There arefive important stages in the manufacture of Portland cement, namely the selection of raw materials, preparation of raw materials, production of clinker, adding additives to the clinker andfinally grinding the mixture. Aflowchart for the process of cement manufacture is shown in Fig.5.1.
5.4.1 Raw Materials Selection
In selecting raw materials for the manufacture of Portland cement, great care must be exercised as they affect the quality of the cement product, the clinker formation process and finally the fuel to use to do the burning. The physical condition, the composition of the substantive minerals and the levels of impurities in the raw material will all have an impact on the selection of processing route and on the quality of the cement product. Raw materials used in the manufacture of cement may be classified into two broad groups. These are the calcareous materials which are rich in lime, and the argillaceous materials which are rich in silica. The most common form of calcium carbonate is limestone. It is used extensively, worldwide, Table 5.6 Specific functions of individual clinker compounds in cement
Compound Function
C3A Causes setting of the reaction mixture
C3S Responsible for early strength (at 7 or 8 days)
C2S and C3S Responsible forfinal strength (1 year) Fe2O3, Al2O3, Mg, and Alkalis Lowers the clinkering temperature
Table 5.7 Nomenclature of
compounds in cement clinker Oxide Notation Oxide Notation
CaO C SO3 Ŝ
SiO2 S Na2O N
Al2O3 A K2O K
Fe2O3 F CO2 Ĉ
MgO M H2O H
TiO2 T
5.3 The Chemistry of Portland Cement Manufacture 143
for the manufacture of cement. Other sources of limestone are marls, chalk, sea sand, seashells and coral reef. Also, by-product calcium carbonate sludge from the paper and pulp industry or from the process of manufacturing ammonium sulfate from gypsum are sometimes used. The argillaceous raw materials may be clay, shale or laterite. Where necessary, the silica modulus of the raw mix may be boosted by the addition of small quantities of bauxite, sandstone and iron ore. The compositions of these raw materials are listed in Tables5.8and 5.9. Another raw material of Portland cement is gypsum. This material, when ground with cement clinker in about 5–6 wt%, acts as a retarder of the setting time of cement. (Note, cement when mixed with water sets immediately, which situation is not wanted in the use of cement for several applications.)
Fig. 5.1 A generalizedflowchart for cement manufacture
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5.4.1.1 Impurities in the Raw Materials and Their Effects
Several impurities also accompany the raw materials and care must be taken as some of these materials are harmful to the cement burning process and the quality of thefinal cement product. Table5.10lists some of the impurities found in the raw materials and the manner they affect the cement and the cement manufacturing process.
5.4.1.2 Clinker Fuel
The fuel used in burning the clinker is also an important raw material for making cement. Coal, oil or gas is the fuel usually used to burn clinker. The raw materials, fuel and the process selected for the burning must be compatible. For instance, in the manufacture of white cement only oil can be used because the Fe2O3in the ash present in coal will contaminate the white cement. Where coal is used as a fuel, Table 5.8 Calcareous raw materials
Composition Raw materials
Limestone Seasand Seashell Marl
SiO2 1.0–15.0 1.0–10.0 0.5–2.0 10.0–20.0
Al2O3 1.0–6.0 1.0–3.0 0.5–1.0 2.0–6.0
Fe2O3 0.2–5.0 0.5–2.0 0.1–0.5 1.0–5.0
CaO 40.0–55.0 45–55.0 53.0–55.0 35.0–45.0
MgO 0.2–4.0 0.2–2.0 0.5–1.0 1.0–4.0
Alkalis 0.2–1.0 1.0–1.5 0.3–0.5 0.5.0–1.0
Chlorides 0.1 1.0–2.0 0.2–0.5 Trace
Sulfates 3.0 0.5–10.0 1.0–0.3 Trace
L.O.I 35.0–44.0 38.0–44.0 42.0–45.0 30.0–38.0
L.O.I = Loss on ignition. The compositions are in % (wt)
Table 5.9 Argillaceous materials Composition Raw materials
Clay Shale Laterite Bauxite Iron-ore Sandstone Coal-ash SiO2 40–70 40–80 10–30 3–15 5–10 85–95 50–70 Al2O3 15–30 15–30 20–40 40–54 2–5 2–5 15–30 Fe2O3 3–10 3–10 20–40 2–10 85–95 1–3 5–10 CaO 1–10 1–10 2–4 2–4 Trace 1–3 2–5
MgO 1–5 1–5 1–2 1–2 Trace 1–3 1–3
Alkalis 1–4 1–4 Trace Trace Trace 1–2 2–4
SO2 2 2 Trace Trace Trace Trace 1–2
L.O.I 5–15 2–5 15–25 20–30 5 2–5 2
TiO2 – – 2–5 2–6 – – –
L.O.I = Loss on ignition. The compositions are in % (wt)
5.4 Production of Portland Cement 145
sometimes small quantities of oil are used to increase the flame temperature and hence the efficiency of the process and quality of the cement.
5.4.2 Raw Material Preparation
The calcareous and argillaceous materials, which are the main raw materials for cement manufacture, when mined are often in the form of large lumps ranging in size from about 200 mm to 1 m and beyond depending on the mode of mining and nature of the raw material. For instance, when mined mechanically limestone comes in lumps of sizes 1 m or more, while when manually mined the lumps sizes are about 200 mm. On the other hand, the mined clay, gypsum, coal, sandstone, etc.
have sizes of about 300 mm. In these forms, they present a physical barrier for use in the burning equipment where they must necessarily be fed to be reacted to form the cement clinker. Such large lumps do not also make efficient use of space in the burning equipment neither do they facilitate intimate mixing of the reactants and heat transfer in the equipment. To make for efficient operation of the clinker for- mation process, the raw materials brought from the mines arefirst worked on before Table 5.10 Impurities in raw materials and their effect on cement
Impurities in raw mix
Effect in cement
SO3 Its presence in the raw material is undesirable. It is difficult to dissociate and drive off the SO3. Sometimes a carbonaceous material has to be added to help the dissociation. SO3is often present in by-product sludge from synthetic ammonium sulfate plants. (As a gas, it forms air pockets in the cement product and weakens the structural strength of the product).
Alkalis Large amounts of alkalis present in by-product sludge from paper and pulp industry is also undesirable. The alkalis react with silica in alkali-silica reaction (ASR), which leads to serious expansion and cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure.
P2O5 If it is in raw mix in excess of 0.5%, the resultant cement may be slow setting and have very low early strengths.
Fluoride Up to 0.5%fluoride is helpful as aflux. However, higher quantities in the raw mix attack the refractory lining of the kiln and increase balling action in the kiln.
Mn2O5 It is undesirable when present in the raw mix in more than 0.5%. It makes the color of the resultant cement very dark. It also affects the early strength of the cement.
TiO2 In raw mix, it acts in the same way as SiO2. In small quantities, it is not harmful to the cement.
MgO When present to the tune of 4% or more in the raw mix, it causes delayed expansion of the resultant cement product with attendant cracks in the products. Because of this, the use of dolomite or dolomitic limestone as a raw material should be avoided.
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feeding into the burning equipment. Generally, the raw material is prepared with six purposes in mind, namely:
i. to reduce the large lumps of the raw limestone and clay intofine particles to facilitate feeding into the burning equipment;
ii. to facilitate intimate mixing of the limestone and the clay (the reactants);
iii. to improve energy transfer in the equipment;
iv. to prepare the raw mix in a suitable physical state for the subsequent burning in the kiln;
v. to improve on the efficiency of the burning process;
vi. to optimize the use of kiln space.
The preparation of the raw materials constitutes two steps. In thefirst step, the lumps are reduced into particulate sizes of about 20 mm using crushers. In the second stage, the products of the crushing operations are mixed and ground in mills to a size of about 90lm, which are then fed to the kiln as a slurry or raw dry meal depending on whether a wet burning or dry burning will be the next operation.
Aflowchart of the raw material preparation process is shown in Fig.5.2.
The type of crusher to use depends on the size of the lumps of raw materials and the nature of the material. For limestones of lump sizes of 200 mm or less and argillaceous materials generally, hammer crushers are used in a single-stage crushing operation. For large factories where the raw limestone is mechanically mined, a two-stage crushing operation is often used. During the first stage, jaw crushers, gyratory crushers or impact crushers crush lump sizes of about 1000 mm
Fig. 5.2 A generalized flowchart for the preparation of raw material for cement manufacture
5.4 Production of Portland Cement 147
to products of sizes 200 mm or less. The product from thefirst stage is subsequently crushed in a second crusher using a hammer crusher, impact crusher or cone crusher. Thefinal product has particle sizes of 20 mm. Larger particles are screened and returned to the second stage to be recrushed. Hammer mills are used to crush shale, gypsum, coal and other argillaceous materials. The crushing processes generate a lot of dust and air-borne particles. Consequently, dust collectors and fans must be installed at the plant to deal with dust pollution.
The products from the crushing operations are usually mixed in steel ball mills where they are ground to particle sizes of 90 microns before being fed in an appropriate form into the burning equipment. Two types of mills are used in the grinding process, namely compartment mills where the mill is divided into two or three chambers with perforated diaphragms in between them or in tube mills. Either of the two employs steel media for the grinding action. Again, the milling may be a closed-circuit or open-circuit mill. In open-circuit mills, the feed enters at one end and thefinished product comes out from the other end. In a closed-circuit mill, the product exiting from the mill is fed to a separator where the coarse particles are separated and recycled to the mill and thefine material sent for storage. The degree of grinding depends on the composition of limestone used. If the limestone is of cement grade composition, the argillaceous constituents are intimately mixed.
The physical state, in which the raw mix is fed to the kiln, depends on the burning process to be used. In the wet process, the raw materials are ground with the addition of sufficient water so that the resultant slurry contains about 30–40%
water. It is this slurry that is fed to the kiln. In the dry process, the raw materials are ground in the dry state and fed in that state to the burning equipment. In the semi-dry process, the ground raw meal isfirst nodulized by the addition of 10–12%
water before feeding to the burning equipment.
5.4.3 Storage of Raw Materials
Limestone in lump form is generally not stored in the factory. Only crushed limestone is stored in the factory. The raw lumps are therefore crushed immediately on reaching the factory and then stored. The argillaceous and other corrective materials, however, may be stored as received or in crushed form. Three types of storage devices are used in the cement industry, which are:
i. Crane gantry with electric overhead traveling (EOT) cranes.
ii. Storage bins with overhead feed belt conveyers and underground reclaiming belt conveyers. This type is more economical than thefirst.
iii. Bed blending system: This is usually used for factories with high capacities. In this facility, crushed limestone from different faces of the mines and correc- tives are spread in pre-determined layers at one end and the material reclaimed with the help of cutter blades at the other end. In this manner, a pre-blended raw mix of approximately the desired chemical composition is fed to the raw mill hopper.
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Ground raw materials must also be stored before use. The mode of storage of the ground materials depends on whether the product of the grinding process is a slurry or dry raw meal. The ground slurry is stored either in silos or in basins with continuous mixing. In the dry process, tall silos are used for the storage of dry raw meals. Mixing silos or homogenizing silos are placed above the storage silos so that ready-mixed and corrected raw mix are fed to the storage silos by gravity. This practice saves power.
5.4.4 Clinker Formation
Burning is the unit operation where limestone and the corrective materials are heated and reacted to form the cement clinker. This operation is the most important step in the cement-making process. Burning is mostly carried out in rotary and shaft kilns, but fluidized bed reactors and sinter grate kilns are also sometimes used.
During burning, the limestone breaks down to produce lime which then reacts with the alumina, silica and ferrite in the clay matter to produce the clinker. The burning takes place in kilns. There are three types of burning processes. First, there is a dry burning process where the raw mix is fed in the dry state to the kiln. Then, there is a wet burning process during which the raw mix is fed to the kiln in the form of a slurry. Finally, there is a semi-dry burning process in which wet, nodulized raw meal is fed to the kiln.
The sequence changes that occur during burning in the long and short rotary kilns are the same irrespective of whether the dry, wet or semi-dry process is at stake. The raw mix is first heated to 100 °C at which temperature the moisture present in the raw mix is removed. At about 400–500 °C, the combined moisture present in the clay matter of the raw mix is dissociated and removed and magne- sium carbonate, if present, is also dissociated. At 900 °C, the calcium carbonate (the main ingredient of the clinker formation process) is dissociated into carbon dioxide and lime. When the temperature reaches about 1200 °C the lime, the silica, alumina and iron oxide in the raw mix begin to react with each other as follows;
first, all the iron oxide present in the raw mix and a part of the alumina combine with lime to form C4AF which is in a liquidus state. At the same time, a part of the silica present starts combining with lime to form C2S. The remaining alumina present combines with lime to form C3A. At this point, the material would have become more liquidus and proper clinker nodule formation begins. Further down the kiln, in a zone called the actual clinkering zone, all the silica present combines with lime to form C3S and the remaining lime combines with C2S to form C3S.
Very little free lime is left at the end of this process. From the clinkering zone, the clinker leaves the kiln and is immediately cooled.
Lime silicates (C2S and C3S) are the true cementations materials. C3S con- tributes toward early strength and C2S contributes toward late strength of cement products. In making clinker, therefore, the objective is to achieve a clinker with the highest possible C3S and lowest possible free lime. This is achieved by judicious
5.4 Production of Portland Cement 149
selection of the raw materials available and the proper control of the burning process.
The composition of the clinker and thefinal burning temperature depend on the composition of the raw mix. For instance, a mixture of pure lime and silica in a proportion of 2.8:1 produces a clinker, which is 100% of tricalcium silicate (Ca3SiO5). However, it will require prolong heating at 1600 °C or more to obtain a perfect combination. Such prolonged heating conditions are in practice not realistic.
What is done in practice, therefore, is to add someflushing materials to bring about a lower temperature and a shorter reaction time. Compounds like alumina, iron oxide, magnesia and alkalis already present in the raw mix act asflushing agents.
Therefore, by measured inclusion of these oxides in the raw mix it is possible to shorten the reaction time to reasonable limits at the prevailing temperatures in actual practice. The reactions that occur during clinker formation are summarized in Table5.11.
The burning process and the quality of the cement product produced are affected by oxides other than CaO, SiO2, Al2O3 and Fe2O3. These other oxides include MgO, Mn2O3, TiO2, K2O, Na2O, SO2 and P2O5. The presence of the oxides in certain percentages could be harmful to both the burning process and the quality of the resultant product. Their specific effects have been previously discussed. The fluoride and chloride compounds when present in higher quantities also affect the burning and the product quality.
5.4.4.1 Changes Occurring During Clinker Formation
In a correctly designed raw mix composed of CaO, SiO2, Al2O3, Fe2O3and MgO, during the process of burning, all the Fe2O3and part of the Al2O3 first combine with CaO to form C4AF; the remaining Al2O3combines with CaO to form C3A.
Then the C4AF and C3A combine to form silicates. All the SiO2 in the mixture merges with a portion of the CaO to produce C2S. The CaO still left (uncombined) then combines with a part of the C2S already formed to form C3S, which is the Table 5.11 Reactions during clinker formation
Temperature (°C)
Reactions Heat change
100 Evaporation of free water Endothermic
500 and above
Evolution of bound water Endothermic
900 and above
Crystallization of amorphous dehydration products of clay Exothermic Evolution of carbon dioxide from calcium carbonate Exothermic 900–1200 Main Reaction between lime and clay Endothermic
1200–1250 Commencement of liquid formation Endothermic
1250 and above
Further formation of liquid and completion of formation of cement compounds
Endothermic
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ultimate desired product. In a correctly designed raw mix, it is expected that no CaO (free) should be left in the clinker. In practice, however, about 0.5–2% free CaO is always left in the clinker.
5.4.4.2 Calculating Raw Mix and Clinker Composition
Formulae exist that can be used to design the composition of the raw mix if the expected clinker composition is known. For a typical Portland cement clinker of the following composition, see Table5.12.
The oxide composition of the raw mix is calculated using Eqs. (5.2) to (5.5) as follows (the chemical formulae represent weight percentages):
Fe2O3¼C4AF0:326 ð5:2Þ Al2O3¼C3A0:377þFe2O30:64 ð5:3Þ CaO¼C3S0:737þC2S0:651þC3A 0:621þC4AF0:456þFree CaO ð5:4Þ SiO2¼C2S 0:26þC3S 0:35 ð5:5Þ Bogue calculations (shown in Eqs.5.5–5.8) enable estimation of the approxi- mate amounts of the four main constituents (i.e. C3S, C2S, C3A and C4AF) in clinker based on the oxide composition as indicated by analysis of the clinker. Note, the chemical formulae represent weight percentages.
ð5:6Þ ð5:7Þ ð5:8Þ ð5:9Þ Bogue’s calculations assume that the four main clinker minerals are all pure minerals. Further, it must be noted that clinker is made by combining lime with silica and also lime with alumina and iron. If some of the lime remains uncombined,
Table 5.12 Composition of
a typical Portland clinker Compound Composition (%wt)
C4AF 10.0
C3A 10.0
C3S 47.0
C2S 28.0
MgO 4.0
Free lime 1.0
5.4 Production of Portland Cement 151
which is often the case, then the uncombined amount must be subtracted from the total lime content before the calculation is done in order to get the best estimate of the proportions of the four main clinker minerals present. Consequently, afigure for the combined free lime must be indicated in any clinker analysis.
Clinker characteristics are often described using the four parameters as follows:
Silica Modulus¼ SiO2
Al2O3þFe2O3 ð5:10Þ The silica modulus is also referred to as thesilica ratio. It measures the relative presence in the clinker of calcium silicates compared to the aluminates and ferrites.
A high silica modulus means there are more silicates in the clinker and less of the aluminates and the ferrites. The value of silica modulus is typically between 2 and 3 for clinker.
Alumina Ratio¼ Al2O3
Fe2O3 ð5:11Þ The alumina ratio (AR) determines the potential relative proportions of alumi- nate and ferrite in the clinker. A high AR implies more aluminate and less ferrite in the clinker. In ordinary Portland cement the value of AR is usually between 1 and 4.
Lime Saturation FactorðLSFÞ ¼ CaO
2:8SiO2þ1:2Al2O3þ0:65Fe2O3 ð5:12Þ The LSF is a ratio of CaO to the other three main oxides, namely, Al2O3, SiO2
and Fe2O3. The LSF accounts for the potential combining power of SiO2, Al2O3 and Fe2O3 with CaO. In Portland cement, the LSF should be between 0.66 and 1.02. Up to a limit, the higher the LSF, the better. If it is potentially near 1.0 in a clinker, it is difficult to burn. The silica modulus is a measure of thefluxes in the clinker. Up to a limit, the lower it is in the raw mix, the better for burning. It should be maintained between 2.0 and 3.0. The optimum values for these ratios are: LSF of 0.91; silica modulus of 2.41; and alumina ratio of 1.80. MgO is necessarily present in the raw mix and hence in cement. It has somefluxing action in the clinker. Its presence can be disadvantageous sometimes. For instance, when burnt at very high temperatures it forms hard burning periclase crystals in the clinker. These crystals hydrate very slowly (over a year) and expand during hydration. Thus, if a clinker contains MgO higher than a maximum specified value, it will be unsound on hydration. In some standards, the MgO content in clinker is specified at 6%.
Ideally, in the manufacture of clinker for any type of cement, e.g. OPC, MHC, LHC, SRC, OWC etc., it will be necessary first to decide upon the compound composition of the cement; from this, the oxide composition of the raw mix can be calculated, andfinally, the quantities of the various raw materials to be used can be estimated. It is, however, noteworthy that beyond the composition of the raw mix
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