Cement Chapter 1

8/11/2019 Cement Chapter 1

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1. Cement ChemistryC E M E N T T E C H N O L O G Y N O T E S 2 0 0 5 2

1 . 1 I N T R O D U C T I O N

1 . 2 R AW M A T E R I A L S

1 . 3 F U E L S

1 . 4 C L I N K E R

1 . 5 G Y P S U M

1 . 6 C E M E N T

1 . 6 . 1 L S F , E T C

1 . 6 . 2 C L I N K E R C O M P O U N D S

1 . 6 . 3 P R I N C I P A L C E M E N T C H A R A C T E R I S T I C S

1 . 7 G Y P S U M - S O L U B L E C A L C I U M S U L P H A TE

1 . 7 . 1 I N T R O D U C T I O N

1 . 7 . 2 D I S S O L U T I O N O F C a S O4

1 . 7 . 3 O P T I M I S AT I O N O F S O L U B L E C A L C I U M S U L P H A T E

1 . 7 . 4 S L U M P L O S S / R E T E N T I O N

contents chapter 2
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1. CEMENT CHEMISTRYC E M E N T T E C H N O L O G Y N O T E S 2 0 0 5 3

1.1 INTRODUCTION

The production of cement can be considered as a chemicalprocess not so different to other chemicals such as SodiumHydroxide and Calcium Chloride. However there are two majordifferences:

- Selling Price- Product Complexity

Portland Cement can contain approximately 10 chemicals/mineralsand is relatively impure when compared to most "chemicals".Part of this impurity arises from the "naturally occurring"nature of the key raw materials, but also from the low sellingprice, which is significantly lower than for most "Chemicals".The impurities can vary quite considerably, both from one plantto another but also within any one plant, and these can have animportant influence on the ultimate cement behaviour.

However, customers of cement place high demands on theperformance of the cement, expecting it to react in a predictablemanner with respect to its handling, workability, setting,hardening and strength development.

The first patent for Portland Cement was granted in 1824. Cements,with a chemistry similar to today's cements, were not reallyproduced until the late 1800's. Shaft or bottle kilns were used.

Rotary kilns were first used around 1900. These becameoperated in a semi-dry manner around the 1950's and themodern dry process kiln appeared in the 1960's and 1970's withthe more efficient pre-calciner process appearing a little later.Wet, semi-wet, semi-dry, dry and pre-calciner process kilns allremain in use throughout the world today. (See Section 2).There are also a significant number of vertical shaft kilns still inoperation, particularly in China.

Portland Cement clinker is manufactured from a calcareousmaterial (e.g. limestone, chalk) and an argillaceous material (e.g.shale, clay). The feed material is finely ground and carefullymixed and heated to a very high temperature (~1500C). Duringthis heating some 25% of the mixture becomes liquid. Thisassists in the chemical reactions and bonds the particles together

in the tumbling action of the rotary kiln to form the well-knowncharacteristic cl inker.

Chemically, clinker consists of a mixture of compounds, whichare made up of various molecules and elements. The mostimportant elements in cement chemistry are shown in Figure 1together with their atomic weights. The most relevant

compounds in Cement Chemistry are shown in Figure 2.

For simplicity, cement chemists have traditionally used a "short-hand" for chemical symbols and these are also shown in Figure 2.

The chemistry of raw materials, fuels and clinker are discussedin more detail in the following sections.

Element Symbol Atomic Weight

Aluminium A1 26.98

Calcium Ca 40.08

Carbon C 12.01

Hydrogen H 1.01

Iron Fe 55.85

Magnesium Mg 24.31

Manganese Mn 54.94

Oxygen O 16.00

Phosphorus P 30.97

Potassium K 39.10

Silicon Si 28.09

Sodium Na 23.00

Sulphur S 32.06

Titanium Ti 47.90

Figure 1. Principal Elements. Figure 2. Principal Compounds.

Compound Formula Shorthand Molecular

Weight

Water H2O H 18.02

Carbon Dioxide CO2 C 44.01

Lime (Calcium Oxide) CaO C 56.08

Magnesia MgO M 40.31

Silica SiO2 S 60.09

Titania TiO2 T 79.90

Alumina Al2O3 A 101.96

Ferric Oxide Fe2O3 F 159.70

Phosphorus Pentoxide P2O5 P 141.94

Sulphur Trioxide SO3 S 80.06

Soda Na2O N 62.00

Potash K2O K 94.20

Calc ium Carbonate CaCO3 CC 100.09

Magnesium Carbonate MgCO3 MC 84.32

Sodium Carbonate Na2CO3 NC 106.01

Calcium Sulphate CaSO4 CS 136.14

Potassium Sulphate K2SO4 KS 174.26

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C E M E N T T E C H N O L O G Y N O T E S 2 0 0 5 4

1.2 RAW MATERIALS

We shall see later that the four most important oxides inPortland Cement are lime (CaO), silica (SiO2), alumina (Al2O3)and iron oxide (Fe2O3). That is, in short-hand C, S, A, and F.

The principal source of lime for cement manufacture islimestone or chalk and this constitutes typically some 80% of

the raw material mix.

In practice the actual materials used for the source of CaOcover the complete range of geological forms. However, thedifferences of most practical relevance involve:

- Chemistry (including impurities)- Hardness- Porosity- Crystal Size- Moisture- Location

and of course - Cost

Some of these are discussed further in Section 2.

The proportion of non-calcareous material, i.e. any siliceous orargillaceous material in the limestone will influence the use ofsecondary materials.

The secondary material providing Al2O3, SiO2, Fe2O3 isprincipally an argillaceous material like shale or clay. In generalthis will contain clay minerals, as well as free silica.

However, sometimes, the principal material may containsufficient levels of S, A, F to produce the desired ClinkerChemistry (origin of Ciments Naturel). More often though, thedesired mix maybe achieved with a mix of high and low grade

limestones. That is, high grade having a high level of CaO (say50%) and low levels of Al2O3, SiO2, Fe22O3 and a low gradematerial having a lower level of CaO but high levels of Al22O3,SiO2 and Fe2O3.

When lower grade limestones or secondary materials are usedthe associated levels of impurities, such as sulphur, alkalis andmagnesia can become important.

Thus, in terms of tonnages, the primary and secondary materialare of most importance with regard to location and cost.

However in addition to these, it maybe necessary to makeadjustments to the mix with sources of silica, alumina and ironoxide. These will generally be at low addition levels, but cansignificantly influence the cost of the final raw material mix.

Whilst the fuel is essentially there to provide the energy forclinker formation, any associated ash (particularly since coalremains the main source of fuel) must be taken into accountwhen designing the raw feed mix for a given clinker chemistry.

Some examples of raw materials are shown in Figure 3, whilsttypical chemical analyses for raw material, raw mix, coal ashand clinker are shown in figure 4.

Figure 3. Raw Materials.

1. CEMENT CHEMISTRY

Figure 4. Typical Chemical Analyses ofMaterials.

Raw Material Source of

Limestone or Chalk, CaCO3 CaO

Shale or Clay Al2O3.Fe2O3.SiO2

Iron Oxide Fe2O3

Bauxite Al2O3.Fe2O3

Sand SiO2

Slag CaO (Al2O3.Fe2O3.SiO2)

Raw

Meal

Raw Mill

S 13.2

A 3.4

F 1.9

C 43.0

Coal

Ash

S 51.7

A 26.4

F 9.5

C 1.6

Clinker

S 20.9

A 5.6

F 3.0

C 65.7

Limestone Shale

S 52.8

A 14.2

F 8.7

C 1.0

S 3.3

A 0.7

F 0.2

C 53.2

Kiln


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Increasingly, the cement industry is considering the utilisation ofless traditional materials, which can often contain significantlevels of impurities (with respect to cement manufacture).

For these (and also for the 'traditional' materials to someextent) it is necessary to carefully assess their potential impacton the industry. This may involve impact in the following areas:

- Raw Material Cost- Availability, Quantities- Capital Requirement- Public Environmental Awareness- Impact on Emission Limits- Material Health and Safety- Influence on Flame Characteristics- Influence on Kiln Performance- Influence on Clinker/Cement Properties- Influence on Emissions- Influence on Product Health and Safety

How individual materials or elements actually influence some ofthese parameters can be quite complex, for example, involving

the degree to which elements are directly retained in the clinker.

A list of the potentially problem causing elements is shown inFigure 5, together with their principal concerns (i.e.environmental, health and safety, process operation or productquality). Recommended maximum input levels in grams pertonne of clinker equivalent are also shown.

Figure 5. Limits for Use of Raw Materials and Fuels

1. CEMENT CHEMISTRY

Element As Limit g/t Principal Reason for Limitation/Notes

Antimony Sb 3000 Environmental Emission Impact

Arsenic As 270 Environmental Emission Impact

Beryllium Be 90 Environmental Emission ImpactCadmium Cd 7 Environmental Emission Impact

Chrome Cr 10010

Product Health & SafetyColour (White Cement)

Fluorine F 500 Environmental Emission Impact. Increases strength at low levels (


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1.3 FUELSThe primary requirement for the fuel is to provide the necessaryenergy involved in the clinker manufacturing process. Thistypically amounts to between 700 and 1500 kcal/kg clinker,with some 420 kcals/kg being the theoretical heat requirement,(i.e. the heat required to convert the raw materials into theclinker minerals). The remainder of the heat is essentially

accounted for by removal of moisture, sensible heat of exit gasesand the shell losses.

The current primary fuels for the cement industry arehydrocarbon fuels, their main constituents being carbon andhydrogen. Coal, oil and natural gas are the most common inuse.

The fuel consumption will of course be dependent on the energycontent of the fuel and the energy requirement of the kilnprocess. For example, for a coal of 7300 kcals/kg and a modernpre-calciner kiln requiring 760 kcals/kg the fuel consumptionwill simply be 760/7300 = 10.4%, i.e. 1 tonne of clinker willrequire 0.104 tonnes of coal. Similarly, for a less efficient wet

process of 1400 kcals/kg the fuel consumption would be 19.2%.

Many factors will influence the ultimate choice of fuel for agiven plant. These will include any potential influence on thefuel preparation (e.g. coal grinding), flame characteristics, ashchemistry (hence clinker quality), kiln build-ups (e.g. Sulphurand Chloride contents), process emissions as well as, of course,price, calorific value, supply and consistency. The choice will beinfluenced by many of the parameters discussed for rawmaterials (See Section 1.2 and Figure 5). Some of the relevantparameters for fuels are shown in Figure 6.

Increasingly, the traditional primary fuels are being replaced (atleast partially) by a large range of waste and by-productmaterials, which have significant energy contents. A list,showing examples of these, is shown in Figure 7.

Figure 7. Examples of some Alternative Fuels.

Heavy Fuel Oil, Natural GasPetroleum CokeWaste Solvents

Waste Oils, OrimulsionRefuse Derived Fuel, BiogasTyres, Recycled Car partsRaw Materials (e.g. Oil Shale)Timber WasteStrawImpregnated SawdustHazardous WasteWater Purification Residues

In recent years there has been particular interest in petroleumcoke, waste solvents and used tyres.

However, as with the primary fuels, the use of alternatives willdepend on a range of factors concerning health and safety (boththe material itself and resultant influence on the cement), fuelpreparation, kiln operation, emissions and clinker/cement

quality and cost and availability. As an example, petroleum coke(See Figure 8) often contains important levels of sulphur whilstwaste solvents often contain appreciable levels of chloride.

Figure 8. Petroleum Coke.

Figure 6. Factors Influencing the Choice of Fuel.

Property Influence on

Calorific Value Fuel Consumption

Price Fuel Cost/clinker tonne

Carbon/Hydrogen ratio Flame characteristics

Ash content & chemistryMix proportioning, Clinker quality,Refractories

Hardness Preparation

Volatiles Flame characteristics

Abrasivity Preparation, Handling

Moisture Flowability, Abrasivity, Preparation

Grading, Size Flowability, Preparation

Sulphur, Chlor ide Kiln build-ups, Corros ion

Nitrogen Environment

Vanadium Refractories

Viscosity Handling, atomisation, Flame

C E M E N T T E C H N O L O G Y N O T E S 2 0 0 5 61. CEMENT CHEMISTRY

Most from United States

Coal replacement

limited by the impact

on:

High Sulphur (~5%, Coal


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1.4 CLINKER

In general, Portland Cement Clinker contains four principalchemical compounds, or clinker minerals as they are usuallyreferred to. These are:

Tricalcium SilicateDicalcium SilicateTricalcium Aluminate

Calcium Aluminoferrite

The composition of these together with their short-handnotation mineral name and typical levels are shown in Figure 9.

The silicate minerals are largely responsible for the strengthdevelopment characteristics of Portland Cement. However, thereaction between lime and silica is difficult to achieve, even athigh temperatures. The reaction (Chemical Combination) isfacilitated by the presence of alumina and iron oxide, as theseassist in the formation of a molten flux through which the limeand silica are able to partially dissolve and then react to formC3S and C2S.

The sequence of reaction is illustrated in Figure 10. A typicalfull clinker chemistry is shown in Figure 11.

Clinkers designed for sulphate resistance require materials withlow levels of Al2O3 so that the clinker contains a low (or zero)level of C3A. In general the C3A content will be less then 1%whilst the C4AF will be higher at 15-20%.

For White Cements materials with low levels of Fe2O3 arerequired so that the clinker contains a low (or zero) level ofC4AF. As a consequence the C3A contents will usually be higherat 12-16%, although it is possible to avoid high C 3A by havingvery high total silicates. In addition the clinker has to be

produced with materials low in Mn2O3 and Cr2O3.

The influence of chemistry is discussed in Section 1.6. Howeverclinker properties can also directly influence:

- Materials Handling- Mill Performance- Storage


Figure 9. Clinker Minerals.

Figure 10. Sequence ofFormation of Calcium

Silicates in a Rotary Kiln.

1. CEMENT CHEMISTRY

Mineral Shorthand Name Formula Formula Typical Range

Tricalcium Silicate C3S Alite 3CaO.SiO2 Ca3SiO5 60% 30-70%

Dicalcium Silicate C2S Belite 2CaO.SiO2 Ca2SiO4 20% 5-40%

Tricalcium Aluminate C3A Aluminate 3CaO.Al2O3 Ca3Al2O5 10% 5-15%

Calcium Aluminoferrite C4AF Ferrite 4CaO.Al2O3.Fe2O3 Ca4Al2Fe2O10 8% 5-15%

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It should also be noted that many plants use stored clinker inaddition to the 'fresh' kiln clinker. Sometimes these maybe fromexternal sources. Therefore the main characteristics influencingthe above can vary significantly.

Materials handling will most notably be influenced by theclinker size grading and the temperature. For stock clinker, the

degree of degradation (i.e. ageing/hydration) will also beimportant

Figure 11. Example of EN197 CEM I Clinker Chemistry.

Size grading will be a function of raw materials, kiln processand kiln operation. In general hard burning conditions can beexpected to produce a dusty clinker (large proportion less than1mm). In fact, it has been shown that free lime can be linked tothe clinker fines content, with low free limes (associated withhard burning) leading to a high fines content. This in turn canresult in difficult handling and a hard grindability.

Conversely, underburned clinkers with a very high free lime canalso contain a high fines content.

Besides the degree of burning (and cooling), that influence themicrostructure, the grindability is strongly influenced by thelevels of C2S and SO3. The latter will affect the level of gypsumrequired to produce a target SO3 content in the cement. Hencethe perceived grindability will be influenced by the proportionof 'softer' gypsum present.

Clinkers with a wide size grading will be subject to moresegregation during storage and hence the material sizes in theclinker feed to a mill can be variable. This in turn can result in avariable mill performance.

Because of the natural size grading of clinkers, sampling is adifficult task. For this reason extreme caution should be takenwhen taking samples (See Section on Cement Grinding) ofclinker (and Gypsum).

1. CEMENT CHEMISTRY

SiO2 21.3

Al2O3 5.6

Fe2O3 3.1

CaO 66.0

MgO 1.3

Mn2O3 0.1

P2O5 0.2TiO2 0.3

LOI 0.1

Na2O 0.8

K2O 0.2

SO3 1.0

Free Lime 1.2

LSF 96.5

S/(A+F) 2.45

A/F 1.81

C3S 60

C2S 16

C3A 10

C4AF 9



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1.5 GYPSUM

Although clinker contains some SO3, arising from the rawmaterials and fuel, additional SO3 is required to produce cementfrom the cl inker.

The SO3 in the clinker is usually combined with the alkalis toform:-

Alkali sulphates K2SO4, Na2SO4Calcium Langbeinite 2 CaSO4. K2SO4Calcium Sulphate CaSO4

In discussions any additional SO3 is usually referred to as'gypsum', but in reality the source of SO3 will contain gypsum,anhydrite and other minerals such as clay, quartz and calcite aswell as free moisture.

A full analysis is required to precisely identify the mineralspresent in any source of SO3. However the most importantparameters concern the total SO3 content and the level ofgypsum and anhydrite. The latter can be derived from the SO 3content and the loss on ignition at 50, 250 and 950C.An example of calculations for gypsum and anhydrite isprovided in Technical Information Sheet (TIS) number MS001.

It has also been traditional to refer to the 'gypsum' as a 'setregulator' or 'set retarder'. However the main role of the addedSO3 is to prevent rapid reaction of the aluminate which wouldresult in early stiffening, loss of workability and early set.

In addition to preventing the rapid reaction of C 3A, thepresence of added SO3 also influences the wider hydrationprocess, which affects setting, workability and strengthdevelopment. These in turn affect the target SO3 content of thecement. However this is typically in the range 2.5 - 3.5%.

The level of SO3 additive ('Gypsum') required is simplycalculated by a mass balance using the 'gypsum', clinker andcement SO3 contents. This is described in TIS No: MS002.

The above calculation often needs to be modified to allow fornon-clinker components, and this is also shown in TIS No:MS002.

Combining the calculations of "gypsum" addition level andgypsum/anhydrite contents provides the levels of addedanhydrite and gypsum in the cement.

Whilst the anhydrite (natural anhydrite) remains unaffectedduring cement grinding, the gypsum largely dehydrates. Theresultant levels of anhydrite and gypsum dehydration products(i.e. hemihydrate and soluble anhydrite) can have a markedinfluence on cement performance, particularly regardingconcrete water demand and workability.

The sources of SO3 in cement, their dissolution rate, theoptimisation of soluble CaSO

4

and concrete slump behaviourare discussed in more detail in Section 1.7.

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1.6 CEMENT

1.6.1 LSF, etc

We have already seen that Clinker, and thus cement, principallyconsists of the calcium silicate minerals, and these derive fromraw materials containing the four principal oxides, namely CaO,SiO2, Al2O3 and Fe2O3.

To assist in the proportioning of the raw materials, the excessCaO available in the limestone (or other), together with theCaO required to saturate the oxides in the shale (or other) needsto be known.

Lea and Parker derived a formula (1935) from the base of100% lime saturation, which permits the calculation of the limerequired for saturation of the other oxides, i.e.

CaO = 2.80 SiO2 + 1.18 Al2O3 + 0.65 Fe2O3

The lime saturation factor (LSF) for any mix of raw materials isthen given by:

LSF = CaO / (2.80 SiO2 + 1.18 Al2O3 + 0.65 Fe2O3)

The ratio is usually expressed as a percentage. It is also appliedto the clinker and the cement, although the calculation can becomplicated by the presence of CaSO4, free lime, alkalis, etc.

For clinker the above formula is usually used, although this canbe misleading if there is a high SO3 content in which some ofthe CaO has combined with the SO3 to leave calciumlangbeinite and calcium sulphate.

For cement it is usual to deduct the lime present in the CaSO 4,before calculating the LSF (See TIS No. MS003).

In proportioning raw materials, at least two materials will berequired to achieve a target LSF. In 'Ciments Natural', a singleraw material chemistry will lead to variation in the clinkerchemistry (and hence LSF). Two materials permit control to asingle target LSF, by varying the component proportions.

Control of LSF will however not control the resultant levels ofaluminate or ferrite. To assist in their control, and targeting, itis usual to use the parameters of silica ratio and alumina ratio.Thus there can often be three target control parameters, i.e.

As you can see for 1 parameter, 2 materials are required, for 2parameters, 3 materials, and for 3 parameters, 4 materials, andso on.

Additional target control parameters, like alkalis, magnesia, willtherefore require more than 4 raw materials.

Typically, clinker will have:

1.6.2 Clinker CompoundsThe main clinker compounds can be determined by x-raydiffraction but are typically estimated according to Bogue(1929). These estimations are shown in TIS No. MS003.

In these calculations the iron oxide is assumed to be present inthe alumina-ferrite phase and all of the alumina not required to

satisfy the iron oxide in this is assumed to be present as C3A. Theremaining lime is then proportioned between the C3S and C2S.

Again, for cement, the quantity of lime present as CaSO4 needsto be taken into account. In addition, for cement, the limepresent which is uncombined (free lime) can also be deducted toprovide a more realistic calculation. However some estimations

are for potential Bogue composition (e.g. ASTM), in which thetotal lime is used.

It is possible to refine these calculations further, for example byallowance for the combination of some of the SO 3 with alkalisrather than CaO.

Calculations according to Bogue should be treated only as theestimation that they are. One area often giving rise to errors isthe assumption of an alumina/iron oxide ratio of 1 for theferrite phase. It is known that this can be variable in thepresence of some minor compounds in the clinker (e.g. fluorine).

1.6.3 Principal Cement Characteristics

1.6.3.1 IntroductionThe most important cement characteristics which are known tohave a significant influence on cement, mortar and concreteperformance include:

- Sil icates- Aluminate- Ferrite- Alkalis- SO3, Clinker- SO3, Cement- Forms of SO3- Free Lime- Fineness (Blaine, PSD)

- Microstructure (Crystallography, Burning, Cooling)- Surface Properties (Hydration, Carbonation)- Composition (Non-Clinker components)

In addition, minor components, such as fluoride, chloride, heavymetals can also be important (See Figure 5).

1. CEMENT CHEMISTRY

CONTROL PARAMETERSNO. OF RAW

MATERIALS REQUIRED

1 LSF 2

2 LSF, SILICA RATIO 3

3LSF, SILICA RATIO,ALUMINA RATIO

4

MEAN RANGE

LSF 95 90 - 98

SILICA RATIO 2.5 2 - 4

ALUMINA RATIO 1.7 1 - 3



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The following sections deal with these in more detail.Additional information concerning cement performance isprovided in Section 7.

1.6.3.2 SilicatesThe total silicate level directly influences the strength potential.A higher lime saturation factor (LSF) will result in a greater

ratio of C3S to C2S and this will tend to produce a higher earlystrength (up to 7 days) for a given 28-day strength. As a guide,a 1% change in C3S content will be equivalent to around 0.35MPa at 2-days and 0.10 Mpa at 28-days (EN196 mortar).

Note should be taken of whether the data for C 3S, etc is"potential" Bogue composition or reflects the actual level of freelime. Higher levels of free lime result in a lower C3S / C2S ratio.

1.6.3.3 AluminateThe principal influence of the C3A content (and itscrystallography) is on the water demand, workability andsetting behaviour. The interaction between C3A and SO3 isimportant and directly influences the water demand andworkability characteristics.

In general high levels and/or reactive forms of C3A could lead toa tendency for "flash set" behaviour. Both of these, which alsodepend on the supply of soluble calcium sulfate, can have anegative influence on the workability and slump retentionbehaviour, which can often be more pronounced when concreteadmixtures are used.

Statistically C3A has been seen to influence strength. As a guide,a 1% change in C3A content will be equivalent to around 0.5MPa at 2-days and 1.0 Mpa at 28-days (EN196 mortar).

1.6.3.4 FerriteIn general, the influence of ferrite content will not be assignificant as other parameters. The most notable effectconcerns colour and cements are manufactured with low levelsfor "off-white" cements and very low levels in white cements.

The C4AF content can however create a barrier to silicatehydration and hence reduce the strength development potential.The effect depends on the burning, cooling and grinding historyof the clinker. Hence, in some cement, the enhancement offerrite hydration can contribute to increased silicate hydration.The role of cement additives is known to be important in thisrespect ("facilitated transport mechanism").

1.6.3.5 AlkalisThe alkali oxides K2O and Na2O have a very strong influenceon cement properties, notably concerning early and latestrength, workability properties and bleeding.

Where there is sufficient or excess SO3 present in the clinker theresultant alkali sulfates will be readily soluble and this has animportant influence on calcium ion solubility and hence on therate of hydration. (See TIS MS004). As a guide, the earlystrength (1-3 days) will be increased by around 0.8 MPa(EN196 Mortar) for every 0.1% increase in the equivalentNa2O level (Na2O + 0.658K2O). At the same time the latestrength (28-days) will be decreased by around 1.7 MPa.

1.6.3.6 SO3 Clinker (See Also Section 1.7)As discussed above, the level of SO3 in the clinker is importantwith respect to the alkali level and whether these are readilysoluble (as sulfate) or enter into the clinker minerals (and affectreactivity).

The level of SO3 in the clinker also directly influences theamount of "gypsum" required to achieve the target cement SO3content. This in turn will directly influence the "grindability"and hence the SSA (Blaine)/mill output relationship. As a guidea 1% increase in "gypsum" addition equates to around 5%increase in mill output at constant Blaine, (or 12m2/kg increase

in Blaine at constant mill output).

Since the level of clinker SO3 influences the amount of"gypsum" added, the levels of the various forms of SO 3 in thecement can also be influenced (see below).

1.6.3.7 SO3 Cement (See Also Section 1.7)The total SO3 content of the cement influences the hydrationprocess and thus effects the strength development, setting andworkability. Also, as discussed, the level of "gypsum" addedinfluences the Blaine. Hence for a constant clinker SO3 content,a varying cement SO3 level indicates variations in added"gypsum". Hence there will be a direct influence on the

Blaine/mill output relationship.

As a guide, a 1% increase in cement SO3 content will equate toaround a 30m2/kg increase in Blaine (constant mil l output) or10-15% increase in mill output (constant Blaine).

In general, higher levels of SO3 content will enhance the earlystrength, extend the setting items and usually decrease theconcrete workability (increase the water demand). As a guide, a0.1% increase in cement SO3 content will equate to 0.5 MPa at2-days and 0.1 Mpa at 28-days (EN196 mortar).

There will be an optimum level of SO3 content with respect tothe 28-day strength. This will depend on many factorsincluding clinker chemistry and whether derived at constantBlaine or constant mill output.

1.6.3.8 SO3 - Forms (See Also Section 1.7)The principal forms of SO3 in cement are:

a) ex. clinker - alkali sulfate, K2SO4, Na2SO4- calcium langbeinite,

2CaSO4 .K2SO4- calcium sulfate, CaSO4

b) ex. added 'gypsum' -natural anhydrite, CaSO4- gypsum, CaSO4 .2H2O

- hemihydrate, CaSO4 .1/2H2O- soluble anhydrite, CaSO4

The major difference between these forms concerns their rate ofdissolution and hence the level of available soluble calciumsulfate in the cement.

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The level of soluble calcium sulfate principally influences thefollowing:

- concrete water demand (concrete slump,workability)

- slump retention or rate of slump loss- silo set or pack set- rate of strength development

1.6.3.9 Free LimeThe level of free lime itself can influence the initial reactivityand setting behaviour, but more importantly, is a guide to thelevel of burning (see below). It can also alleviate problems ofsilo set (acts as a desiccant). (See also section 1.6.3.2 - Silicates).

Free lime target is typically between 1 and 2%. Higher levelsrepresent incomplete reaction between lime and silica (hence lesssilicates for strength development), whilst lower levels indicate amore complete reaction, but usually at the expense of hardburning conditions and reduced strength development potential.(See also section 1.6.3.11).

1.6.3.10 Fineness - Blaine, PSDThe cement fineness is naturally influenced by the mill outputbut, as already discussed, can be influenced by otherparameters, such as SO3 level, filler level, etc. The principalmeasure is the Blaine (m2/kg) but a more important one is theparticle size distribution (PSD). Often, this is not available butthe Blaine, in combination with a residue level (e.g. Alpine 45-micron), can provide a good estimate of the PSD. (See TIS no.MS005 and Section 3.3)

In general, high levels of fineness will result in enhancedstrengths. The Blaine is a better indication of early strength,whilst the residue is a good guide to the 28-day strength.

As a general guide, for OPC type cements (Type I), a 10m2/kgincrease will be equivalent to around 0.30 Mpa (all ages) and a1% increase in 45-micron residue will be equivalent to around -0.35 Mpa at 2-days and -0.40 Mpa at 28-days. (See also TIS no.MS006).

It should be noted that if influenced by parameters such as SO 3,prehydration, filler, etc, than these guidelines will not be valid.

It has also been noted that the more efficient milling systemswill result in a narrower PSD, i.e. lower 45-micron residue for agiven Blaine. (See also section 3).

1.6.3.11 MicrostructureThe clinker microstructure can have a very strong influence oncement performance but is less well defined on a routine basis.It is influenced principally by:-

- raw material mineralogy- raw feed fineness- burning regime and temperature- fuel type and fineness- cool ing

The microstructure is studied by microscopic examination.

As a guide, hard burning and/or slow cooling will lead to largecrystal sizes which can adversely influence both strength

development and grindability. In the extreme, for clinkers with ahigh MgO content, the burning and cooling regime could leadto expansive properties, due to large periclase crystals.

Clinker Microstructure Characteristics:

1.6.3.12 Surface PropertiesThe cement surface characteristics can be expected to influencethe early age performance, for example, water demand,workability, setting and early strength.

Significant levels of prehydration or carbonation, e.g. due to theutilisation of outside stock clinker or high water injection rates

can result in a high fineness, but with a misleading influence onstrengths.

The only guide to surface properties readily available is the losson ignition (LOI). In general a 1% increase in the LOI will leadto around a 3 Mpa reduction in 28-day strength and a 0.50Mpa reduction at 2-days (EN 196 mortar).

The absence of any moisture can lead to high initial reactivity (apossible contribution to the high reactivity of cements producedin a finish roll press) and resultant reduced workability.

1.6.3.13 Cement CompositionNaturally, besides the clinker, other components can have a

significant influence on the final cement properties. Variations inlevel and quality of non-clinker components could directlyinfluence setting, workability and strength performance. Forexample, as a guide, a 1% change in limestone can influence 28day strength by around 1%. See also section 9.

Non-clinker components can also effect the fineness/mill outputrelationship and hence influence the evaluation of millperformance.

Insoluble residue (IR) can provide a guide to the presence ofsome non-clinker components.

1. CEMENT CHEMISTRY

Alite Sizes, Typically 15-100 microns

Smaller (60) - less reactive

Burning Temperature Lower - better crystal sizes

Cooling Regime Faster - smaller crystal size

Crystal Impuriti es Purer crysta ls - less reactive

Reduction Loss of SO3, Flowability problemsRaw Feed Fineness Coarser - diff icult combinability

Raw Feed homogeneity

Siliceous - lower strengthCalcareous - better strengthHeterogeneity - difficultcombinability

Porosity Lower - difficult grindability



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1.6.3.14 Other PropertiesOther parameters that can also have an influence on cementperformance include:

MgO (e.g.. an expansive behaviour)Fluoride (e.g.. on setting and late strength)Chloride (e.g.. on setting, early strength and

corrosion)P2O5 (e.g.. on setting behaviour)Strontium, Barium (e.g.. on C2S stabilisation)Heavy Metals (e.g.. Pb, Zn) (e.g. on setting

behaviour)Transition Metals (e.g. Cr, V, Mn) (e.g.. on colour)

(See also Figure 5).

Apart from MgO (and Mn2O3 and P2O5 where XRF analysis isavailable) it is not common to have this data readily available.

However there are specific examples where trace levels can beimportant and relevant.

1.6.3.15 ConclusionIt will not always be possible to acquire all of this data, but thefollowing should usually be readily available:

- Silicates (either as C3S, C2S or calculated fromSiO2, Al2O3, Fe2O3 and CaO, SO3and free lime)

- C3A (either as C3A or calculated as above)- C4AF (either as C4AF or calculated as

above)- Alkalis (Na2O and K2O)- SO3 (for the cement and possibly the

clinker)

- Free Lime- Fineness (Blaine and possibly the Alpine, or

other, residue)- Composition (i.e. non-clinker components)

The alkalis, SO3, fineness, silicates and composition are likely tobe the most relevant parameters in understanding relativecement performance. A table of the typical relationships forsome of these parameters is shown in Figure 12.

Figure 12. "Rules of Thumb" Relationships.

Data concerning important properties, such as the clinkermicrostructure, surface properties and the forms of SO3 in the

cement will be virtually impossible to readily acquire.

It is therefore recommended, where possible, that when samplesare received and during plant trials data including the fullchemical analysis is recorded. This will usually be in the form:-

SiO2 LSF Composi tion (e .g. non clinker l evel s)Al2O3 S/(A+F) Gypsum details (e.g. Gypsum, anhydrite)Fe2O3 A/FCaO

MgO C3SMn2O3 C2STiO2 C3A

P2O5 C4AFSO3 Free LimeNa2OK2OLOI

Insoluble ResidueBlaine (SSA)Residue (e.g.. 45 micron)

C E M E N T T E C H N O L O G Y N O T E S 2 0 0 5 131. CEMENT CHEMISTRY

Parameter UnitIncremental

change

Effect on

strength

2d

MPa

28d

MPa

SSA m2/kg +10 0.30 0.30

45-micron residue % +1 -0.35 -0.40

Eq. Na2O % +0.1 0.80 -1.70

LOI % +1 -0.50 -3.00

C3S % +1 0.35 0.10

Free Lime % +1 0.50 -1.50

SO3 % +0.1 0.50 0.10

C3A % +1 0.50 1.00



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1.7 'GYPSUM' - SOLUBLE CALCIUM SULPHATE

1.7.1 Introduction

The principal sources of SO3 in cement are:a) ex clinker, i.e. alkali sulphates K2SO4, Na2SO4

calcium langbeinite 2Ca SO4 .K2SO4calcium sulphate CaSO4

Note: The calcium sulphate present in clinker, formed wherethere is a large excess of SO3 over alkalis, will be equivalent toanhydrite (anhydrite I - slowly soluble).

b) ex the added calcium sulphate ('gypsum'), which can bepresent as: natural anhydrite CaSO4

gypsum CaSO4 .2H2Ohemihydrate CaSO4 .

1/2H2Osoluble anhydrite CaSO4

Note: (1)Natural anhydrite or anhydrite II.(2)Hemihydrate and soluble anhydrite (anhydrite III)

can be present as and forms. The forms aremore reactive (in plaster properties terms).

The forms predominate where gypsum is processedin a less than water saturated atmosphere atatmospheric pressure, although some forms willalso be present.

To produce only the forms, processing has to becarried out at elevated pressures or at atmosphericpressure in a water-saturated atmosphere.

Hence it follows that in cement milling the formswill be preferentially produced.

Natural anhydrite is unaffected by the temperatures involved incement milling and storage.

Gypsum will dehydrate to hemihydrate and soluble anhydrite.The degree of this conversion will principally depend ontemperature, residence time and atmosphere.

It is difficult to distinguish between soluble anhydrite and naturalanhydrite by analysis. (XRD can provide some indication).

As a guide, the following is likely:

In cement milling, below 80C, little gypsum will dehydrate.Between 80-100C, up to around 50% of the gypsum will

dehydrate. Between 100-120C, more than 50% will dehydrateand some soluble anhydrite will be formed. Above 120C, littleor no gypsum will remain and a greater proportion of solubleanhydrite and hemihydrate will be found.

In storage, any retained gypsum (ex the cement mill) can slowlydehydrate at temperatures above around 70C.

Thus a combination of relatively low milling temperatures butmoderately high storage temperatures (e.g. milling at 100C,storage above 70C) can result in silo problems as a result ofgypsum dehydration, water migration and cement hydration(i.e. silo set and lump formation).

Figure 13a provides a range of scenarios for a cement SO 3content of 3.0% and a clinker SO3 content of around 1.0%(assuming a typical level of alkalis of around 0.6 eq. Na2O).

Figure 13a. Examples of Cement Sulfate Forms.

Cement A: No anhydrite, high mill temperature - produces highlevel of D.SO3 (i.e. hemihydrate + solubleanhydrite).

Cement B: No anhydrite, moderate milling temperature -produces a moderate level of D.SO3.

Cement C: Anhydrite present, high mill temperature - producesa moderate level of D.SO3.

Cement D: Anhydrite present, low milling temperature -produces a low level of D.SO3.

1.7.2 Dissolution of CaSO4The principal differences between the various forms of CaSO4present in cement concern their rate of solubility.

Gypsum gradually dissolves up to the level that represents asaturated solution of CaSO4. Natural anhydrite dissolves moreslowly. Hemihydrate or soluble anhydrite dissolve very rapidlyand form a super-saturated solution with respect to CaSO4. Therates of solubility are shown in Figure 13b.

Figure 13b. Dissolution Rates for Calcium Sulfate Forms

Dissolution rates can be influenced by the presence of certainconcrete admixtures and by other sources of SO4, e.g. from thealkalis.

1. CEMENT CHEMISTRY

Cement Type: A B C D

Sources of SO3:

Cement 3.0 3.0 3.0 3.0

Clinker 1.0 1.0 1.0 1.0

Natural Anhydrite 0.0 0.0 1.0 1.0

Gypsum 0.1 1.0 0.1 1.0

Hemihydrate + Soluble Anhydrite 1.9 1.0 0.9 0.0



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1.7.3 Optimisation of Soluble Calcium Sulphate

The availability of soluble CaSO4 is required to 'satisfy' theinitial reactivity of the C3A. The 'optimum' supply of solubleCaSO4 will therefore depend on the initial 'reactivity' of theC3A. This will principally depend on:

- l eve l of C3A (actual, rather than just according to Bogue)

- finenesssurface freshness (e.g. presence of prehydration)- alkali/sulphate balance- crystal size (burning, cooling, etc.)

An excess of alkalis in the clinker can result in alkali modifiedC3A which is more reactive.

Balanced retardation of the C3A will result in the formation offine grained ettringite (C3A.3CaSO4.32H2O). This providesmaximum mobility in fresh concrete thereby optimisingworkability:

- BALANCED RETARDATION

An excess of soluble CaSO4 over that necessary will provide asupersaturated solution of CaSO4 and resultant precipitation ofgypsum crystals. The morphology of these will not providemaximum mobility and hence workability (initial) will not beoptimised. This can be more pronounced where the cementcontent is high. Prolonged mixing is generally accepted as ameans to counter this loss of workability:

- FALSE SET

Insufficient soluble CaSO4 will allow the formation ofmonosulphate (as opposed to tri-sulphate [= ettringite]) or evenC3A hydrates (4CaO.Al2O3.13H2O). Again, the morphology ofthese will prevent maximum mobility being obtained. More

importantly, the rate of slump loss will be severe:- FLASH SET

A schematic of the above scenarios is shown in Figure 14.

Therefore, for any clinker, there is an optimum level of solubleCaSO4 (i.e. D.SO3) which provides maximum initial workability(or minimum water demand). This relationship is illustrated inFigure 15 showing concrete water demand versus D.SO3.

Figure 15. Concrete Water Demand versus Soluble Calcium Sulfate.

In general:- the optimum level of D.SO3 will be around 1.0% SO3

(i.e. typically in the range 0.7 - 1.3% SO3).- for more reactive clinkers the optimum D.SO3 will be higher.- for less reactive clinker the optimum D.SO3 will be lower.- for less reactive clinkers the curve will be flatter in the

region below the optimum D.SO3 but steeper in the

region above the optimum (i.e. tendency for false setbehaviour more important).- for more reactive clinkers the curve will be flatter in the

region above the optimum D.SO3, but steeper in theregion below the optimum (i.e. tendency for flash setbehaviour more important).

1.7.4 Slump Loss/Retention

The preceding points mainly refer to the initial slump orworkability or water demand in concrete. However a consistent,and predictable, slump loss behaviour over the initial period of,say, up to one hour is more important.

In general, low levels of D.SO3 and/or reactive clinkers (e.g.

excess alkalis over SO3) will result in a moderate to low initialslump and severe rate of slump loss.

Conversely high levels of D.SO3 and/or less reactive clinkers,whilst also producing a moderate to low initial slump, canprovide an overall better retention of slump behaviour.

With very high levels of D.SO3 (i.e. false setting) the initialslump can be very low. But after mixing the slump can besubstantially improved.

In general, high w/c ratios (e.g. as a result of poor initial slump)result in less of a slump loss. Conversely low w/c ratios can

produce a more severe slump loss.

Figure 16 shows slump behaviour over 60 minutes for twocements. Slumps were determined at 6, 20, 40 and 60 minutes,in each case after a short period of re-mixing.

Figure 14. Optimisation of Cement Sulfate.

1. CEMENT CHEMISTRY



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Figure 16. Slump Loss.

A - Cement with Mill temperature ~100C D.SO3 ~ 1.0%B - Cement with Mill temperature ~80C D.SO3 ~ 0.4%C - Laboratory heat treated Cement B D.SO3 ~ 1.6%

The cement milled at 95-100C had a level of D.SO3 of around1.0% and gave slumps of 40 and 17mm at 6 and 60 minutesrespectively. In order to achieve slumps of 40mm, w/c ratios of0.60 and 0.64 respectively would be necessary. Therefore theslump loss was equivalent to an increase in water demand of7%.

The cement milled at 80-85C had a lower level of D.SO3 ofaround 0.4% and gave slumps of 25 and 7mm respectively.Again, to achieve slumps of 40mm, w/c ratios of 0.62 and 0.69respectively would be necessary. Therefore the slump loss was

equivalent to an increase in water demand of 11%.

The third curve shows the results of this latter cement afterlaboratory heat treatment. The D.SO3 was raised to around1.7%. The cement now false sets and there is a very low initial

slump of only 16mm. However, the slump at 60 minutes is now18mm, i.e. back to the level of the first cement.

A summary of the principal points concerning cement SO3 areshown in Figure 17.

Figure 17. Key Points concerning Soluble Calcium Sulfate.

1. CEMENT CHEMISTRY

1 SO3 present in cement as:Ex. clinker -

alkali sulfatecalcium langbeinitecalcium sulfate

Ex. added "gypsum" -natural anhydritegypsumhemihydratesoluble anhydrite

2 Gypsum dissolves slowlyNatural anhydrite dissolves very slowly

Hemihydrate & soluble anhydrite dissolve very rapidly

3 SO3 ex. hemihydrate & soluble anhydrite has been referred as "D.SO 3"

4 Clinker reactivity is strongly influenced by:C3A levelCement finenessSurface "freshness"Alkali:sulfate balanceThermal history (e.g. crystal sizes)

5 Excess alkalis over sulfate results in a more reactive clinker

6 Need to consider availability of soluble calcium sulfateNeed to consider clinker "reactivity"

Need to match sulfate availability to clinker reactivity

7 Balanced retardation of C3A provides optimum water demandExcess D.SO3 produces "FALSE SET" tendencyInsufficient D.SO3 produces "FLASH SET" tendency"FLASH SET" tendency probably more negative


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