaced by carbonate or% probably% partly

replaced by two hydroxyl ions% although in practice neither of these is oftenobserved.

4n a concrete made from cement containing ust clinker and gypsum% ettringite formsearly on in the hydration process% but gradually replaced by monosulfate. This isbecause the ratio of available alumina to sulfate increases with continued cement


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hydrationB on first contact with water% most of the sulfate is readily available to dissolve%but much of the ",$ is contained inside cement grains with no initial access to water."ontinued hydration gradually releases alumina and the proportion of ettringitedecreases as that of monosulfate increases.

4f there is eventually more alumina than sulfate available% all the sulfate will be as

monosulfate% with the additional alumina present as hydroxyl1substituted $!m phase(hydroxy1$!m). 4f there is a small excess of sulfate% the cement paste will contain amixture of monosulfate and ettringite. Aith increasing available sulfate% there will bemore ettringite and less monosulfate% and at even higher levels of sulfate there will beettringite and gypsum.

4f fine limestone is present% carbonate ions become available as some of the limestonereacts. The carbonate displaces sulfate or hydroxyl in $!mB the proportion ofmonosulfate or hydroxy1$!m therefore decreases as the proportion of monocarbonateincreases. The displaced sulfate typically combines with remaining monosulfate to form

ettringite% but if any hydroxy1$!m is present% the sulfate will displace the hydroxyl ionsto form more monosulfate. The key here is the balance between available alumina on

the one hand% and carbonate and sulfate on the other.

,-ro+arnet:hydrogarnet forms mainly as the result of ferrite or ",$ hydration.

Gydrogarnets have a range of compositions% of which ",$Gis the most common phaseforming from normal cement hydration and then only in small amounts. $ wider range

of hydrogarnet compositions can be found in autoclaved cement products.

T!e a%o*e t!ree parameters are t!ose most commonl, use-6 A "ourt!;

t!e


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cement with which most people are familiar. #ther types of Portland cement includeAhite Portland "ement and ulfate ;esisting Portland "ement (;P").

Clinker:Portland cement is made by grinding clinker and a little added gypsum. "linkeris a nodular material before it is ground up. The nodules can be anything from +mm to/mm in diameter.

Cement:Esually taken to mean Portland "ement% but could mean any other type ofcement% depending on the context.

?ifferencebetween Cement(left)and clinker(right). Thecoin is a EKone1pound coin about

,mm across.

A++re+ate:"obbles%

pebbles% gravel% sand and silt 1 the DrockD component of all particle si@es in concrete.

Concrete:ynthetic rock made using cement (usually% but not necessarily% Portland

cement) mixed with aggregate and water.

Mortar:8ixture of cement and fine aggregate% mainly sand. Esed typically to bond

bricks and building stone.

Grout:8ixture of cement (possibly of various types) and other fine material such as

fine sand. Esed in a wide range of applications from filling the gaps between bathroomtiles to oil wells.

Composite cements:ome types of cement are mixtures of Portland cement withother material% such as blastfurnace slag from iron production and pulverised fuel ashfrom coal1fired electricity power stations. These widely1used mixtures are calledDcompositeD cements.

Non$2ortlan- cements

#f course% there are other types of cement apart from Portland cement. 4mportant

examples include:

"alcium aluminate cements

Lime concrete'mortar

Cxpansive cements


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Calcium aluminate cements (CACs

These may also be termed D"iment !onduD and used to be called Dhigh aluminacements.D They are made from lime or limestone mixed with bauxite (aluminium ore) or

other high1alumina material.

"oncretes made with "$"s develop strength 9uickly and are resistant to chemicalattack. "$"s have a wide range of compositions% mainly with different ratios of lime toaluminaB strictly%


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