cement formationâ--a success story in a black box- high temperature phase formation of portland...

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Cement FormationA Success Story in a Black Box: HighTemperature Phase Formation of Portland Cement ClinkerSamira Telschow, Flemming Frandsen, Kirsten Theisen, and Kim Dam-Johansen*,

Technical University of Denmark, Department of Chemical and Biochemical Engineering, Soltofts Plads Building 229, DK-2800Kgs., Lyngby, DenmarkFLSmidth A/S, Vigerslev Alle 77, 2500 Valby, Denmark

ABSTRACT: Cement production has been subject to several technological changes, each of which requires detailed knowledgeabout the high multiplicity of processes, especially the high temperature process involved in the rotary kiln. This article gives anintroduction to the topic of cement, including an overview of cement production, selected cement properties, and clinker phaserelations. An extended summary of laboratory-scale investigations on clinkerization reactions, the most important reactions incement production, is provided. Clinker formations by solid state reactions, solidliquid and liquidliquid reactions arediscussed, as are the influences of particles sizes on clinker phase formation. Furthermore, a mechanism for clinker phase

formation in an industrial rotary kiln reactor is outlined.

1. INTRODUCTION

Higher! Faster! Further! is not just a motto of the OlympicGames; it is a theme, describing progress during the twentiethand twenty-first centuries. The construction world especiallyhas committed itself to the first: Higher! The heights of

buildings particularly, always a symbol of power, economicalpotential, and wealth, have increased drastically over the last100 years (Figure 1).

One factor, among others, that makes high constructionspossible is reinforced concrete. Around 47 000 m3 of concrete

was needed for the construction of the Empire State building(U.S.A.).1 The construction of the Petronas Towers (Malaysia)required approximately 160 000 m3 of concrete,2while the BurjKhalifa (U.A.E.), the tallest building of the world to date, wasconstructed using ca. 330 000 m3 of concrete.3 These numberssymbolize the importance of concrete, and it is not surprisingthat the production of its key compound, cement, is not onlyone of the oldest but also one of the biggest industrial sectors,and it is still growing. World production increased by a factor of

10 between 1950 and 2006. Within only 5 years (20012006),the production rose by 35%.4 In 2006, a total of 2.54 billiontonnes of cement was produced worldwide, with 47.4% inChina, 10.5% in the European Union, 6.2% in India, and 3.9%in the U.S. (Figure 2).4 This ongoing success is accompanied byconstantly changing and improving production technology tomeet demand for the cement quality and quantity, fuelefficiency, pollution limitations (emission control), and, last

but not least, production profitability. At the heart of cementproduction are high temperature processes (up to 2000 C gastemperatures), where raw materials undergo several chemicalchanges to form clinker crystal phases as the main constituentsof most cement types.541 The reactor (cement kiln) in whichthese processes occur has been subject to changes over the last

few decades. Whereas the first kilns were bottle shaped batchreactors with limited production capacity,42 clinker is nowadaysproduced mainly in continuously operating rotary kilns with acapacity of 30005000 tonnes of clinker per day, or even up to10 000 tonnes per day in some Asian plants.4

Over the last few decades, rotary kiln technology has beenimproved: the incremental shift from the wet- to the dry-

Received: August 16, 2011Revised: July 25, 2012Accepted: July 26, 2012Published: July 26, 2012

Figure 1. Nontrue to scale comparison of buildings: (left to right) theEmpire State Building (U.S.A.), the Petronas Towers (Malaysia), andthe Burj Khalifa (U.A.E.).

Review

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production process, the separation of the location of differentchemical reactions (raw meal calcination clinkerization), gas

bypass systems, energy recovery systems, and state-of-the-art

emission control systems are some of the changes that haveincreased production capacity greatly and reduced energyconsumption and pollution. Nonetheless, cement productionremains a highly energy consuming and polluting sectors withinthe industrial world. The energy required for cementproduction is provided by the combustion of different kindsof fossil fuels, biomass fuel, and waste. The demand for thermalenergy equals 30004000 MJ/tonne of clinker for the widelyused dry process (including cyclone preheater and calciner).4

Additionally, around 324540 MJ/tonne cement of electricalenergy is required for processes such as grinding (raw materialsand cement) and exhaust fans.9 The main air-pollutantsubstances related to cement production are, among others,CO2, CO, nitrogen compounds (e.g., NOx), and sulfur

compounds (e.g., SO2). About 9001000 kg CO2 per tonneof clinker (gray cement clinker) is released, of which 60% isrelated to calcination of CaCO3, while the rest accounts for fuelcombustion.4 NOx emissions (mainly NO and NO2) amount to0.34.7 kg/tonne of clinker. The main sources for NOxformation are the combustion of fuel and the reaction ofnitrogen in combustion air with oxygen at high kilntemperatures.4 Sulfur is brought into the process by rawmaterials and fuel, and it is mostly combined with CaO andalkalis as CaSO4 and alkali-sulfates in the clinker product anddust. Organic sulfur and sulfidic sulfur is emitted as SO2 viaexhaust gas. The emission depends on utilized raw materialsand fuels and differs from one cement works to the other.4

Systematic changes and improvements of the cement

production process require detailed knowledge about theclinker formation process itself. The research in that topic can

be dated back to the end of the nineteenth century, andsubstantial progress has been made in understanding the varietyof individual clinker phase formations. Due to the difficulty andhigh costs of investigating reactions and reaction conditionsdirectly in an industrial reactor, research has been mostlycarried out in the laboratory.1037 Several books7,3840 andsome review papers9,41 have summarized clinker formationreactions. Laboratory data have been compared with theexperiences of cement plant operations. Therefore, it is, ingeneral, possible today to predict the properties of clinker andcement obtained at the end of the process, dependent on

starting conditions such as raw meal composition, type of fueland air/gas mixtures (determining the temperature profile inthe kiln), and kiln dimensions (determining the retention time

of the solid material). However, only a few attempts, to theauthors knowledge, have been made to present the reactionmechanisms of clinker phase formations as they emerge in theindustrial process.9

In this paper, the vast amount of research on clinker phaseformation at temperatures between 900 and 1500 C isevaluated and summarized. It provides a detailed description ofthe formation of the silicate, aluminate, and ferrite phases fromthe four major compounds CaO, SiO2, Al2O3, and Fe2O3 oftypical cement raw meals. Results from laboratory-scale studiesare discussed with respect to chemical processes occurring in anindustrial rotary kiln reactor.

The influence of minor and trace compounds on clinkerformation/clinker quality will not be addressed in this article,

unless stated diff

erently, as this topic has already beenintensively reviewed elsewhere.38,39

2. PORTLAND CEMENT PRODUCTION

Cement is a hydraulic binder, forming a paste with water, whichsets and hardens due to hydration reactions.5 It creates thestrength, durability, and soundness of concrete and mortar.Cement is either classified by its composition or in respect toits performance-related properties by national or internationalinstitutions. However, the most common classifications arerelated to the European Standards for Common Cements (e.g.,EN 197-1) defined by the European Committee for Stand-ardization43 and the ASTM Standards (e.g., C150/C150 M andC595/595M) defined by the American Society for Testing and

Materials.44 A detailed overview of the different cementclassifications is given in ref 5.

Up to now, Portland cements are the most producedcements yearly. The main constituent of these cements isclinker (often termed as Portland cement clinker), which itselfis composed of 4080 wt % C3S, 1050 wt % C2S, 015 wt %C3A, and 020 wt % C4AF.

4

Therefore, the following description of industrial process isbased on the Portland cement production.

2.1. Portland Cement Clinker. Clinker, as the mainconstituent of cement, is composed of various crystal phases,the following of which are the most important: alite, belite,aluminate, and ferrite. Alite and belite are calcium silicate

Figure 2. Overview of cement production worldwide in 2006.4

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phases. Consisting only of CaO and SiO2, alite is a tricalciumsilicate phase (Ca3SiO5) and belite is a dicalcium silicate phase(Ca2SiO4). The aluminate phase, formed by pure CaO andpure Al2O3, is a tricalcium aluminate phase (Ca3Al2O6), and theferrite phase, formed by pure CaO, Al2O3, and Fe2O3, is atetracalcium aluminoferrite phase (Ca4Al2Fe2O10). It iscommon to abbreviate the chemical formulas as shown in

Table 1).

Pure oxides are only available for laboratory investigations ofclinker formation. In the industrial process, the raw material

contains various impurities,5,7

which form clinker phasesincorporating impurities or forming solid solutions withminor compounds. Therefore, it is more appropriate to usethe phase names alite, belite, and so on, since these expressthe clinker phases including impurities.

2.2. Charge Materials. The main constituents of the rawmaterials required for cement production are calcium oxide(CaO), silicon dioxide (SiO2), aluminum oxide (Al2O3), andiron oxide (Fe2O3).

45 Typical sources of these oxides arelimestone, chalk, marl, clays (kaolinite, Illite, feldspar) or shale,tuff, oil shale, bauxite, and iron ore.39,45 These materials oftencontain alkalis, earth alkalis, heavy metals, sulfate, sulphide,phosphate, fluoride, and chloride compositions in lowerconcentrations.7,39,45,46 Besides natural materials, waste prod-

ucts from other industrial processes, such as lime sludge or flyash from coal combustion, can be added. The addition ofrelatively pure limestone, sand, or iron ore might be necessaryto adjust for absent chemical compounds and to achieve thestandards required for cement.5,45 A typical chemicalcomposition of four-component raw meal is listed in Table2.5 The chemical composition of the raw materials is constantlycontrolled in plant laboratories.

Several stages of cement production occur at elevated or hightemperatures such as the drying of raw materials, theendothermic decarbonization reaction of limestone, and finally,

clinker formation reactions at up to 1500 C. The heat requiredis provided by the combustion of fuel, namely black coal,lignite, heavy and light fuel oil, fuel gas, and/or petroleum coke(Table 3). These traditional fuels are nowadays increasingly

substituted by alternative fuels such as scrap tires, waste oil,fullers earth, wood waste, plastic waste, and fractionizedcommercial and domestic waste,4760 thus disposing waste and

reducing the deletion of natural resources. Noncombustiblefractions of alternative fuels are utilized for the clinkerization,as, for example, the steel carcasses of tires are a significantsource of iron oxide. Thus, alternative fuels are completelysalvaged.45

2.3. Raw Meal Pretreatment. Mined raw materials arecrushed and, depending on fluctuations in the chemicalcomposition, separately prehomogenized to provide constancyof cement product quality (Figure 3a). Further raw mealpreparation was originally distinguished mainly into dry and

wet pretreatment, depending on the moisture content of theraw materials.5,39,45A feedstock with a moisture content of >15

wt % was often prepared under wet conditions,5 where waterwas added in order to achieve a slurry with a water content of2550 wt %.5,39 The advantage of this method is theachievement of high raw meal uniformity. However, theapplication of the wet method decreased over the years sincethe removal of the water is energy-intensive and, due to fuelprices, expensive. If a wet preparation cannot be avoided, partsof the water are removed by filtration, decreasing the watercontent to ca. 20 wt %.5

The more cost-efficient dry pretreatment method is by farthe most employed process in cement production, which is whythe following process descriptions focus only on the drymethod-based cement plants. There, raw materials are oftenground and dried simultaneously. The blending of the differentmaterials, including secondary raw material to obtain clinkerraw meal, is carried out either by simultaneous grinding or afterthe grinding step by air agitation in large silos.5,45 Thereby,

comparable homogeneities with respect to the wet method areachieved.5,45

2.4. Preheating. Clinker raw meal is heated in the cyclonepreheater (Figure 3b, Figure 4a). The preheater is basically aseries of cyclones arranged in 50120 m high towers. Hot gases(1000 C) from the calciner and rotary kiln heat the raw mealin counter-current flow, increasing the feed temperature from200 to 800 C in less than a minute.7,39

High contents of inorganic volatiles in the gases, mostlysodium and potassium sulfates and chlorides, are problematic

when released from the raw material or fuel in the rotary kiln,62

as they may condense at the preheater walls forming soliddeposits, which limits the motion of gas and material.7

Table 1. Abbreviation of the Chemical Formulas Used in theCement Field7

chemicalcompound abbreviation clinker phase

clinker phaseabbreviation

CaO C Ca3SiO5 C3S

SiO2 S Ca2SiO4 C2S

Al2O3 A Ca3Al2O6 C3A

Fe2O3 F Ca4Al2Fe2O10 C4AF

Ca12Al14O33 C12A7

Table 2. Typical Chemical Composition of a FourComponent Raw Meal5

limestone(wt %)

shale(wt %)

sand(wt %)

ironoxide

(wt %)

kiln feedacomposition

(wt %)

dry materialused

73 22.5 4.2 0.3

SiO2 1.4 37.9 95.0 2.7 20.1

Al2O3 0.5 16.5 1.4 6.6 6.3

Fe2O3 0.2 5.1 1.3 84.0 2.4

CaO 53.7 15.4 1.0 2.7 64.4

CaCO3 95.9 27.5

minorb

compounds2.0 13 2.3 6.7 6.8

aDecarbonized material. bModification of the original.

Table 3. Fuel Types and Consumption at Cement Plants inthe European Union in 20064

fuel type consumption (%)petcoke (fossil) 38.6

coal (fossil) 18.7

petcoke and coal (together) 15.9

fuel oil 3.1

lignite and other solid fuels 4.8

natural gas 1.0

waste fuel 17.9




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2.5. Calcination. The separation of the calcination processof calcareous materials from processes in the rotary kilnimproved the cement production significantly (Figure 4b).63,64

The rotary kiln decreased in size, or with unchanged kilndimensions, production capacity increased, thus decreasinginvestment and operational costs.7,45

The hot raw meal enters the calciner at about 800 C fromthe preheater. The energy required for the endothermicdecarbonization reaction (calcination) is obtained from hot

rotary kiln gases and additional combustion of fuel. Most often,hot air from the clinker cooler, so-called tertiary air, is utilizedas combustion air (Figure 4f). The consumption of fuel forcalcination can account for up to 60% of fuel needed forcement production.45 The degree of decarbonization con-stitutes 9095% and solid material enters the rotary kiln with atemperature of about 900 C.7,45

2.6. Clinkerization. Rotary kilns vary greatly in length anddiameter depending on the design of the whole pyro-processkiln system (includes all from raw meal preheating until clinkercooling). Typical dimensions for rotary kiln systems withcyclone preheater and calciner are 50100 m in length and 37 m in diameter (length to internal diameter ratio 1116).64,65 These kilns operate typically at a tilt of 13 from

horizontal and with a rotation velocity of 24.5 rpm (Figure4c), which results in material retention times in the kiln of ca.2040 min.64 The precalcined solid meal is fed into the kiln atthe higher end, solid or liquid fuel, along with primary air, is

blown into the kiln and combusted at the lower end of thekiln,7 creating a flame with temperatures of around 2000 C.66

Additionally, secondary air from the clinker cooler is also drawninto the kiln for fuel combustion (Figure 4e).7 The calcinedmeal moves counter current to the hot gases toward the hotregion (sintering zone) of the kiln, thereby achieving heattransfer between gases, solids, and the kiln walls. The calcinedfeed is heated to 1500 C, leading to several chemicalreactions and mineralogical changes in the different zones of

Figure 3. Overview of a cement production plant: (a) pretreatment of the raw materials; (b) high temperature process; (c) post-treatment ofclinker/cement.61 Copyright 2012 FLSmidth A/S. All rights reserved.

Figure 4. In-line calciner preheater system: (a) preheater tower, (b)precalciner, (c) rotary kiln, (d) clinker cooler, (e) burner andsecondary air inlet, (f) tertiary air pass.63




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the kiln: decomposition of the remaining uncalcined CaCO3and, formation of the clinker phases. At temperatures up to1250 C, solid state reactions occur, and the belite, aluminate,and ferrite phases are gradually formed. In addition, at thesetemperatures, inorganic volatiles are released from the feedmaterial and fuel consisting mainly of alkalis, sulfates, andchlorides, which condense and form deposits as rings inside thekiln. At higher temperatures (13001500 C), a liquid phase isformed from the aluminate, ferrite, and partly belite phase,

which leads to solid particles sticking (granulation/noduliza-tion). Further, free CaO and belite react to form alite.7,39

2.7. Clinker Cooling. Controlled clinker cooling is veryimportant for clinker quality and for the energy efficiency of theoverall production process, since it allows heat recovery (Figure4d). It is essential that the hot clinker is cooled quickly to below12001250 C, as fast cooling causes the recrystallization ofthe finely grained aluminate phase,7which results in a desirableslow, controllable hydration reaction during the setting processof cement. Conversely, a coarsely grained aluminate phaseformed due to low cooling rates, causes overly rapid setting ofcement.5,45 More important is the effect of rapid cooling on theimportant alite clinker phase, which is thermodynamicallystable only at temperatures above 1250 C (see Figure 8 inSection 3.2). Below 1250 C, alite decomposes into belite andcalcium oxide (Figure 5).22,67,68 Fast cooling to temperatures

below this critical temperature results in metastable alite.

The clinker is cooled by air flowing through the hotgranulated solids. The heated air is recirculated as combustionair (secondary air) into the burner of the rotary kiln (Figure 4e)

and/or into the burner of the calcination chamber (tertiary air)(Figure 4f). The higher the cooling rate, the higher the heatrecovery and therefore the energy efficiency of the cementproduction process.5,45

2.8. Cement Blending and Grinding. For the preparationof cements, clinker needs to be ground and blended withadditives (Figure 3c), the most important of which are gypsum,

basserite, or anhydrite, which control the setting of concreteduring hydration. Other additives, depending on the desiredcement type, are, for example, blastfurnace slag, limestone, orfly ash.5,9,39,69

3. PHYSIO-CHEMICAL PROCESSES OF PORTLANDCEMENT CLINKER FORMATION

3.1. Clinker Phases. 3.1.1. Alite (C3S). Alite is the majorclinker phase in Portland cement clinker and controls mainlythe initial and ultimate strength of cement. Portland cementclinker consists of ca. 5070 wt % of alite,4,7,45 which contains7175 wt % CaO, 2428 wt % SiO2 and 34 wt % substitutedions.7 Typically incorporated ions within the alite crystal latticeare Mg2+, Al3+, and Fe3+. The impurities in alite stabilize hightemperature polymorphs at low temperatures (below therelated decomposition temperature).21 So far, there existseven known polymorphs between room temperature and1070 C: three triclinic (denoted with T), three monoclinic(M) and one rhombohedral (R) polymorph (Figure 6).7,39,69

Due to incorporations in the alite crystal lattice, M1 and M3

polymorphs are present mostly in industrial clinker. Coolingclinker from 1450 C, inversion of the R polymorph to M3 and

furthermore to M1 occurs, forming small crystals (M3) rich insubstituents or large crystals, poor in substituted ions (M1).Especially, high MgO concentrations promote high nucleation,resulting in formation of small automorphic M3- crystals.

7,21

The different polymorphs do not show significant differences inthe hydraulic properties.39

3.1.2. Belite (C2S). The second largest clinker phase inPortland cement is belite. Its hydration product developssimilar strength in cement as alite, only much more slowly.Belite makes up between 15 and 30 wt % of Portland cementclinker and consists of 6065 wt % CaO, 2935 wt % SiO2,and 46 wt % substituted oxides, mainly Al2O3 and Fe2O3, butalso K2O, Na2O, MgO, SO3, and P2O5.

7 Belite crystallizes infive polymorphs: -belite, H-belite, L-belite, -belite (H =high and L = low temperature) and -belite (Figure 7),

which differ in structural and hydraulic properties. The -polymorphs are the most hydraulic forms of belite, whereas -

belite is a nonhydraulic polymorph and does not account forthe setting and hardening of cement. -belite is also a hydraulicpolymorph, but it is less hydraulic than the - polymorphs. It isthe most common polymorph in industrial Portland cementclinker. A phenomenon that needs to be prevented by tracecompounds inclusions is disintegration (dusting) of clinker,

which happens if-C2S is not stabilized during cooling and/orby inclusions, affording a part -C2S inversion. -C2S crystalsare less dense (more voluminous) than -C2S crystals, whichcauses cracking of other -C2S crystals, forming a voluminouspowder and dust.7,39

3.1.3. Tricalcium Aluminate (C3A). C3A is the most reactivephase of the four clinker phases in Portland cement clinker,

which contains 510 wt % of the phase. Pure C3A consists of

62 wt % CaO and 38 wt % Al2O3 and does not exhibittemperature dependent polymorphs. However, ion substitutionof Ca2+ in the structure of the pure C3A causes changes incrystal structure. Typically, Ca2+ is substituted by Mg2+, 2 K+,and 2 Na+, and Al3+ by Fe3+ and Si4+, but only the alkali metalsaffect the structural changes7,39 from a cubic crystal structure(pure C3A) to orthorhombic and monoclinic structures viaintermediate structures of lower symmetry.

In industrial clinker products, orthorhombic and cubicstructures are commonly present polymorphs. The ortho-rhombic form features dark, prismatic crystals, whereas thecubic polymorph forms fine grains with dendritic ferritecrystals.3

Figure 5. Formation and decomposition reaction of alite.

Figure 6. Temperature dependence of the seven polymorphs of puretricalcium silicate (alite).7

Figure 7. Temperature dependence of the five polymorphs of puredicalcium silicate.7




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3.1.4. Calcium Aluminoferrite (C4AF). Calcium aluminofer-rite constitutes 515 wt % of Portland cement clinker. Thepure phase contains 46 wt % CaO, 21 wt % Al 2O3, 33 wt %Fe

2O

3, but in industrial clinker up to 10 wt % of incorporated

oxides appear (mostly MgO). This phase is composed of anysolid solution composition of Ca2(AlxFe1x)2O5, with 0 < x 57 wt % CaO. This shows the importance of highcement raw meal homogeneity. Poor homogeneity, caused forexample by poor mixing or too coarse raw material particles(see detailed discussion in Section 3.4.4), could lead to localareas of different chemical composition than the overallcomposition, potentially causing the formation of product

phases other than the typical clinker phases.Clinker phase relations are best shown in three- and four-component phase diagrams. The most relevant system is theCaOSiO2Al2O3Fe2O3 phase diagram shown in Figure 9a)and the subsystem related to Portland cement clinker (CaOC2SC12A4C4AF) is shown in Figure 9b).

72,74 It should bepointed out that this is not an isothermal diagram, whichcomplicates the reading of the diagram. Nevertheless, a fewimportant details are demonstrated. The quaternary phasediagram (Figure 9b) is built up of four ternary systems, whichrepresent the surfaces of the quaternary system: (1) CaOC12A7C2S, (2) CaOC2SC4AF, (3) CaOC12A7C4AF,and (4) C2SC4AFC12A7. Phase boundaries and phase fields

Figure 8. Ternary equilibrium phase diagrams of pure reactants: the CaOSiO2Al2O3 system showing the typical Portland cement clinkercomposition (). The area marked by the square denotes the typical composition of these three reactants in Portland cement clinker. Compositionsdue to inhomogeneities in Portland cement clinker, as denoted by the rectangle, could result in the formation of a melt phase at lower temperatures(e.g. 1170 C).7,74,107




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known from binary and ternary phase systems become surfacesand phase volumes. Two invariant points (P7 and P8) (i.e.points where the thermodynamical degrees of freedom arezero) occur in the four-component phase diagram (Figure 9b).

At the first invariant point (P7), the solids CaO, C3S, C3A, andC4AF coexist in equilibrium at 1341 C with the liquid phasehaving a composition of 55 wt % CaO, 22.7 wt % Al2O3, 5.8 wt% SiO2 and 16.5 wt % Fe2O3.

75,76 The second invariant point(P8) is found at 1338 C and the composition of the liquid inequilibrium with the solids C3S, C2S, C3A, and C4AF is 54.8 wt% CaO, 22.7 wt % Al2O3, 6.0 wt % SiO2, and 16.5 wt %Fe2O3.

7577

The most important phase volume is the tetrahedron C3SC2SC3AC4AF (Figure 9c). Any raw meal composition

within this phase volume results in the formation of all four

major clinker crystal phases for Portland cement clinker. A rawmeal composition to the right of this tetrahedron results in theformation of C3A, C4AF, C12A7, and C2S, but no C3S, and rawmeal compositions to the left of the tetrahedron result in aclinker composition with C3S, C3A, C4AF, and free CaO. Bothcases are undesirable in cement production. C2S does notfeature the same desired initial strength of a cement paste asC3S, and it is therefore not the preferred main silicate phase incements, except in special cases such as low energy belitecement. High concentrations of free CaO, cause concreteunsoundness, while from an economical point of viewremaining overly high concentrations of CaO are a waste ofraw material and energy, and it is therefore important to

carefully adjust the raw meal composition with respect to theamount of lime. Note, however, that small amounts of free CaOin the clinker product are usually left intentionally by choice ofthe process conditions (i.e. burning temperature and time)since economic constraints make full conversion to zero CaOlevels unreasonable. In other words, considering the case of araw meal composition with in the C3SC2SC3AC4AFtetrahedron, all CaO in the raw meal should react to form aclinker phase. However, full reaction of CaO in the industrialprocess is not always desired, since small amounts of free CaOmight not impair the clinker/cement quality significantly.However, time and energy, thus operational cost, can be saved,

when the clinkerization reactions are stopped by cooling beforeall CaO has reacted. In this case, the acceptable concentrationof free lime depends highly on the raw meal CaO concentration

and thus the theoretical C3S formation. Each percent free CaOin the clinker product correlates to a C3S loss of4 wt %, as ithas not been formed by the reaction of CaO with C2S. For rawmeal composition on the CaO richer side within the C3SC2SC3AC4AF tetrahedron, a loss of 812 wt % of C3S (i.e.,23 wt % free CaO in clinker) may be acceptable, but forcompositions on the CaO lower side within the tetrahedron,the loss is significant.

The phase volumes shown in Figure 9b are primary phasefields. A primary phase field denotes all compositions where arelated phase crystallizes first from a melt. This will beexplained on the case of C3S. The primary phase field of C3S isframed by points P1P6 in Figure 9b or by both gray-shaded

Figure 9. Quaternary phase diagram. (a) CaOSiO2Al2O3Fe2O3 phase diagram. The dotted line denotes the variety of ferrite phasecompositions. (b) The relevant part of a for Portland cement clinker compositions, showing the primary phase fields of different phases.7 (c) Thephase volumes of different clinker phases. (d) Primary phase fields and phase volumes of the different clinker phases. (Modified with permission forref 72. Copyright 2007, Butterworth-Heinemann).




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areas (together) in Figure 9d. This primary phase fieldsuperimposes the phase volumes of C3SC3AC2SC4AFand C2SC3AC12A7C4AF (Figure 9d). The part of theprimary phase field superimposing the former one will beregarded as volume 1 (framed by the dark grayish area) and thepart superimposing the latter as volume 2 (framed by the lightgrayish area). First, it is assumed that the raw meals of anycomposition in both volumes are completely molten. Adecrease in temperature results in both volumes initially inthe crystallization of only C3S from the melts.

Volume 1: Decreasing the temperature further results notonly in the crystallization of more C3S but also in phasesC2S, C4AF, and C3A. At the end of the cooling process, aproduct is obtained, containing all four solid phases.

Volume 2: Further cooling leads to the crystallization ofC2S, C3A, C12A7 ,and C4AF, while C3S remelts. The finalcomposition does not contain the C3S solid phase, butrather the other four phases.

Consequently, primary phase fields only provide informationabout the crystallization path of the melt of a given

composition. The composition of thefi

nal product is notdetermined by the primary phase fields.The previous discussion is only valid under equilibrium

conditions, which are often not achieved or even desired inindustrial processes. For example, rapid cooling of volume 2compositions prevents the complete remelting of C3S, resultingin a product containing C3S, the most important clinker phase.On the other hand, fast cooling could also result in undesiredeffects. In this case, the white shaded volume (volume 3) on theleft side of volume 1 is accounted for (Figure 9d). The primaryphase field of CaO joins directly on to the left side of the C 3Sprimary phase field (Figure 9b), which superimposes partly

with the phase volumes of the C3SC3AC2SC4AFtetrahedron (Figure 9d). The cooling of a melt of anycomposition in volume 3, results first in the crystallization ofCaO. Similar to the scenario described before, CaO remelts

with further cooling, under equilibrium conditions, forcompositions of the superimposed zone and the C3S, C3A,C2S, and C4AF phases crystallize. However, fast cooling resultsin remaining free CaO, decreasing the quality of the clinkerproduct. This discussion is only partly relevant for state-of the-art cement production, since clinker is only partly formed frommelt. However, initial attempts have been made to obtainclinker from a completely molten phase.78 There, it is crucial touse raw meal composition similar to those of volume 1 to avoideither high CaO concentrations (in the case of volume 3compositions) or high C2S (and no C3S) concentration (in thecase of volume 2 compositions).

3.3. Clinker Phase Related Equations. Based on phase

relations in the four component system, several equations havebeen derived to describe the quality and quantity of Portlandcement clinker of a known raw material composition. In allequations, the chemical compositions are expressed in wt %.

The quality of clinker is often referred to as the amount offree (nonreacted) CaO in the sample, which reduces thestrength of concrete.39 Up to now, many theoretically andempirical derived equations have been developed to calculatethe so-called Lime Saturation Factor (LSF) (eq 1).7,72 It isused to quantify the amount of CaO in the raw material thatcan be combined with SiO2, Al2O3, and Fe2O3 to form the mainclinker phases C3S, C2S, C3A, and C4AF. For satisfactory clinkerquality, LSF should be in the range of 9298%.72 A common

equation is given in eq 1. Other parameters are the silica ratio(SR or Ms) (eq2) and the alumina ratio (AR or Ma) (eq3).The SR, usually in the range 23, describes the proportion ofthe silica phases to aluminate and ferrite phases and reflects theratio of solid phases (the silica phases) to the liquid phase,formed by aluminate and ferrite, in clinker. AR expresses theratio between the aluminate phase and ferrite phase andindicates which of these two phases is forming the melt phase(for further details see Section 3.4.2.1).7

=

+ +

LSFCaO

2.8SiO 1.18Al O 0.65Fe O100

2 2 3 2 3 (1)

=

+

SRSiO

Al O Fe O

2

2 3 2 3 (2)

=ARAl O

Fe O

2 3

2 3 (3)

A method used to determine the potential quantities of C3S,C2S, C3A, or C4AF from the four major oxides contained in

cement raw meal is given by the Bogue calculations,

7,79

which isonly mentioned here for the sake of completeness. Detailedinformation can be found in refs 7 and 79.

3.4. High Temperature Clinker Phase Formation. Themechanisms of the clinker formation process are very complex.The chemical composition of the raw meal controls the qualityand quantity of the final clinker product phases, together withraw meal properties like mineralogical composition, particle sizedistributions, and homogeneity, as well as process conditionssuch as maximum burning temperature and retention time ofthe material in the kiln.

The formation of the different clinker phases is subject totwo or three reaction types: solid state reactions,19,80,81 solidliquid and/or liquid state reactions.23 Considering a Portlandcement clinker raw meal composed only of the four majoroxides (CaO, SiO2, Al2O3, and Fe2O3) in a pure form, reactions

below 1250 C proceed solely via solid state reactions. In anabsolutely homogeneous mixture, no liquid phases are formed

below 1338 C, where a invariant point in the four componentsystem exists. Nevertheless, due to inhomogeneities, smallamounts of melt occur locally at temperatures below 1338 C.It is assumed that at temperatures above 1325 C enough meltexists to wet most of the remaining solid particles, in which casemainly solidliquid or liquidliquid reactions occur. Between1250 and 1325 C, all three reaction types exist simulta-neously.16 Of course these absolute temperature values are notstrictly valid, since the occurrence of melt depends highly onthe actual composition, including minor compounds, and thedegree of inhomogeneity of each raw meal mixture. As an

example, most of the belite phase, the aluminate as well asferrite phase formation occur below around 1250 C via solidsolid interactions, whereas alite is formed by all three reactiontypes above 1250 C. Compared to reactions in the presence ofa liquid phase, solid state reactions always proceed between twocompounds only, whereby the product is formed at the originalphase boundary between the solids. Diffusion is thefundamental prerequisite for solidsolid reactions.82,83 Itoccurs due to crystal lattice defects (Figure 10a) and proceedsin two basic steps (Figure 10b), namely, self-diffusion of thediffusing species (e.g., Ca2+ in CaO) and diffusion through theproduct layer toward the reaction interface.82,83A critical role inthe rate of reaction is the contact made by the solid particles.




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In the clinkerization process, the four possible diffusing ionspecies found are Ca2+, Si4+, Al3+, and Fe2+/Fe3+. Thedominating diffusion species are often determined by acomparison of the activation energies of different clinkerformation reactions with the activation energies for species self-diffusion.20,84,85 In a more accurate method, platinum markersare utilized in clinker formation experiments with pressedpowder compacts, which are assembled from two annealedspecimens, each containing one of the pure oxides (Figure11).18,19,86,87 The marker is positioned at the phase boundary

between the compacts. After the clinkerization experiments anevaluation is made as to which side of the marker the product

was located. In the example shown in Figure 11, the powdercompact was assembled from a CaO specimen and a SiO2specimen. After the clinkerization reaction takes place, themarker was located at the boundary phase between CaO andthe C2S (Ca2SiO4) product. Similar tests were carried out in theCaOAl2O3 system.

87 These experiments revealed clearly the

higher mobility of Ca2+ cations. The only exception arises in thecase of ferrite phase formation (C4AF; Ca4Al2Fe2O10), in whichFe3+/Fe2+ is the major diffusing species.85

In the following sections of this article, the formation of thefour main clinker phases is discussed. The order of the sectionsis orientated around the order of the clinker phase occurrencein the industrial process for temperatures of 900 C up to 1500C: belite, aluminate, ferrite > melt > alite. The influence of theparticle size distribution is subsequently discussed.

3.4.1. Clinker Phase Formation between 950 and 1250 C.3.4.1.1. Belite (C2S) Formation. In industrial cementproduction, belite is formed in the rotary kiln at 9001250C.15 Although the process in the kiln is complex and depends

on many parameters, the basis of belite formation is thereaction between solid CaO and SiO2 particles. Therefore, itcan be described by reactions of the two pure oxides only. The

basic understanding of belite formation was established in the1920s and 1930s80,81 and has been verified in laterinvestigations utilizing advanced analysis techniques.15,1820 Inthe reaction of pure CaO and SiO2, belite is one product, withothers being C3S2 (Ca3Si2O7) and CS (CaSiO3) at temper-atures between 950 and 1450 C, and, in the case of CaOexcess and temperatures above 1250 C, alite also (C3S;discussed later). Of these phases, only C2S and C3S are ofinterest, since the other do not exhibit hydraulic properties.The formation of the different phases is determined by diffusingCa2+ through solid matter, which is dependent on temperature,the concentration of the two compounds and on the methodsused to prepare the raw meal. In solidsolid reactions, theproduct occurs as a layer between the original raw materials.

Belite (C2S) is the favored formed product phase in the caseof CaO excess.19,19,80,81 In investigations concerning theinfluence of CaO and SiO2 concentration, the C2S concen-tration was found to be higher, with an increasing CaO/SiO2molar ratio in the raw meal mixture. In raw mixes of CaO/SiO2molar ratios of 4:1 or 3:1, belite have been the only productphase formed. Decreasing CaO concentration (i.e. to CaO/SiO2 = 1:1 or 1:2) causes the formation of additional silicatephases, C3S2 and CS, while the formation of belite exhibits amaximum concentration, and a decrease with time, until no

belite is left. It has been concluded that C2S is the first phaseformed in the presence of CaO excess at the interface of theCaO and SiO2 solids. At longer reaction times, [CaO]decreases, and therefore, Ca2+ diffusion through the productlayer decreases. The more acidic phases C3S2 (Ca3Si2O7) andCS are also formed at the interface of C2S and SiO2.

18,80,81Withthe total consumption of CaO, the [Ca2+] at the CaO/C2Sphase boundary becomes low and C2S-formation stagnates.Finally, Ca2+ diffusion occurs only from the product phases richin CaO in the direction of any remaining SiO2. Here, more CSis formed at the CS/SiO2 interface. The diffusion of Ca

2+

decreases CaO concentration in C2S and C3S2, with the formertransforming into C3S2 and the latter into CS. Therefore, [CS]

Figure 10. (a) Draft of a crystal lattice. X represents cations and Oanions. The ideal order is shown on the left side, and the latticeexhibiting defect sites is shown on the right side. Important crystallattice defects are vacant sites and the occupation of interstitialpositions.82 (b) A sketch of X-ions diffusion through solids. Diffusionthrough solid X represents the self-diffusion of X-ions. This is followedby the diffusion of X through a newly formed product layer Y. At theinterface between product Y and solid Z, the reaction occurs and moreY is formed. The reaction interface moves in the direction of solid Z.

Figure 11. Determination of the diffusing species in the CaOSiO2system utilizing marker experiments. A platinum marker is positionedbetween two compacts of pure raw material. After clinkerization, thelocation of the marker is evaluated. Since it is found between the CaOcompact and the newly formed product layer, Ca2+ is assumed to bethe dominating diffusing species. As a consequence of electro-neutrality, O2 diffusion through the solid or transport through thegas phase occurs.20,86




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increases at the cost of other phases (Figure 12).81 Thecomplete conversion of C2S to CS is a slow process and takes,

even for raw meal mixes with low [CaO], up to 18 h.81

Considering this time frame, the conversion of C2S to CS doesnot play a major role in the industrial process, since theresidence time of the reactants in the kiln is 60 min maximum.

Higher temperatures increase the mobility of diffusingspecies, and thereby accelerate the clinker formation reaction.

Analogous results were reported by Weisweiler et al.,18 whoestablished that an increase in temperature from 1000 to 1250C increased the reaction rate constant of C2S formation by afactor of 75.18 The increase of the Ca2+ diffusion is attributed toa higher number of disordered ions and vacant sites in the

crystal lattice with increasing temperature and to an increase inthe number of mobile Ca2+ ions.82

Besides the diffusion rate, the temperature also influences theconversion of belite polymorphs. High temperature in situ XRDstudies enables the tracking of belite polymorph conversionsduring the reaction:15

At 950 C, belite exists as a L-C2S polymorph.

Up to 1055 C, the transformation of the L-C2Spolymorph to H-C2S is completed.

Complete conversion into the -C2S polymorph occursat 1380 C, but during the cooling process, thistransformation proceeds in a reversed way and iscompleted at 1240 C.

In diffusion controlled reactions, the diffusion distance of aspecies significantly influences the rate of the reaction. Inpowdery raw meals, the diffusion distance to the reactioninterface depends on the fraction of particle fines and isexpressed by the surface/volume ratio. The higher the ratio, theshorter the diffusion distance, in general. Comparisons of rawmeal powder samples ground in a mortar for 15 and 150 min,respectively, have shown a drastic increase in the reaction rateof the CS formation when increasing treatment time. C2S-formation reaches its maximum concentration earlier and theformation of C3S2 as well as CS starts earlier. The conversion ofC2S is completed four times faster than in the case of shortergrinding treatment.81Although these results were not discussedfurther by Jander et al., they can be attributed first to anincreased homogenization of the powder mixture and second toan increase of particle fineness (i.e., an increase in the surface/

volume ratio). Investigations comparing the reaction rate of rawmeal mixes containing CaCO3 or CaO have shown similarresults.80 In the case of the use of CaCO3, the completereaction to CS was faster than when using CaO. Theprecalcined CaO featured larger particle sizes than the original

Figure 12. Formation of C2S by solidsolid reactions. (a) A highconcentration of Ca2+ diffuses toward SiO2. (b) At the interface, C2S isformed, and the product phase grows. (c) The concentration of Ca2+

decreases. Less CaO rich phases, C3S2 and CS, are formed. (d) CaO iscompletely consumed. Ca2+ diffusion occurs only in the formed

product phases. CS is formed at the cost of C2S and C3S2.

Figure 13. C2S formation in particles. (a) A layer of C2S crystals is formed at the interface of CaO and SiO2. (b) With the further diffusion of Ca2+

toward SiO2, more C2S crystals are formed. Simultaneously, the SiO2 particle shrinks. (c) C2S crystals grow in size due to coalescence.




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CaCO3 particles,80 which caused an increase in the diffusion

distance. Similar results have been obtained by varying the SiO2particle size.80 Although these results concerned CS formation,the same correlation of raw material particle size with thediffusion rate and, respectively the reaction rate, is valid,

because each CS formation precedes C2S formation. A moredetailed discussion about the influence of particles sizes on theclinker phase formation will be given later in the text.

In the industrial process, a typical raw meal compositionconsists of 68.6 wt % CaO, 22.6 wt % SiO2, 5.6 wt % Al2O3, and3.2 wt % Fe2O3 (LSF = 95, SR = 2.6, AR = 1.8) excludingminor compounds.5 Belite formation often begins in theprecalciner at around 900 C, and continuous in the rotarykiln.15 The reaction of CaO and SiO2 at the particle interfacecauses the formation of the belite polymorph L-C2S (Figure13a).15 The formed product layer is not a rigid uniform crystallayer, but consists offine, packed belite crystals.23,87,99 FurtherCa2+ diffuses through the newly formed product layer, andreacts at the phase boundary between the product and SiO2particles (Figure 13b). The continuous Ca2+ diffusion causes anincrease in belite layer thickness, while the SiO2 particle coreshrinks simultaneously (Figure 13c)). In addition, the belite

crystals grow in size mainly by coalescence.88 Besides theformation of the product layer a stepwise transformation of the

belite polymorphs occurs simultaneously. At ca. 1055 C theL-C2S polymorph is completely converted into H-C2S.

15

The belite formation reaches its maximum (ca. 60 wt %) ataround 1250 C. Above 1250 C most of the belite isconsumed due to the formation of alite. The remaining H-C2Sis converted at higher temperatures in the more reactivepolymorph -C2S.

89 It should be mentioned here that duringclinker cooling belite polymorph transformation occurs in theopposite direction and the L-C2S polymorph and -C2S aremost likely contained in the final clinker product.88

3.4.1.2. Aluminate (C3A) and Ferrite (C4AF) Formation.C3A and C4AF are of particular importance in the clinkerization

formation, as these phases form a molten phase at lowertemperatures, which influence the development of the chemicaland physical properties of the final clinker product greatly.90

The formation of C3A and C4AF occurs in three steps:

1. Between 950 and 1250 C, the crystalline phases C3A

and C4AF form.2. Above 1250 C, the crystalline phases melt.

3. Recrystallization occurs during clinker cooling.

Here, the focus will be on the first steps only, as the secondstep will be outlined in Section 3.4.2.1. The recrystallizationprocess is thoroughly described elsewhere.91 The formation ofthe C3A and C4AF phases will be described only by reference tothe reaction among CaO, Al2O3, and Fe2O3.

92

Tricalcium Aluminate (C3A) Formation. In the solidsolidreaction of CaO with Al2O3, several calcium aluminate phasescould, in principle, be formed, including C3A, C12A7, C2A, CA,CA2, and CA6.

15,84,87,89101As discussed earlier, the occurrenceof a phase, and the phase formation sequence, strongly dependson experimental conditions, that is, the concentration of theraw materials, temperature, particle size, particle contactpattern, and the extent of crystal lattice defects within thereactants. An overview of possible formation paths is given inFigure 14.

Some general conclusions on the formation of the C3Aphase, that is, the phase of interest in Portland cement clinker,can be drawn. In general, the dominantly diffusing species is

Ca2+ (Figure 15), although Al3+ diffusion also occurs to a smallextent.87,98 Occurring phases are formed as layers between CaOand Al2O3.

In most investigations, the C12A7 phase was initiallyformed,84,87,92,95,96 while in some cases this also applied tothe CA or CA2 phases.

87,92,95 The final product depends,analogous with belite formation, on the concentration of CaOand the CaO/Al2O3 molar ratio, respectively. An excess of CaOresults in the formation of C3A,

9496 while any deficiency inCaO causes the formation of Al2O3 rich phases.87,97,99

Higher temperatures accelerate the diffusion of Ca2+ due toan increase in the number of mobile Ca2+ ions and a highernumber of defect sides in the crystal lattice, respec-tively.87,93,96,97

In the CaOAl2O3 system, mechanochemical activationhas been studied.95,96,99,100 This term describes the increase inthe reactivity of solids due to mechanical treatment, e.g.grinding, which is not related to particle size effects.101

Mechanical stress on a crystal lattice causes distortion anddeformation ofthe crystal structure101 and an increase of crystaldefect sites.95,96 In the case of aluminate phase formation, ball-milled raw materials feature reaction rate acceleration of thedesired phase, governed by the CaO/Al2O3 ratio, especially at

lower temperatures (1100 C) compared to raw materials,which are not mechanically pretreated. Similar to the effect ofhigher temperatures, the increase of defect sites in the crystallattice of the raw materials enhances Ca2+ cations diffusion andtherefore product formation.95,96,99 The study of thismechanochemical activation for clinker formation is still in itsinfancy. Further investigation needs to be focused especially onthe significance of the reaction rate increase due tomechanochemical activation compared to the increase of thereaction rate due do a decrease of particle sizes by grinding(mechanical treatment).

Tetracalcium Aluminoferrite (C4AF) Formation. In situsynchrotron powder diffraction of the clinkerization reactions

Figure 14. Formation of aluminate phases. The most importantaluminate phase in Portland cement clinker is C3A, which is formed bythe direct reaction of CaO and Al2O3 or previously formedintermediate phases such as C12A7, C2A, CA, and CA2. CA6 is abyproduct. Bold arrows indicate the favored direction of equilibriumreactions.

Figure 15. The order of aluminate phases formation because of thedirection of Ca2+ diffusion toward Al2O3 (indicated by the arrow).CaO rich phases are formed close to CaO and Al2O3 rich phases onthe opposite side.




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indicates the consumption of C3A when forming the ferritephase C4AF with Fe2O3.

15 The concentration of C4AF increasesup to approximately 20 wt %, while the concentration of C3Adecreases. As soon as the ferrite-phase reaches a constant levelat around 20 wt %, the C3A concentration increases again. Theferrite phase C4AF is formed by the reaction of C3A with Fe

3+/Fe2+ or with phase CF (CaFe2O7), which is also formed in a

CaO

Al2O3

Fe2O3 system.

85

The former reaction is thepreferred option of the two; therefore, Fe2+/Fe3+ ions diffuseinto the aluminate phase and replace Al3+ ions. If all free ironoxide is consumed, the reaction of C3A with CF occurs (atlower temperatures of ca. 1100 C). At temperatures greaterthan 1100 C the CF phase disproportionates into C2F andCF2 phases. These phases form in a side reaction with C3A andCaO more C4AF. An overview of the ferrite phases is given inFigure 16.

C3A and C4AF phase formation starts after the calcined rawmaterials enter the rotary kiln (Figure 17). At the phase

boundary between the CaO and Al2O3 particles, theintermediate phase C12A7 is formed. With increasing temper-ature, due to the motion of the solid materials along the kiln,Ca2+ diffusion is enhanced. At the interface between the CaOparticles and the C12A7 crystals, C3A is formed at the cost ofC12A7. As a side reaction to C3A formation, C4AF formationoccurs. Fe2O3 particles are in contact with the freshly formedC3A. Fe

3+/Fe2+ ions diffuse into the aluminate phase and partlyreplace Al3+ ions in the crystal lattice. At this stage (i.e. 10501200 C), both C3A and C4AF are formed simultaneously. Theconcentration of C4AF increases steadi ly, whereas C3Aconcentration remains nearly constant, due to consumptionfor the formation of the former phase.15At the end of this step(i.e. the formation of the crystalline phases), the concentrationof C3A reaches around 12 wt % and C4AF ca. 7 wt % for the

same raw meal composition used for the example in Section3.4.1.1.5

3.4.2. Clinker Phase Formation between 1250 and 1450C. 3.4.2.1. Liquid Phase Formation. In systems consisting ofonly CaO, SiO2, Al2O3, and Fe2O3, with typical Portlandcement compositions, melting C3A and C4AF crystal phasescommences at the invariant point at 1338 C.15,75,76 This isonly valid in an absolutely homogeneous mixture. Inhomoge-neities in the raw meal mixture cause a shift of the invariantpoint toward lower temperatures (and different compositions).

As an example, local composition in an Portland cement rawmeal mix of 23 wt % CaO, 15 wt % Al2O3, and 62 wt % SiO2melts at a temperature of ca. 1170 C (Section 3.2, Figure 8,

).10 In addition, all natural raw materials contain minorcompounds, which decrease the melting point of a certaincomposition. Therefore, it is common that a molten phaseoccurs in industrial raw meal mixes at temperatures lower than1338 C. Actually, de la Torre et al. observed the melting ofC3A and C4AF at 1280 C through in situ studies of clinkerformation.15 To simplify, the following discussion of melt

formation will be described for a homogeneous system of pureoxides at a temperature of 1338 C. The reader should keep inmind that temperatures might shift by 50100 C in moretypical Portland cement raw meals.

The composition of the melt phase at the invariant point is54.8 wt % CaO, 22.7 wt % Al2O3, 16.5 wt % Fe2O3, and 6.0 wt% SiO2. Preformed crystalline C3A and C4AF melt to provide

Al2O3 and Fe2O3, as well as CaO for the melt phase. SiO2 isobtained from free SiO2 particles or, if all has been consumedfor belite formation, from partially liquefied belite (Figure 19ac). The extent of C3A and C4AF melting at 1338 C dependson the total clinker raw meal composition, that is, on the ratioof Al2O3/Fe2O3 in the total composition. This can be visualizedin Figure 18, which shows the part of interest for Portland

cement clinker compositions in the phase diagram for the fouroxides. Since only C3A and C4AF melt, the amount of all theother phases, mainly CaO and C2S, is fixed. It should beemphasized here that, for all three of the following discussedcases, the Al2O3/Fe2O3 ratio of the molten phase is always 1.38,as it is for the composition at the invariant point. In the firstcase, a total clinker raw meal composition with an Al2O3/Fe2O3ratio of 1.38 is considered. The total composition can bedescribed by the triangular plane CaOC2SP8 in Figure 18.The Al2O3/Fe2O3 ratio is constant over the whole surface ofthe plane.76 Since it is the same ratio as in the molten phase atthe invariant point, all crystalline C3A and C4AF melts.

11A totalraw meal composition with an Al2O3/Fe2O3 ratio less than 1.38is represented by any triangular plane between CaOC2SP8

and CaO

C2S

C4AF. In this case, Fe2O3 exists in higherconcentrations, as in the previous case, and therefore, morecrystalline C4AF is formed. When melting commences, C3Amelts completely, but only parts of C4AF melt to provide themelt phase with 16.5 wt % Fe2O3 (Fe2O3 concentration atinvariant point). The other part of C4AF remains crystalline.Finally, an Al2O3/Fe2O3 ratio higher than 1.38 in the total rawmeal composition is considered, which is described by everyplane between CaOC2SP8 and CaOC2SC12A7. Here, thesituation is reserved. More Al2O3 exists in the raw meal than inthe first case, so more C3A is formed and the amount of C4AFis limited, which results in the complete melting of the ferritephase, while only parts of the aluminate phase melt.11,76

Crystalline C3A is left.

The liquid phase fulfi

lls two important tasks in the clinkerburning process:

1. Acceleration of the clinker phase formation.

2. Prevention of clinker dust formation.

This results in the macroscopic effect of nodule formation. Inclinker nodules solid raw material particles as well as formedcrystal phases are held together by the liquid phase. Furthercrystal phase formations are accelerated due to the diffusion ofCa2+ through the melt, which is faster than diffusion throughsolids.24 Strong agglomerates are formed, when melt is present,around 1520 wt % filling out most of the void space betweenparticles.9 The amount of nongranulated material increases with

Figure 16. Formation of the C4AF phase. C4AF is formed directly dueto the reaction of C3A with Fe2O3 or from intermediate ferrite phasessuch as CF, CF2, and C2F.




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a decrease of liquid, since less particles are moistened by theliquid.102

The granulation proceeds in three steps:

1. agglomeration and regrouping of the particles2. granule growth

3. solidification and crystallization of the melt12,102

The first step occurs rapidly. Primary particles are moistenedby droplets of liquid, which are then quickly drawn and held

together to form porous granules (Figure 19d). Small granuleshave an excess of liquid at their outer boundaries, so moreparticles can be incorporated in the granule.102 Denseragglomerates are formed by regrouping grains (Figure 19e).The melt enters the pores in and between the agglomeratedparticles, and carries weakly bound particles into the interior,

which is accommodated by particle shrinkage. This process isstrongly dependent on the surface tension and viscosity of theliquid,12,102 but independent of rotational speed of the kiln,retention time in the kiln, or material loading.103A high surfacetension is required to ensure sufficient adhesion of the particlesto each other and a lower viscosity supports particle transport.It was found by Timiashev et al.,13 that the final size of clinkernodules is directly proportional to the surface tension of the

melt. In this context, the effect of adding minor compounds isof great importance. Melts of pure cement clinkers exhibitsurface tensions of ca. 0.5 N/m (comparable with the surfacetension of Hg) and a viscosity of 0.10.2 Pas (similar to the

viscosity of olive oil). The second stage of the nodulization ischaracterized by granule growth and the formation of thecrystal phases C2S and C3S (Figure 19f).

12,102 Therefore, alite issurrounded by more melt than belite, since it develops a lessdense crystal packing.14 This stage becomes dependent on time,rotational velocity, and loading, while inertial and gravitationalforces become important, with the tendency for granules to

break down, counteracting growth.102 In the third stage, theliquid crystallizes to form the aluminate and ferrite phase at a

Figure 17. Aluminate and ferrite phase formation. (a) Ca2+ cations diffuse results in the formation of the intermediate phase C12A7 at the interface ofCaO and Al2O3 particle. (b) The aluminate phase is formed due to further diffusion of Ca

2+ and a reaction with the previous formed intermediatephase. Fe3+ cations penetrate the formed aluminate phase causing the formation of the ferrite phase. (c) Growing aluminate and ferrite crystals andconsuming Fe2O3, CaO, Al2O3, and C12A7.

Figure 18. Melt formation from C3A and C4AF. Point P8 denotes the

invariant point at 1338 C in the quaternary system. The Al2O3/Fe2O3ratio at this point is 1.38. The scattered triangle, swinging around theCaOC2S axis, represents a constant CaO and C2S composition. Forany position of the scattered triangle between points C4AF and P8, theAl2O3/Fe2O3 ratio is 1.38, indicating a complete melting of C4AF.

7,72




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temperature of 1250 C,15 or if the liquid cools down toorapidly, it forms a glass residue.7577,79

3.4.2.2. Alite (C3S) Formation. The alite crystal phase is thepreferred clinker phase in Portland cement clinker, since itfeatures the highest hydraulic strength. The formation of alite is

very complex, since it can proceed via solidsolid,20 solid

liquid,90

or liquidliquid21,23

reactions. Mostly, the solidsolidreaction can be neglected, since diffusion through solids isminimal compared to diffusion through a liquid.23

Alite is only formed in the presence of excess CaO (i.e., atCaO/SiO2 > 1) and is thermodynamically stable above 1250C.21 Small amounts are already formed at lower temperatures,and therefore, the belite and alite formation overlap, whichcomplicates the kinetic description.81 The formation reaction is

believed to be controlled by the diffusion of Ca2+, through theliquid phase.9,17,2326 CaO dissolves partly in the melt, andCa2+ ions diffuse in the direction of the belite product, wherethe formation reaction occurs. Nonetheless, it is notinvestigated in detail, whether the belite phase liquefies partlyand C3S crystallizes out of the melt or if the solid belite is

converted into alite by penetration of Ca2+

into defect sites ofthe solid C2S (Figure 20a).

23

Nevertheless, the new alite crystals form layers between theCaO agglomerates and belite crystals (Figure 20b). The alitecrystal layer is not rigid, and the void between the crystals isfilled with liquid. Successive Ca2+ diffusion occurs through thealite crystal/melt layers to the interface between alite and belite.

Although the reaction boundary is located away from CaO, thealite layer functions as a connection to the CaO agglomeratesdue to the inner motion of the liquid (Figure 20c). DecreasingCaO concentration during the continuing process results in thegrowth of alite crystals instead of forming new seed crystals.23

The consumption of CaO during the alite formation could

result in pores of the size of the original CaO agglomerates,surrounded by a product layer.26 Advanced diffusion studies inthe ternary system CaOSiO2Al2O3 show the correlation

between the reaction rate for the alite formation and theamount of liquid phase. A 4-fold increase of the melt up to 30

wt % in the clinker mixture, increases the reaction rate by a

factor of about 13 at a temperature of 1500 C. Although thisinvestigation considered only isothermal conditions, the findingcan also be applied correspondingly to nonisothermalconditions in industrial clinker production.17 The nature ofthe reaction rate increase has not yet been discussed, but itmight be related to an increase in the number of solid particles,in this case CaO and belite, being in contact with liquid. Themacroscopic effect is the formation of strong nodules, asdescribed.102 The increased contact surface between the twophases increases the number of diffusing species (Ca2+)penetrating through the interface into the belite phase, wherealite formation occurs.

3.4.3. Overview of Clinker Formation in the IndustrialRotary Kiln. The calcined solid material mixture enters the

rotary kiln at a temperature of approximately 900 C. A typicalmaterial composition, excluding minor compounds, consists of68.6 wt % CaO, 22.2 wt % SiO2, 5.6 wt % Al2O3, and 3.2 wt %Fe2O3. At first, the low temperature clinker phases tricalciumaluminate and calcium aluminoferrite (Figure 21b) are formedsimultaneously, and belite formation continues from the firstformation in the precalciner (Figure 21a). These reactions aremainly solid state reactions. A product layer of fine, packed

belite crystals is formed at the interface of CaO and SiO2(Figure 21a). Belite crystal formation and growth occurs in thedirection to the SiO2 particle, as a result of Ca

2+ diffusion intothe first crystal layers of the SiO2 particles. The aluminate phaseis formed by the reaction of CaO and Al2O3 (Figure 21b). Ca

2+

Figure 19. Melt and granules formation. (a) The clinker consists of crystalline phases in a four-oxide system until temperatures between 1250 and1338 C are reached. (b) At the beginning of the melt formation, C 3A, C4AF, and SiO2 melt. (c) C2S melts partially if all free SiO2 is consumedcompletely. (d) The CaO and SiO2 particles are moistened by the melt and form porous granules. (e) Denser granules are formed, due to smallerparticles regrouping into the interior of the granule. (f) The granules grow and the clinker phases C 3S and C2S are formed.




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diffuses in the direction of Al2O particles forming aluminatephase crystals at the interface of the reactant particles. The

formation often occurs via the intermediate phase C12A7. Atapproximately 10501200 C, Fe3+ ions diffuse into previouslyformed aluminate phase crystals, partly replacing Al3+ ion in thecrystal lattice and forming the ferrite phase. At this point, bothreactions, the aluminate and ferrite phase formation, occursimultaneously. The concentration of formed belite, aluminate,and ferrite reaches a maximum at around 1250 C. Meltformation (ca. 1520 wt %) occurs at temperatures between1250 and 1338 C, after which the aluminate and ferrite phase,remaining SiO2 and/or belite then melt (Figure 21c). Theliquid phase accelerates all further clinker formation reactionsand prevents the formation of clinker dust due to the formationstrong agglomerates (Figure 21d). The formation of alite

occurs at temperatures greater than 1250

C either by solid

liquid reactions, liquid phase reactions or both (Figure 21e).CaO dissolves partly in the melt, and Ca2+ ions diffuse in thedirection of the belite product, where the formation reactionoccurs. Subsequently, either the belite phase is molten and alitecrystallizes out of the melt (liquid phase reactions) or the solid

belite is converted into alite by Ca2+ penetration into the defectsides of the solid belite (solidliquid reaction).

The hot clinker exits the kiln and is cooled rapidly to atemperature below 1250 C to prevent the decomposition ofalite to belite and CaO. The final clinker product consists ofabout 5070 wt % alite, 1530 wt % belite, 510 wt %aluminate, and 515 wt % ferrite phase.7

3.4.4. Influence of Raw Material Particle Size Distributions.The fineness of raw materials is an essential factor in the quality

of the clinker/cement product, and therefore, it requires carefultreatment in order to carry out cement production in aneconomically efficient way. As a rule of thumb, with finer rawmaterial particles, the burnability of the raw meal improves,

which allows clinker burning at lower temperatures and/or atshorter retention times.27,28 Actually, particle fineness affectsthe homogeneity of the product, clinker crystal sizes and theconversion degree of the reacting compounds. Coarse particlesizes cause inhomogeneities in cement raw meal, resulting inlocal volumes with chemical compositions deviating from theaverage composition. Therefore, the particle sizes of pure rawmaterials such as limestone and sand are of more crucialinfluence than other mixed raw materials such as marl, which

feature certain homogeneity of their own. In general, particlesize effects are discussed in the literature only in connectionwith the calcareous and siliceous compounds of the raw meal,but not for the Al2O3 and Fe2O3 compounds. The latter twocompounds are added mostly to the cement raw meal in theform of mixed raw materials and are therefore relatively fineand uniformly distributed.

The following discussions about the influence of particle sizesand size distributions are based on the assumption of a constantoverall chemical composition of raw meals. Differences existonly due to variations in particle size fractions. For a betterunderstanding, raw meal compositions with an overall LSF of100% are assumed, so that theoretically, all CaO in the raw

Figure 20. Alite formation. (a) Diffusion of Ca2+ ions in the direction of belite, which is either crystalline (upper C2S particle) or partly molten(lower C2S particle). (b) C3S formation at the interface between the liquid and C2S particle (upper C2S particle) and/or in the belite melt phase(lower C2S particle). (c) Growth of alite crystal at the expense of belite.




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meal is consumed completely for the formation of the clinker

phases.The influence of coarse particles on clinker properties hasbeen qualitatively described by Johansen26,28 utilizing theCaOSiO2Al2O3 phase diagram at 1500 C (Figure 22). Thisphase diagram shows several areas of interest:

two-phase areas (A1, A3, and A5) and three-phase areas(A2 and A4)

area A1 denotes all compositions of CaO in equilibriumwith the melt of any composition on the isothermbetween points M1 and M2.

A3 area indicates compositions of alite in equilibriumwith the melt of M2M3,

in area A5 belite is in equilibrium with melt of M3M4 the three-phase area A2 marks all compositions of CaO

and alite in equilibrium with the melt of composition M2 in area A4 alite and belite are in equilibrium with the

melt of M3 composition.

The following discussion should be understood as anexample of one out of many possible raw meal compositionsand is not claimed to complete.

As a starting condition for the following discussion, a typicalcement raw meal composition, denoted by point P in area A4,

will be assumed. It can be understood as an average or overallraw meal composition with an LSF of 100%.34 Due toinhomogeneities, caused by coarse particles and by poor

blending, the raw meal also contains areas (local volumes)represented by oxide compositions (CaO, SiO2, Al2O3) related

to the A2 and A5 areas, which are characterized by LSFs

diff

erent from the average LSF: higher than the average LSF inarea A2 (161%) and lower than the average LSF in area A5(e.g., 66%).34 It is assumed that the concentration of Al2O3 andFe2O3 are constant, the local volumes differ only in [CaO] and[SiO2].

The starting conditions are shown in Figure 23a. The figurein the center represents the average raw meal composition offinely ground, homogeneously mixed CaO and SiO2 particles(A4). For the sake of simplification, other compounds such as

Al2O3 are considered present only in the melt phase. Thepicture on the left side shows the case of coarse SiO2 particlescharacterized by area A5, and the picture on the right sideindicates compositions containing coarse CaO particles as inarea A2. Both areas are in contact with A4. It should be borne

in mind that the starting condition is not represented by thephase diagram of Figure 22, since only the oxides and noreaction products or melt are present. Further discussions deal

with the case of isothermal heating at 1500 C, but qualitatively,they are also valid for nonisothermal heating from temperaturesabove 1338 C, where the occurrence of a melt phase sets in.Heating the raw meal results in clinkerization reactions withinthe local volumes, reaching local equilibrium.23,2629 In the caseof the raw meal composition of A4 (center figure in Figure23b), CaO and SiO2 particles react to form a phase of alite and

belite crystals, which is in equilibrium with a melt denoted bypoint M3 in Figure 22. Coarse SiO2 particles from area A5 leadto the formation of belite clusters (Figure 23b (left)). Diffusing

Figure 21. Overview of clinker phase formation in an industrial rotary kiln. (a) Formation of belite; (b) formation of the aluminate and ferrite phase;(c) melt formation; (d) formation of agglomerates; (e) formation of alite.




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Ca2+ reacts with the SiO2 particles resulting in belite clustersroughly twice the size of the former SiO2 particle.

33 The beliteparticles are in equilibrium with a melt of a compositionsituated on the isotherm somewhere between the point M4 andM3 (Figure 22). In contrast, the presence of coarse CaOparticles (area A2 in Figure 22) results in residual CaO, whichhas not been consumed for the alite reaction. The free CaOparticles are either the remains of former coarse CaO particlesor form from melt recrystallized CaO agglomerates.23 Never-theless, CaO and alite are in equilibrium with the melt phase(related to A2 in Figure 23b), as indicated by M2 (Figure 22).

All further reactions can be understood as diffusioncontrolled reactions between the different local volumes.26,28

The melt phase (M3-M4) related to the area A5 is poorer inCaO than the melt phase (M3) of A4. CaO diffusion occurs inthe melt phase in the direction of M4 (Figure 22), so alite fromphase A4 decomposes, forming belite and CaO, which aredissolved in the melt phase. As a result, the local volume withcompositions of area A5 increases (growth of the belite

cluster), whereas A4, containing alite and belite, decreases(Figure 23c). On the other hand, the melt phase of the A2 areais rich in CaO (M2), causing diffusion of CaO in the directionof melt phase M3 of A4, where it reacts with belite crystals toform alite (area A3) (Figure 23c (right)). Both processescontinue with increasing heating time. The formation of alite(A3) stops, when all free CaO has been consumed. Local

volumes with a composition of area A2 disappear (Figure 23d(right)). In contrast, the dissolution and diffusion of CaO fromdecomposed alite in area A4 only stops, when the melt phasecomposition of area A5 has reached M3 (Figure 23d (left)). Inthis case, all A4 alite has been decomposed and A4 degeneratedto a boundary layer between A5 and A3.

The formation of the clinker phases is a diffusion controlledprocess and therefore final product composition is stronglydependent on temperature and time. For any raw meal of acomposition of LSF less than or equal to 100% a completereaction of CaO with other raw meal compounds to form the

clinker phases, regardless of the degree of inhomogeneity dueto coarse particle size, is theoretically possible, if adequatereaction times and maximum temperatures (excluding the caseof total melt formation) are chosen. Higher temperaturesincrease the amount of CaO dissolving in the melt, as well asthe diffusion of Ca2+, while longer reaction times allow diffusionof the different compounds, especially Ca2+, even over longdistances (>0.3 mm)30 and through tightly packed crystalstructures until no more diffusion gradient persists between thelocal volumes.

Usually, industrial burning times are shorter than indicatedby the sketch, resulting in clinker distributions similar to thosehighlighted in Figure 23c), with both areas dominated by belite

Figure 22. Part of the isothermal ternary phase diagram CaOSiO2Al2O3 relevant for compositions typical for Portland cement clinker

showing the composition of different areas. A1 compositions withCaO in equilibrium with the melt of any composition M1M2; A2compositions with CaO and C3S in equilibrium with the melt of M2;A3 compositions with C3S in equilibrium with the melts of anycomposition M2M3; A4: compositions with C3S a n d C2S inequilibrium with the melt of M3; A5: compositions with C2S inequilibrium with the melt of any M3M4 composition.26 (Modifiedwith perm issi on from ref 72. Copyright 2007, Butterworth-Heinemann).

Figure 23. Formation of alite and belite depending on the particle sizeof CaO and SiO2 particles. (a) A5: coarse SiO2 particles aresurrounded by fine CaO particles. A4: finely ground, homogenously

mixed CaO and SiO2 particles. A2: coarse CaO particles surroundedbyfine SiO2 particles. (b) Formation of belite and alite. A5: formationof the belite cluster. A4: belite and alite are homogenously distributed.A2: formation of alite and free CaO is left. (c) A4: Diffusion of CaOinto the melt and decomposing of alite. A5: the belite clusters grow insize due to the decomposition of alite (A4). A2: diffusion of CaO inthe direction of the belite phase of A4, forming alite (A3). (d)Continuing growth of belite clusters (A5) and distributed alite crystals(A3) until belite (A4) is consumed.




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clusters, fairly uniform distributed alite and belite areas, andareas with nonreacted CaO within clinker phases. Theinhomogeneity of the raw meal due to coarse particle sizesoften leads to the development of big belite clusters embeddedin a matrix of alite, instead of homogeneous distribution of aliteand belite. Belite has a slower hardening behavior than aliteduring the hydration reaction in the cement product. A

homogeneous distribution of belite in cement is required toensure a uniform hardening of cement,103 so inhomogeneousclinker requires more effort in grinding and homogenizationafter the clinker cooling. Additionally, belite features a higherhardness than all other clinker phases.104 Clinker granulescontaining high amounts of big belite clusters are harder togrind than small, uniformly distributed crystals. All in all, theclinker grinding of an inhomogeneous product, as described,results in higher energy consumption (electrical energy) and

wear on the grinding mills. Conversely, if the temperature and/or the retention time of the material in the rotary kiln areincreased to allow the conversion of CaO and belite cluster toalite, belite and alite crystals in other more homogeneous localareas will grow in size. Clinker products rich in big alite and

belite crystals could cause problems during clinker grinding,even though energy consumption and material wear would besomewhat lower than in the case of high concentrations of

belite clusters. Furthermore, the increasing costs of fuel in orderto achieve higher maximum temperatures and longer burningtimes need to be opposed to the costs of the energyconsumption of clinker grinding and maintenance of theclinker grinding mill. A third consideration needs to be takeninto account owing to the fact that a small concentration of freeCaO (2 wt %) in the final clinker is acceptable and potentialhigher concentrations can be counteracted by reducing theCaCO3 concentration in the raw meal. Of course, this wouldnot affect the possible formation of belite nests in the case ofcoarse siliceous particles. All of these considerations show how

complex the correlation between the chemical and physicalcomposition of raw meal is and how careful adequate processsteps need to be considered, that is, raw material grinding cost

vs fuel costs vs electrical energy and maintenance costs.The term particle fineness refers to particle sizes as well as

particle size distributions. Particle size distributions are used, ifthe burnability of a raw meal is classified, whereas specificparticle sizes (diameters) are referred to when the influence ofone compound on clinker formation and product properties isdiscussed. The influence of particle size distributions on

burnability was extensively investigated in laboratory tests byHeilmann.27 As a general tendency, it could be concluded thathigher weight fractions of small particles in raw meal result in

better burnability. It has been found that the weight fraction of

particles below 0.015 mm should be ca. 35 wt %. Incomparison, decreasing the amount of this particle fraction ina raw meal resulted in a significantly lower burnability. On thecontrary, an increase in the weight fraction did not improve

burnability. For each raw material (CaCO3 and SiO2 indifferent raw materials such as sand, clay, marl, slag andmarble), a significant fraction needs to be smaller than 0.05mm, otherwise burnability is negatively affected. Of course,precise values are not applicable to each industrial raw meal, butthe tendencies are. As a matter of fact, as unique as raw meals ofeach works are, as individual is also the optimal particle sizedistribution of each raw meal. In each case, burning tes ts arerequired to determine the most suitable particle fineness.27

Critical particle sizes refer to the maximum acceptableparticle diameters of the different mineralogical calcareous andsiliceous compounds in raw meal to prevent impairment of

burnability. Typical (approximate) values for critical particlesizes given in the literature are 0.15 mm,27 0.125 mm,23,32,33

0.09 mm,27 0.63 mm,31 and 0.044 mm23,32,33 (although theactual mesh size would be 0.045 mm). They have been

obtained through laboratory clinker burning tests at 1400Cfor 30 min utilizing ordinary sieves of standard mesh sizes to

fractionize the raw meal compounds. The first two values arerelated to the calcareous compounds in the raw meal, the latterthree to the siliceous compounds. The values of the maximumdiameter of 0.15 mm and 0.09 mm are based on the extensivestudies of Heilmann.27 Both were replaced by other values(0.125, 0.063, and 0.044 mm).23,3133

Particle sizes of pure calcite larger than 0.125 mm i ncreasethe local LSF drastically, for example, by LSF 25,34 andresult in dense nonreacted CaO clusters36 in the clinkerproduct within normal reaction times (3045 min) and formaximum cement plants burning temperatures. Furthermore,the concentration of the clinker phases, most likely of alite,

since it is formed in the final step, will be lower thantheoretically possible from the overall chemical composition,

because less dissolved CaO is available for the formationreaction. Particles of calcareous marl greater than 0.125 mmlead often to clusters of small alite crystals surrounded bynonreacted CaO.36 Small alite crystals are formed due to thenatural occurrence of SiO2 in marl. If a marl particle has acementitious composition, e.g. LSF = 9298%, SR = 23%,and AR = 23%, then alite crystals, small amounts of belite and

very small amounts of nonreacted CaO are formed.36

Therefore, large calcareous marl particles are problematic, ifthe chemical composition is somewhat different from normalcementitious compositions. Coarse CaCO3 particles in therotary kiln feed are not only a result of poor raw materialgrinding. Fine CaCO3 (resulting in dusty CaO due tocalcination) causes coarsening of the particle sizes in the kilnfeed. Dusty CaO entering the rotary kiln is easily floated by thekiln gases and transported out back to the calciner andpreheater cyclones. Early liquid formation causes the formationof agglomerates, which are separated from the gases in thecyclone system and enter kiln feed often as particles above thecritical size (up to 1 mm).35

Adjusting the particle size of siliceous compounds is of moreimportance than for calcareous compounds, since the resultsare not only nonreacted CaO clusters and lower concentrationsof alite, but also dense belite clusters that cause numerousdownstream problems such as poor grindability. Most crucial

are quartz,fl

int, and slag particles greater than 0.044 mm.31,36

Here, large belite crystals ( 0.6 mm;31,36 in the modern kilnsystems 0.2 mm117) are formed and compacted in denselypacked belite clusters, often with an acid insoluble siliceouscore (for particle greater than 0.2 mm).31,36 Particles of othersiliceous materials such as marl, shale, and feldspar (low inCaCO3 concentration) should exist in the raw meal as particlesless than 0.063 mm; otherwise, irregular belite clusters withcrystals of 0.30.4 mm are formed. The undesired effect of

belite clusters on clinker quality has been described previously.An overview of the effect of specific mineralogical compound

particles exceeding the critical diameter on the final clinkerproduct is given in Table 4.31,36



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