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Cement

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CEMENT , TYPES OF CEMENTS , PORTLAND CEMENT …

CEMENT , TYPES OF CEMENTS , PORTLAND CEMENT
TYPES OF PORTLAND CEMENT, GENERAL FEATURES OF THE MAIN TYPES OF PORTLAND CEMENT, ORDINARY PORTLAND CEMENT (OPC), RAPID HARDENING PORTLAND CEMENT, SPECIAL TYPES OF RAPID HARDENING PORTLAND CEMENT, MANUFACTURE OF PORTLAND CEMENT, Raw Materials, Crushing & Grinding of Raw Materials,Type of cement processes, Wet Process, Dry process, Burning Process, Grinding, storage, packing, dispatch,CEMENT CHEMISTRY,Chemical Compositions,Bogue’s Equations, Fineness of cement

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  • 1. Topic 5: CEMENT A short series of lectures prepared for the Third year of Geology, Tanta University 2013- 2014 by Hassan Z. Harraz hharraz2006@yahoo.com 13 March 2014 Prof. Dr. H.Z. Harraz Presentation Cement
  • 2. OUTLINE OF TOPIC 5: CEMENT TYPES OF CEMENTS PORTLAND CEMENT TYPES OF PORTLAND CEMENT GENERAL FEATURES OF THE MAIN TYPES OF PORTLAND CEMENT ORDINARY PORTLAND CEMENT (OPC) RAPID HARDENING PORTLAND CEMENT SPECIAL TYPES OF RAPID HARDENING PORTLAND CEMENT MANUFACTURE OF PORTLAND CEMENT:- 1) Raw Materials 2) Crushing & Grinding of Raw Materials 3)Type of cement processes: a)Wet Process b)B) Dry process 4) Burning Process 5) Grinding 6) storage, packing, dispatch CEMENT CHEMISTRY  Chemical Compositions  Bogue’s Equations  Fineness of cement 13 March 2014 Prof. Dr. H.Z. Harraz Presentation Cement 2
  • 3. CEMENT DEFINATION:  Cement is the mixture of calcareous, siliceous, argillaceous and other substances.  Cement is a hydraulic binder and is defined as a finely ground inorganic material which, when mixed with water, forms a paste which sets and hardens by means of hydration reactions and processes which, after hardening retains it's strength and stability even under water. 13 March 2014 Prof. Dr. H.Z. Harraz Presentation Cement 3  Cement used in construction is characterized as hydraulic or non-hydraulic. 1) Hydraulic cements (e.g., Portland cement) harden because of hydration, chemical reactions that occur independently of the mixture's water content; they can harden even underwater or when constantly exposed to wet weather. The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water-soluble. 2) Non-hydraulic cements (e.g. Gypsum plaster) must be kept dry in order to retain their strength.  TYPES OF CEMENTS: 1)Portland cements 2)Natural cements 3)Expansive cements 4)High-alumina cements
  • 4. PORTLAND CEMENT  Portland Cement is a hydraulic cement that hardens in water to form a water- resistant compound. The hydration products act as binder to hold the aggregates together to form concrete. A hydraulic cement made by finely pulverizing the clinker produced by calcining to incipient fusion a mixture of argillaceous and calcareous materials Portland cement is the fine gray powder that is the active ingredient in concrete Limestone + Shale/Clay + Heat = Clinker +CKD + Exit Gas Material Temperatures Exceed 2700 oF Pulverized Clinker + Gypsum = Portland Cement Cement is powder so fine that one pound contains 150 billion grains The name Portland Cement comes from the fact that the colour and quality of the resulting concrete are similar to Portland stone, a kind of limestone found in England. 13 March 2014 Prof. Dr. H.Z. Harraz Presentation Cement 4
  • 5. TYPES OF PORTLAND CEMENT According to the ASTM standard, there are five basic types of Portland Cement:. Regular cement, general use, called Ordinary Portland cement (OPC) – Type Ι Moderate sulphate resistance, moderate heat of hydration, C3A < 7% , Modified cement - Type ΙΙ Rapid-hardening Portland cement , With increased amount of C3S, High early strength – Type ΙΙΙ Low heat Portland cement – Type ΙV High Sulfate-resisting Portland cement – Type V It is possible to add some additive to Portland cement to produce the following types:  Portland blastfurnace cement – Type ΙS  Pozzolanic cement - Type ΙP  Air-entrained cement - Type ΙA  White Portland Cement (WPC)  Colored Portland Cement Note:  sulphates can react with C4ASH18 to from an expansive product. By reducing the C3A content, there will be less C4ASH18 formed in the hardened paste.  Physically and chemically, these cement types differ primarily in their content of C3A and in their fineness.  In terms of performance, they differ primarily in the rate of early hydration and in their ability to resist sulfate attack. • The general characteristics of these types are listed in Table 2. • The oxide and mineral compositions of a typical Type I Portland cement were given in Tables. 13 March 2014 Prof. Dr. H.Z. Harraz Presentation Cement 5
  • 6. GENERAL FEATURES OF THE MAIN TYPES OF PORTLAND CEMENT ASTM Type Classification Characteristics Applications Type I General purpose Fairly high C3S content for good early strength development General construction (most buildings, bridges, pavements, precast units, etc) Type II Moderate sulfate resistance (Modified cement) Low C3A content (<8%) Structures exposed to soil or water containing sulfate ions Type III High early strength (Rapid-hardening) Ground more finely, may have slightly more C3S Rapid construction, cold weather concreting Type IV Low heat of hydration (slow reacting) Low content of C3S (<50%) and C3A Massive structures such as dams. Now rare. Type V High sulfate resistance Very low C3A content (<5%) Structures exposed to high levels of sulfate ions White White color No C4AF, low MgO Decorative (otherwise has properties similar to Type I) 13 March 2014 Prof. Dr. H.Z. Harraz Presentation Cement 6 Chemical composition of the main types of Portland cement
  • 7. ORDINARY PORTLAND CEMENT (OPC)  This type of cement use in constructions when there is no exposure to sulfates in the soil or groundwater.  The chemical composition requirements are listed as shown below: Lime Saturation Factor (L.S.F.) = {CaO-0.7(SO3)}/ {2.8(SiO2)+1.2(Al2 O3)+0.65(Fe2O3)} L.S.F. is limited between 0.66-1.02 Where each term in brackets denotes the percentage by mass of cement composition. This factor is limited – to assure that the lime in the raw materials, used in the cement manufacturing is not so high, so as it cause the presence of free lime after the occurrence of chemical equilibrium. While too low a L.S.F. would make the burning in the kiln difficult and the proportion of C3S in the clinker would be too low. Free lime – cause the cement to be unsound.  Percentage of (Al2O3/Fe2O3) : is not less than 0.66  Insoluble residue: not more than 1.5%  Percentage of SO3 : limited by 2.5% when C3A≤7%, and not more than 3% when C3A>7%  Loss of ignition L.O.I. : 4% (max.)  Percentage of MgO : 5% (max.)  Fineness : not less than 2250 cm2/gm 13 March Prof. Dr. H.Z. Harraz Presentation Cement 7
  • 8. Typical compound composition of Ordinary Portland Cement (OPC) Chemical Name Oxide Formula Cement Notation Mineral Name Weight (%) Tricalcium Silicate 3CaO.SiO2 C3S Alite 50 Dicalcium Silicate 2CaO.SiO2 C2S Belite 25 Tricalcium Aluminate 3CaO.Al2O3 C3A Aluminate 12 Tetracalcium Aluminoferrite 4CaO.Al2O3.Fe2O3 C4AF Ferrite 8 Calcium sulfate dihydrate CaO.SO3.2H2O CSH2 Gypsum 3.5 13 March Prof. Dr. H.Z. Harraz Presentation Cement 8
  • 9. 13 March 2014 Prof. Dr. H.Z. Harraz Presentation Cement 9  The differences between these cement types are rather subtle. All five types contain about 75 wt% calcium silicate minerals, and the properties of mature concretes made with all five are quite similar. Thus these five types are often described by the term “Ordinary Portland Cement”, or OPC.  Types II and V OPC are designed to be resistant to sulfate attack. Sulfate attack is an important phenomenon that can cause severe damage to concrete structures. It is a chemical reaction between the hydration products of C3A and sulfate ions that enter the concrete from the outside environment. The products generated by this reaction have a larger volume than the reactants, and this creates stresses which force the concrete to expand and crack. Although hydration products of C4AF are similar to those of C3A, they are less vulnerable to expansion, so the designations for Type II and Type V cement focus on keeping the C3A content low. There is actually little difference between a Type I and Type II cement, and it is common to see cements meeting both designations labeled as “Type I/II”. The phenomenon of sulfate attack will be discussed in much more detail in Sections 5.3 and 12.3, but it should be noted here that the most effective way to prevent sulfate attack is to keep the sulfate ions from entering the concrete in the first place. This can be done by using mix designs that give a low permeability (mainly by keeping the w/c ratio low) and, if practical, by putting physical barriers such as sheets of plastic between the concrete and the soil.  Type III cement is designed to develop early strength more quickly than a Type I cement. This is useful for maintaining a rapid pace of construction, since it allows cast-in-place concrete to bear loads sooner and it reduces the time that precast concrete elements must remain in their forms. These advantages are particularly important in cold weather, which significantly reduces the rate of hydration (and thus strength gain) of all Portland cements. The downsides of rapid-reacting cements are a shorter period of workability, greater heat of hydration, and a slightly lower ultimate strength.  Type IV cement is designed to release heat more slowly than a Type I cement, meaning of course that it also gains strength more slowly. A slower rate of heat release limits the increase in the core temperature of a concrete element. The maximum temperature scales with the size of the structure, and Type III concrete was developed because of the problem of excessive temperature rise in the interior of very large concrete structures such as dams. Type IV cement is rarely used today, because similar properties can be obtained by using a blended cement.  White Portland cement (WPC) is made with raw ingredients that are low in iron and magnesium, the elements that give cement its grey color. These elements contribute essentially nothing to the properties of cement paste, so WPC actually has quite good properties. It tends to be significantly more expensive than OPC, however, so it is typically confined to architectural applications. WPC is sometimes used for basic cements research because the lack of iron improves the resolution of nuclear magnetic resonance (NMR) measurements.
  • 10. RAPID HARDENING PORTLAND CEMENT  This type develops strength more rapidly than ordinary Portland cement. The initial strength is higher , but they equalize at 2-3 months  Setting time for this type is similar for that of ordinary Portland cement  The rate of strength gain occur due to increase of C3S compound, and due to finer grinding of the cement clinker ( the min. fineness is 3250 cm2/gm (according to IQS 5)  Rate of heat evolution is higher than in ordinary Portland cement due to the increase in C3S and C3A, and due to its higher fineness  Chemical composition and soundness requirements are similar to that of ordinary Portland cement Uses a)The uses of this cement is indicated where a rapid strength development is desired (to develop high early strength, i.e. its 3 days strength equal that of 7 days ordinary Portland cement), for example: i) When formwork is to be removed for re-use ii) Where sufficient strength for further construction is wanted as quickly as practicable, such as concrete blocks manufacturing, sidewalks and the places that can not be closed for a long time, and repair works needed to construct quickly. b) For construction at low temperatures, to prevent the frost damage of the capillary water. c) This type of cement does not use at mass concrete constructions. 13 March Prof. Dr. H.Z. Harraz Presentation Cement 10
  • 11. SPECIAL TYPES OF RAPID HARDENING PORTLAND CEMENT A) Ultra High Early Strength Cement  The rapid strength development of this type of cement is achieved by grinding the cement to a very high fineness: 7000 to 9000 cm2/g. Because of this, the gypsum content has to be higher (4 percent expressed as SO3). Because of its high fineness, it has a low bulk density. High fineness leads to rapid hydration, and therefore to a high rate of heat generation at early ages and to a rapid strength development ( 7 days strength of rapid hardening Portland cement can be reached at 24 hours when using this type of cement). There is little gain in strength beyond 28 days.  It is used in structures where early prestressing or putting in service is of importance.  This type of cement contains no integral admixtures. B) Extra Rapid Hardening Portland Cement  This type prepare by grinding CaCl2 with rapid hardening Portland cement. The percentage of CaCl2 should not be more than 2% by weight of the rapid hardening Portland cement.  By using CaCl2:  The rate of setting and hardening increase (the mixture is preferred to be casted within 20 minutes).  The rate of heat evolution increase in comparison with rapid hardening Portland cement, so it is more convenient to be use at cold weather.  The early strength is higher than for rapid hardening Portland cement, but their strength is equal at 90 days.  Because CaCl2 is a material that takes the moisture from the atmosphere, care should be taken to store this cement at dry place and for a storage period not more than one month so as it does not deteriorate. 13 March 2014 Prof. Dr. H.Z. Harraz Presentation Cement 11
  • 12. MANUFACTURING OF CEMENT 13 March Prof. Dr. H.Z. Harraz Presentation Cement 12 1) Quarry 2) Raw Material 3) Mixing and crushing of raw materials: a) Dry process b) Wet process 4) Burning 5) Grinding 6) Storage 7) Packing 8) Dispatch
  • 13. Step in the Manufacture of Portland Cement 1. BLASTING : The raw materials that are used to manufacture cement (mainly limestone and clay) are blasted from the quarry. Quarry face 1. BLASTING 2. TRANSPORT quarry 3. CRUSHING & TRANSPORTATION 2. TRANSPORT : The raw materials are loaded into a dumper. crushing conveyor dumper storage at the plant loader Typical Quarry operation:  Typically shale provides the argillaceous components: Silica (SiO2, Aluminum (Al2O3) & Iron (Fe2O3)  Limestone provides the calcareous component: Calcium Carbonate (CaCO3 )  Raw materials may vary in both composition and morphology.
  • 14. THE CEMENT MANUFACTURING PROCESS 1. RAW GRINDING : The raw materials are very finely ground in order to produce the raw mix. 1. RAW GRINDING Raw grinding and burning 2. BURNING 2. BURNING : The raw mix is preheated before it goes into the kiln, which is heated by a flame that can be as hot as 2000 °C. The raw mix burns at 1500 °C producing clinker which, when it leaves the kiln, is rapidly cooled with air fans. So, the raw mix is burnt to produce clinker : the basic material needed to make cement. conveyor Raw mix kiln cooling preheating clinker storage at the plant Raw mill
  • 15. THE CEMENT MANUFACTURING PROCESS 1.GRINDING : The clinker and the gypsum are very finely ground giving a “pure cement”. Other secondary additives and cementitious materials can also be added to make a blended cement. 1. GRINDING Grinding, storage, packing, dispatch 2. STORAGE, PACKING, DISPATCH 2. STORAGE, PACKING, DISPATCH :The cement is stored in silos before being dispatched either in bulk or in bags to its final destination. clinker storage Gypsum and the secondary additives are added to the clinker. silos dispatch bags Finish grinding
  • 16. MANUFACTURE OF ORDINARY PORTLAND CEMENT  Ordinary Portland Cement (OPC) is one of several types of cement being manufactured throughout the world.  OPC consists mainly of lime (CaO), silica (SiO2) , alumina (Al2O3) , iron (Fe2O3) and sulphur trioxide (SO3). Magnesium (MgO) and other Oxide elements are present in small quantities as an impurity associated with raw materials.  When cement raw materials containing the proper proportions of the essential oxides are ground to a suitable fineness and then burnt to incipient fusion in a kiln, chemical combination takes place, largely in the solid state resulting in a product named clinker.  This clinker, when ground to a suitable fineness, together with a small quantity of gypsum (SO3) is Portland Cement. SO3 is added at the grinding stage to retard the setting time of the finished cement. Basic Chemical Components of Portland Cement:  Calcium (Ca)  Silicon (Si)  Aluminum (Al)  Iron (Fe) Typical Raw Materials:  Limestone (CaCO3)  Sand (SiO2)  Shale, Clay (SiO2, Al2O3, Fe2O3)  Iron Ore/Mill Scale (Fe2O3) 2/3 calcareous materials (lime bearing) - limestone 1/3 argillaceous materials (silica, alumina, iron)- clay 13 March Prof. Dr. H.Z. Harraz Presentation Cement 16
  • 17. 2) Raw Materials for Cement Manufacture  The first step in the manufacture of Portland Cement is to combine a variety of raw ingredients so that the resulting cement will have the desired chemical composition.  These ingredients are ground into small particles to make them more reactive, blended together, and then the resulting raw mix is fed into a cement kiln which heats them to extremely high temperatures.  Since the final composition and properties of Portland Cement are specified within rather strict bounds, it might be supposed that the requirements for the raw mix would be similarly strict. As it turns out, this is not the case. While it is important to have the correct proportions of calcium, silicon, aluminum, and iron, the overall chemical composition and structure of the individual raw ingredients can vary considerably. The reason for this is that at the very high temperatures in the kiln, many chemical components in the raw ingredients are burned off and replaced with oxygen from the air.  Table 1 lists just some of the many possible raw ingredients that can be used to provide each of the main cement elements. Table 1: Examples of raw materials for Portland Cement manufacture Calcium Silicon Aluminum Iron Limestone Clay Clay Clay Marl Marl Shale Iron ore Calcite Sand Fly ash Mill scale Gypsum Shale Aluminum ore refuse Shale Marly limestone Fly ash Phyllite Blast furnace dust Sea Shells Rice hull ash slate slag Cement kiln dust Silica Chalk Sand 13 March Prof. Dr. H.Z. Harraz Presentation Cement 17
  • 18. Typical Composition of Raw Materials 13 March Prof. Dr. H.Z. Harraz Presentation Cement 18
  • 19.  The ingredients listed above include both naturally occurring materials such as limestone and clay, and industrial byproduct materials such as slag and fly ash. From Table 1 it may seem as if just about any material that contains one of the main cement elements can be tossed into the kiln, but this is not quite true.  The raw materials used in the manufacture of cement are limestone, shale, sand and iron ore, typical chemical compositions of which are given in the table below.  Limestone makes up approximately 80% of the raw material requirements, composes of mainly calcium carbonate with small intrusions of magnesium carbonate. Quarrying operations are geared to minimizing the intrusions. MgO in the cement, if present in sufficient quantities will cause expansion upon hydration thus resulting in unsoundness in the concrete.  Materials that contain more than minor (or in some cases trace) amounts of metallic elements such as magnesium, sodium, potassium, strontium, and various heavy metals cannot be used, as these will not burn off in the kiln and will negatively affect the cement.  Another consideration is the reactivity, which is a function of both the chemical structure and the fineness. Clays are ideal because they are made of fine particles already and thus need little processing prior to use, and are the most common source of silica and alumina. Calcium is most often obtained from quarried rock, particularly limestone (calcium carbonate) which must be crushed and ground before entering the kiln. The most readily abundant source of silica is quartz, but pure quartz is very unreactive even at the maximum kiln temperature and cannot be used. 2) Raw Materials for Cement Manufacture (Cont.) 13 March Prof. Dr. H.Z. Harraz Presentation Cement 19
  • 20. Raw Material Proportioning  The raw materials are extracted from the hoppers via weigh-feeders. The materials are conveyed to the grinding mill and are ground to a suitable fineness, called raw meal at this stage. This is then stored in a blending silo and blended to ensure homogeneity.  The proportions of the 4 components are controlled by the continuous sampling and testing of this raw meal.  The raw meal chemical composition is determined by the use of an x-ray fluorescence analyzer. This is linked to the computer which will automatically adjust the weigh-feeders, so that the resultant raw meal stored in the blending silo meets the preset parameters. After blending this material is then discharged into the storage silos ready for the next phase of production.  The parameters used in the control of the raw meal are lime saturation factor, silica modulus and iron modulus. These are actually proportions of the various chemical components which are desired in the resultant clinker.  As coal is used as a fuel the coal ash, a combustion product of the coal, has to be treated as an individual raw material component and the appropriate corrections made at the weigh-feeder stage. 13 March Prof. Dr. H.Z. Harraz Presentation Cement 20
  • 21. 2) Crushing & Grinding of Raw Materials  Due to the variable nature of these components, they are pre-blended prior to their use. It is crushed and stored in a pre-blending hall, utilizing the chevron pile stacking method. In this method, stacking takes place at one end of the pile. At the other end of the pile the material is reclaimed and then stored in a feeding hopper which is ready for use.  The limestone is crushed to less than 25mm in size.  Grinding and blending prior to entering the kiln can be performed with the raw ingredients in the form of a slurry (the wet process) or in dry form (the dry process). The addition of water facilitates grinding. However, the water must then be removed by evaporation as the first step in the burning process, which requires additional energy. The wet process, which was once standard, has now been rendered obsolete by the development of efficient dry grinding equipment, and all modern cement plants use the dry process. When it is ready to enter the kiln, the dry raw mix has 85% of the particles less than 90 µm in size 13 March Prof. Dr. H.Z. Harraz Presentation Cement 21
  • 22. The raw materials used for manufacturing Portland Cement are limestone, clay and Iron ore. a) Limestone (CaCO3) is mainly providing calcium in the form of calcium oxide (CaO) CaCO3 (1000oC) → CaO + CO2 b) Clay is mainly providing silicates (SiO2) together with small amounts of Al2O3 + Fe2O3 Clay (1450oC) → SiO2 + Al2O3 + Fe2O3 + H2O c) Iron ore and Bauxite are providing additional aluminum and iron oxide (Fe2O3) which help the formation of calcium silicates at low temperature. They are incorporated into the raw mix. d) The clinker is pulverized to small sizes (< 75 μm). 3-5% of gypsum (calcium sulphate) is added to control setting and hardening. The majority particle size of cement is from 2 to 50 μm. (Note: “Blaine” refers to a test to measure particle size in terms of surface area/mass) 13 March Prof. Dr. H.Z. Harraz Presentation Cement 22
  • 23. a) Dry process:  In this process calcareous material such as lime stone (calcium carbonate) and argillaceous material such as clay are ground separately to fine powder in the absence of water and then are mixed together in the desired proportions.  Water is then added to it for getting thick paste and then its cakes are formed, dried and burnt in kilns. This process is usually used when raw materials are very strong and hard.  In this process, the raw materials are changed to powdered form in the absence of water.  Dehydration zone requires a somewhat shorter distance than wet process.  74% of cement produced.  kilns less fuel requirements b) Wet process: In this process, the raw materials are changed to powdered form in the presence of water. In this process, raw materials are pulverized by using a Ball mill, which is a rotary steel cylinder with hardened steel balls. When the mill rotates, steel balls pulverize the raw materials which form slurry (liquid mixture). The slurry is then passed into storage tanks, where correct proportioning is done. Proper composition of raw materials can be ensured by using wet process than dry process. Corrected slurry is then fed into rotary kiln for burning. This process is generally used when raw materials are soft because complete mixing is not possible unless water is added. Actually the purpose of both processes is to change the raw materials to fine powder. dehydration zone would require up to half the length of the kiln easiest to control chemistry & better for moist raw materials high fuel requirements - fuel needed to evaporate 30+% slurry water The kiln is a continuous stream process vessel in which feed and fuel are held in dynamic balance Type of cement processes
  • 24. 3) Burning in a Kiln – Formation of Cement Clinker  The next step in the process is to heat the blended mixture of raw ingredients (the raw mix) to convert it into a granular material called cement clinker.  This requires maximum temperatures that are high enough to partially melt the raw mix. Because the raw ingredients are not completely melted, the mix must be agitated to ensure that the clinker forms with a uniform composition.  This is accomplished by using a long cylindrical kiln that slopes downward and rotates slowly.  To heat the kiln, a mixture of fuel and air is injected into the kiln and burned at the bottom end. The hot gases travel up the kiln to the top, through a dust collector, and out a smokestack. A variety of fuels can be used, including pulverized coal or coke, natural gas, lignite, and fuel oil. These fuels create varying types and amounts of ash, which tend to have compositions similar to some of the aluminosilicate ingredients in the raw mix. Since the ash combines with the raw mix inside the kiln, this must be taken into account in order to correctly predict the cement compassion. There is also an increasing trend to use waste products as part of the fuel, for example old tires. In the best-case scenario, this saves money on fuel, reduces CO2 emissions, and provides a safe method of disposal. 13 March Prof. Dr. H.Z. Harraz Presentation Cement 24
  • 25. Kiln Feed Preparation  Proportioning of feed stock.  Size reduction to < 125μ.  Control of moisture.  Blending to reduce standard deviation.  Uniform delivery rate of feed to the Kiln. 13 March Prof. Dr. H.Z. Harraz Presentation Cement 25
  • 26. Manufacturing control criteria in the Kiln Silica Modulus (SM) : Rate of reactions SiO2 SM = -------------------- Al2O3 + Fe2O3 2.3 to 3.5 (desired at least 3.0), slow reaction if SM is high 13 March Prof. Dr. H.Z. Harraz Presentation Cement 26
  • 27. Alumina Modulus (AM): controls melt temperature Al2O3 AM = -------------- Fe2O3 AM ~2 13 March Prof. Dr. H.Z. Harraz Presentation Cement 27
  • 28. Lime Saturation factor (LSF): Designed to insure against equilibrium free lime CaO LSF = ----------------------------- SiO2 + Al2O3 + Fe2O3 LSF : 0.92-0.96 Or CaO LSF = --------------------- Al2O3 + Fe2O3 C/(A+F) should be equal to 2 13 March Prof. Dr. H.Z. Harraz Presentation Cement 28
  • 29. CEMENT KILNS And the Hottest  High temperature  Long residence time  Natural alkaline environment  CKD is only by-product of the process  Thermal stability the Largest Moving Equipment in any Manufacturing Operation
  • 30. Kiln Process Thermochemical Reactions Kiln Process Process Temperature (oC) Reactions Chemical Transformation Drying 20 - <100 Escape of free water (i.e., Free Water evaporates) Pre-heat 100 - 300 Escape of adsorbed water (i.e., Crystallization water driven out) 400 - 750 Chemical water driven out, Decomposition of shale., with formation of metakaolinite Al4Si4O10(OH)8  2(Al2O3.2SiO2) + 4H2O 600 - 900 Decomposition of metakaolinite and other compounds, with formation of reactive oxide mixture Al2O3.2SiO2  Al2O3.+ 2SiO2 Calcining 600 - 1000 Decomposition of limestone, CO2 Driven out, Formation of Free lime , with formation of CS (CaO.SiO2) and CA (CaO.Al2O3) 3CaCO3  3CaO + 3CO2 3CaO + 2SiO2 + Al2O3  2(CaO.SiO2) + CaO.Al2O3 Sintering Clinkering 800 – 1550 (1350 exothermic) Uptake of lime by CS and CA, Formation of Liquid Phase, Formation of: Belite (C2S), Aluminates (C3A) and Ferrites (C4AF) CS + C  C2S 2C + S  C2S CA + 2C  C3A CA + 3C + F  C4AF Cooling 13 March Prof. Dr. H.Z. Harraz Presentation Cement 30
  • 31. 13 March Prof. Dr. H.Z. Harraz Presentation Cement 31
  • 32. Clinker Burning  For the production of cement clinker, the raw meal which is known as kiln feed at this stage has to be heated to a temperature of about 1550 oC in the long cylindrical rotating kiln.  The kiln feed enters the system at the top of the pre-heater and fall until the lower end of the kiln.  The heat exchange occurs during this process when the hot gases from the kiln end rise up to the top of the pre-heater.  The clinker formation process is divided into four parts: drying, calcining, sintering and cooling.  As the kiln feed moves towards the lower end of the kiln it undergoes successive reactions as shown in the table: CLINKER • Clinker is what comes out of the kiln • 3 to 25 mm in diameter • 20 – 25 % Molten 13 March Prof. Dr. H.Z. Harraz Presentation Cement 32
  • 33. Clinker Microstructure C3S crystals magnified 3000 timesDark, Rounded – C2S Light, Angular – C3S 13 March Prof. Dr. H.Z. Harraz Presentation Cement 33
  • 34. Burning Process  This description refers to a standard dry-process kiln. Such a kiln is typically about 180 m long and 6 m in diameter, has a downward slope of 3-4%, and rotates at 1-2 revolutions per minute.  The raw mix enters at the upper end of the kiln and slowly works its way downward to the hottest area at the bottom over a period of 60-90 minutes, undergoing several different reactions as the temperature increases. It is important that the mix move slowly enough to allow each reaction to be completed at the appropriate temperature. Because the initial reactions are endothermic (energy absorbing), it is difficult to heat the mix up to a higher temperature until a given reaction is complete.  The general reaction zones are as follows: 1) Dehydration zone (up to ~ 450˚C) 2) Calcination zone (>450˚C – 900˚C) 3) Solid-state reaction zone (>900˚ - 1300˚C) 4) Clinkering zone (>1300˚C – 1550˚C) 5) Cooling zone 13 March Prof. Dr. H.Z. Harraz Presentation Cement 34
  • 35. Reaction Zone Temperature (oC) Characteristics Dehydration up to ~ 450 This is simply the evaporation and removal of the free water Even in the “dry process” there is some adsorbed moisture in the raw mix.  Although the temperatures required to do this are not high, this requires significant time and energy. In the wet process, the dehydration zone would require up to half the length of the kiln, while the dry process requires a somewhat shorter distance. Calcination 450˚C – 900 The term calcination refers to the process of decomposing a solid material so that one of its constituents is driven off as a gas. At about 600˚C the bound water is driven out of the clays, and by 900˚C the calcium carbonate is decomposed, releasing carbon dioxide. By the end of the calcination zone, the mix consists of oxides of the four main elements which are ready to undergo further reaction into cement minerals. Because calcination does not involve melting, the mix is still a free-flowing powder at this point. Solid-state reaction >900˚ - 1300 This zone slightly overlaps, and is sometimes included with, the calcination zone. As the temperature continues to increase above ~ 900˚C there is still no melting, but solid- state reactions begin to occur. CaO and reactive silica combine to form small crystals of C2S (dicalcium silicate; Belite), one of the four main cement minerals. In addition, intermediate calcium aluminates (C3A) and calcium ferrite (C4AF) compounds form. These play an important role in the clinkering process as fluxing agents, in that they melt at a relatively low temperature of ~1300˚C, allowing a significant increase in the rate of reaction. Without these fluxing agents, the formation of the calcium silicate cement minerals would be slow and difficult. In fact, the formation of fluxing agents is the primary reason that Portland (calcium silicate) Cements contain aluminum and iron at all. The final aluminum- and iron-containing cement minerals (C3A and C4AF) in a Portland Cement contribute little to the final properties. As the mix passes through solid-state reaction zone it becomes “sticky” due to the tendency Burning Process (Cont.)
  • 36. Reaction Zone Temperature (oC) Characteristics Clinkering >1300 – 1550 This is the hottest zone where the formation of the most important cement mineral, Alite (C3S), occurs. The zone begins as soon as the intermediate calcium aluminate (C3A) and ferrite (C4AF) phases melt. The presence of the melt phase causes the mix to agglomerate into relatively large nodules about the size of marbles consisting of many small solid particles bound together by a thin layer of liquid. Inside the liquid phase, Alite (C3S) forms by reaction between Belite (C2S) crystals and CaO. (C2S + C  C3S) Crystals of solid Alite (C3S) grow within the liquid, while crystals of Belite (C2S) formed earlier decrease in number but grow in size. The clinkering process is complete when all of silica is in the C3S and C2S crystals and the amount of free lime (CaO) is reduced to a minimal level (<1%). Cooling As the clinker moves past the bottom of the kiln the temperature drops rapidly and the liquid phase solidifies, forming the other two cement minerals C3A (aluminate) and C4AF (ferrite). In addition, alkalis (primarily K) and sulfate dissolved in the liquid combine to form K2SO4 and Na2SO4. The nodules formed in the clinkering zone are now hard, and the resulting product is called cement clinker. The rate of cooling from the maximum temperature down to about 1100˚C is important, with rapid cooling giving a more reactive cement. This occurs because in this temperature range the C3S can decompose back into C2S and CaO, among other reasons. It is thus typical to blow air or spray water onto the clinker to cool it more rapidly as it exits the kiln.  Rapid cooling of the clinker is essential as this hampers the formation of crystals, causing part of the liquid phase to solidify as glass.  The faster the clinker cooling the smaller the crystals will be when emerging from the liquid phase.
  • 37. Dual Line Preheater Planetary Cooler
  • 38. i) Generalized Diagram of a Long Dry Process Kiln Reaction Material Temperature Gas Temperature The kiln exit gas temperature will depend on the process Zone Exhaust GasesRaw Feed Clinker Out
  • 39. Suspension Preheaters and Calciners  The chemical reactions that occur in the dehydration and calcination zones are endothermic, meaning that a continuous input of energy to each of the particles of the raw mix is required to complete the reaction. When the raw mix is piled up inside a standard rotary kiln, the rate of reaction is limited by the rate at which heat can be transferred into a large mass of particles. To make this process more efficient, suspension preheaters are used in modern cement plants to replace the cooler upper end of the rotary kiln. Raw mix is fed in at the top, while hot gas from the kiln heater enters at the bottom. As the hot gas moves upward it creates circulating “cyclones” that separate the mix particles as they settle down from above. This greatly increases the rate of heating, allowing individual particles of raw mix to be dehydrated and partially calcined within a period of less than a minute.  Alternatively, some of the fuel can be burned directly within the preheater to provide even more heating to the suspended particles. The area of the preheater where fuel is burned is called a precalciner. With a precalciner, the particles are nearly completely calcined as they enter the rotary kiln. Preheaters and precalciners save on fuel and increase the rate at which the mix can be moved through the rotary kiln. 13 March Prof. Dr. H.Z. Harraz Presentation Cement 40
  • 40. ii) General Diagram of Preheater/Precalciner Reactions
  • 41. Dry Process Preheater/Precalciner System Preheater Precalciner Kiln 13 March Prof. Dr. H.Z. Harraz Presentation Cement 42
  • 42. Kiln Process Control  Critical Parameters: Fuel, Feed, Kiln Speed, Gas Flow  Kiln Temperatures - Burning Zone  Kiln Stability  Chemistry  Instrumentation 13 March Prof. Dr. H.Z. Harraz Presentation Cement 43
  • 43. 4) Grinding and the Addition of Gypsum  Now the final process is applied which is grinding of clinker, it is first cooled down to atmospheric temperature.  Grinding of clinker is done in large tube mills.  After proper grinding gypsum (Calcium sulphate CaSO4) in the ratio of 01-04 % is added for controlling the setting time of cement.  Finally, fine ground cement is stored in storage tanks from where it is drawn for packing.  Once the nodules of cement clinker have cooled, they are ground back into a fine powder in a large grinding mill. At the same time, a small amount of calcium sulfate such as gypsum (calcium sulfate) is blended into the cement. The calcium sulfate is added to control the rate of early reaction of the cement  Cement is produced by grinding clinker with gypsum (calcium sulfate) in the finish-grinding mill to a required fineness.  A small quantity of gypsum, about 3 to 5 %, is needed to control the setting time of cement produced.  The amount of gypsum being used is determined by the Sulphuric anhydride (SO3) contents in cement. Cement Grinding
  • 44. • Clinker, gypsum, and optional additives are weighed to proper proportions and ground in the cement mills. • Additives may include: Fly-ash, Limestone….. 13 March Prof. Dr. H.Z. Harraz Presentation Cement 45
  • 45. Schematic of a Grinding Mill 13 March 2014Prof. Dr. H.Z. Harraz Presentation Cement46
  • 46. Cement Storage & Distribution • At this point the manufacturing process is complete and the cement is ready to be bagged or transported in bulk away from the plant After the grinding process, cement is pumped into the storage silos. • This silo is preventing the moisture to react with cement. • When needed cement from the silos is packed into bags or loaded into road tankers and rail wagons for dispatch. • However, the cement is normally stored in large silos at the cement plant for a while so that various batches of cement can be blended together to even out small variations in composition that occur over time. 13 March Prof. Dr. H.Z. Harraz Presentation Cement 47
  • 47. 13 March Prof. Dr. H.Z. Harraz Presentation Cement 48 CEMENT CHEMISTRY
  • 48. CHEMICAL COMPOSITIONS Oxide Notation CaO C SiO2 S Al2O3 A Fe2O3 F SO3 H2O H MgO M Na2O N S The properties of cement during hydration vary according to:  Chemical composition  Degree of fineness It is possible to manufacture different types of cement by changing the percentages of their raw materials. 13 March Prof. Dr. H.Z. Harraz Presentation Cement 49 1) Cement chemistry notation based on oxides
  • 49. Chemical Name Chemical Formula Oxide Formula Cement Notation Mineral Name Tricalcium Silicate Ca3SiO5 3CaO.SiO2 C3S(40-60%) Alite Dicalcium Silicate Ca2SiO4 2CaO.SiO2 C2S(16-30%) Belite Tricalcium Aluminate Ca3Al2O6 3CaO.Al2O3 C3A(7-15%) Aluminate Tetracalcium Aluminoferrite Ca2AlFeO5 4CaO.Al2O3.Fe2O3 C4AF(7-12%) Ferrite Calcium hydroxide Ca(OH)2 CaO.H2O CH Portlandite Calcium sulfate dihydrate CaSO4.2H2O CaO.SO3.2H2O CSH2 Gypsum Calcium oxide CaO CaO C Lime Compound Composition of Clinker / Cement 13 March Prof. Dr. H.Z. Harraz Presentation Cement 50
  • 50. Phase Diagram Tricalcium Silicate Tricalcium Aluminate SiO2 CaO CaOCaO CaO Al2O3 CaOCaO 13 March Prof. Dr. H.Z. Harraz Presentation Cement 51
  • 51. Bogue’s Equations – Compound composition  To calculate the amounts of C3S, C2S, C3A, and C4AF in clinker (or the cement) from its chemical analysis (from the mill certificate)  Assumptions in calculations:  Chemical equilibrium established at the clinkering temperature  Components maintained unchanged through the rapid cooling period  Compounds are “pure” • A simple estimate of the phase composition of a Portland Cement can be obtained from the oxide composition if one assumes that the four main cement minerals occur in their pure form. • With this assumption, all of the Fe2O3 is assigned to C4AF and the remaining Al2O3 is assigned to C3A. • This leaves a set of two linear equations to be solved for the amounts of C2S and C3S. • This method is named after the cement chemist R.H. Bogue. A standardized version of this simple method is given in ASTM C 150. There are two sets of equations, based on the ratio of A/F in the cement (both inputs and outputs are in weight percent): 13 March Prof. Dr. H.Z. Harraz Presentation Cement 52
  • 52. Case 1 : If A/F >=0.64  C3S = 4.071C - 7.6S - 6.718A - 1.43F - 2.852S  C2S = 2.867S - 0.7544C3S  C3A = 2.65A - 1.692F  C4AF = 3.043F Case 2 : If A/F < 0.64  C3S = 4.071C - 7.6S - 4.479A – 2.859F - 2.852S  C2S = 2.867S - 0.7544C3S  C3A = 0  C4AF = 2.10A + 1.702F 13 March Prof. Dr. H.Z. Harraz Presentation Cement 53
  • 53. Fineness of cement • Grinding is the last step in processing • Measures of fineness Specific surface Particle size distribution • Blaine’s fineness  Measure of air permeability • Typical surface areas   350m2 / kg (Normal cements)  ~ 500 m2 / kg (High early strength cements) Significance of fineness  Finer cement = Faster reaction  Finer cement = Higher heat of hydration  Large particles do not react with water completely  Higher fineness Higher shrinkage  Reduced bleeding  Reduced durability  More gypsum needed 13 March Prof. Dr. H.Z. Harraz Presentation Cement 54

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