Earlier, people did not have sophisticated homes; therefore, they used simple things found in the environment to build houses. But today there are many advanced materials and equipment, which assist in constructions. Cement is a marvellous material among them. Before developing high standard cement, which is in the market today, there were primitive types of cement made out from limestone. Earlier, types of cement were not that stable, and they were not a great binding agent. However, today cement has evolved in such a way it has become a reliable building material.
Clinker is the material that we use as the binder of cement, and it is a nodular material. Usually, the lumps or nodules of clinker has its size in the range of 3 millimetres to 25 millimetres in diameter and are dark grey in colour. This material forms during the cement production, inside the kiln. There, clinker forms as a result of sintering limestone and aluminosilicates such as clay during the cement kiln step. Above all, we produce cement via adding gypsum to clinker and grinding finely.
Furthermore, we can store this material for a long period in a dry condition. There, the storing does not degrade the quality of clinker. When considering the composition of this material, there are two major groups as mineral components and chemical components. There, the four major components are alite, belite, aluminate and ferrite.
Cement is an important substance that we use in constructions as a binder to adhere materials to other materials. Often we use cement along with sand and gravel rather than using it alone. We can use this material mainly in two purposes, as mortar in masonry and as concrete; there, we can produce mortar by mixing cement with fine aggregates whereas we can produce concrete by mixing cement with sand and gravel.
The cement that we use in construction purposes is inorganic; manufacturers use lime or calcium silicate in producing this type of cement. We can characterize this material as either hydraulic and non-hydraulic cement, depending on the ability of this material to set in the presence of water or the absence of water respectively. Therefore, non-hydraulic cement sets as it dries and reacts with carbon dioxide. Moreover, it is chemical resistant after setting.
Cement is an important substance that we use in constructions as a binder to adhere materials to other materials. Clinker is a component in cement. It is the active binding component in cement. Therefore, the key difference between clinker and cement is that clinker appears as marble-like nodules, whereas cement is a very fine powder. Moreover, particles in clinker size are in the range of 3 millimetres to 25 millimetres in diameter while in cement there are very fine particles. Apart from that, clinker forms inside the kiln during the cement manufacturing whereas we can produce cement via adding gypsum to clinker and grinding finely.
Cement is a major building material that we use in constructions. Clinker is a major component in cement. The key difference between clinker and cement is that clinker appears as marble-like nodules, whereas cement is a very fine powder.
Madhu is a graduate in Biological Sciences with BSc (Honours) Degree and currently persuing a Masters Degree in Industrial and Environmental Chemistry. With a mind rooted firmly to basic principals of chemistry and passion for ever evolving field of industrial chemistry, she is keenly interested to be a true companion for those who seek knowledge in the subject of chemistry.
Clinker is a nodular material produced in the kilning stage during the production of cement and is used as the binder in many cement products. The lumps or nodules of clinker are usually of diameter 3-25 mm and dark grey in color. It is produced by heating limestone and clay to the point of liquefaction at about 1400C-1500C in the rotary kiln. Clinker, when added with gypsum (to control the setting properties of cement and ensure compressive strength) and ground finely, produces cement. Clinker can be stored for long periods of time in a dry condition without degradation of quality, hence it is traded internationally and used by cement manufacturers when raw materials are found to be scarce or unavailable.
The raw materials entered into the kiln are taken at room temperature. Inside the kiln, the temperature continues to rise and when it reaches its peak, clinker is produced by rapid cooling. Though the reaction stages often overlap, they can be expressed in a sharply-defined sequence as follows:
The most common type of clinker is produced for Portland cement and its blends. The types of clinker vary depending on the type of cement for which the clinker is produced. Aside from the Portland cement blends, some special types of cement clinker are listed below:
It contains 76% alite, 5% belite, 2% tricalcium aluminate, 16 % tetracalcium aluminoferrite, and 1% free calcium oxide. Its production has decreased in recent years because sulfate resistance can easily be obtained by using granulated blast furnace slag in cement production.
It contains 29% alite, 54% belite, 2% tricalcium aluminate and 15 % tetracalcium aluminoferrite, with very little free lime. It is no longer produced because cement produced from ordinary clinker and ground granulated blast furnace slag has excellent low heat properties.
It contains 76% alite, 15% belite, 7% tricalcium aluminate, no tetracalcium aluminoferrite, and 2% free lime, but the composition may vary widely. White clinker produces white cement which is used for aesthetic purposes in construction. The majority of white cement goes into factory-made pre-cast concrete applications.
Reduction of alkali content in clinker is done by either replacing the raw-mix alumina source with another component (thus obtaining a more expensive material from a more distant source), or installing an "alkali bleed", which involves removing some of the kiln system's high temperature gases (which contain the alkalis as fume), resulting in some heat wastage.
This concept is used in producing a type of clinker with up to 30% less carbon dioxide emission. Energy efficiency improves and the electricity costs for the manufacturing process are about 15% lower as well.
Clinker, combined with additives and ground into a fine powder, is used as a binder in cement products. Different substances are added to achieve specific properties in the produced cement. Gypsum added to and ground with clinker regulates the setting time and gives the most important property of cement, compressive strength. It also prevents agglomeration and coating of the powder at the surface of balls and mill wall. Some organic substances, such as Triethanolamine (used at 0.1 wt.%), are added as grinding aids to avoid powder agglomeration. Other additives sometimes used are ethylene glycol, oleic acid, and dodecyl-benzene sulphonate. The most notable type of cement produced is Portland cement, but certain active ingredients of chemical admixtures may be added to clinker to produce other types of cement, such as:
Clinker is primarily used to produce cement. Since it can be stored in dry condition for several months without noticeable deterioration, it is traded internationally in large amounts. Cement manufacturers buy clinker for their cement plants in areas where raw materials for cement are scarce or unavailable.
Please note that the information in Civiltoday.com is designed to provide general information on the topics presented. The information provided should not be used as a substitute for professional services.
Cement and clinker are not the same material. Cement is a binding material used in construction whereas clinker is primarily used to produce cement. The main differences between clinker and cement are given below.
Please note that the information in Civiltoday.com is designed to provide general information on the topics presented. The information provided should not be used as a substitute for professional services.
Portland cement clinker is nodules (diameters, 525mm) of sintered material produced by heating a homogeneous mixture of raw materials in a kiln to a sintering temperature of approximately 1450C for modern cements.
Portland cement is a fine powder produced by grinding Portland cement clinker (more than 90%), a limited amount of gypsum (calcium sulphate dehydrate CaSO4.2H2O, which controls the set time) and other minor constituents which can be used to vary the properties of the final cement. The standard Portland cement is often referred to as Ordinary Portland Cement, and European Standard EN197-1 gives the following description:
Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3CaO.SiO2 and 2CaO.SiO2), the remainder consisting of aluminium and iron containing clinker phases and other compounds. The ratio of CaO to SiO2 shall not be less than 2.0. The magnesium oxide content (MgO) shall not exceed 5.0% by mass.
Hydraulic cement (cement that not only hardens by reacting with water but also forms a water-resistant product) produced by pulverising clinkers consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulphate as an inter-ground addition.
Portland cement clinker is nodules (diameters, 525mm) of sintered material produced by heating a homogeneous mixture of raw materials in a kiln to a sintering temperature of approximately 1450C for modern cements. The resulting clinker consists of four main minerals:11
The aluminium oxide and iron oxide are present in the main as a flux and contribute little to the mechanical strength of the final concrete. The proportions of each mineral in the clinker are important in determining the properties of the resulting cement. For example, in some special cements, such as Low Heat (LH) and Sulphate Resistant (SR) types, it is necessary to limit the amount of tricalcium aluminate (3CaO-Al2O3) that is formed.
Portland cement clinker contains four principal chemical compounds, which are normally referred to as the clinker minerals. The composition of the minerals and their normal range of levels in current UK and European Portland cement clinkers are summarized in Table1.1.
It is the two calcium silicate minerals, C3S and C2S, which are largely responsible for the strength development and the long-term structural and durability properties of Portland cement. However, the reaction between CaO (lime from limestone) and SiO2 (silica from sand) is very difficult to achieve, even at high firing temperatures. Chemical combination is greatly facilitated if small quantities of alumina and iron oxide are present (typically 5% Al2O3 and 3% Fe2O3), as these help to form a molten flux through which the lime and silica are able to partially dissolve, and then react to yield C3S and C2S. The sequence of reactions, which take place in the kiln, is illustrated in Figure1.5.
The reaction requiring the greatest energy input is the decarbonation of CaCO3, which takes place mainly in the temperature range 7001000C. For a typical mix containing 80% limestone the energy input to decarbonate the CaCO3 is approximately 400 kCal/kg of clinker, which is approximately half of the total energy requirement of a modem dry process kiln.
When decarbonation is complete at about 1100C, the feed temperature rises more rapidly. Lime reacts with silica to form belite (C2S) but the level of unreacted lime remains high until a temperature of ~1250C is reached. This is the lower limit of thermodynamic stability of alite (C3S). At ~1300C partial melting occurs, the liquid phase (or flux) being provided by the alumina and iron oxide present. The level of unreacted lime reduces as C2S is converted to C3S. The process will be operated to ensure that the level of unreacted lime (free lime) is below 3%.
Normally, C3S formation is effectively complete at a material temperature of about 1450C, and the level of uncombined lime reduces only slowly with further residence time. The ease with which the clinker can be combined is strongly influenced by the mineralogy of the raw materials and, in particular, the level of coarse silica (quartz) present. The higher the level of coarse silica in the raw materials, the finer the raw mix will have to be ground to ensure satisfactory combination at acceptable kiln temperatures.
Coarse silica is also associated with the occurrence of clusters of relatively large belite crystals around the sites of the silica particles. Figures1.6(a) and 1.6(b) are photomicrographs of a normal clinker containing well-distributed alite and belite and clinker produced from a raw meal containing relatively coarse silica.
As the clinker passes under the flame it starts to cool and the molten C3A and C4AF, which constitute the flux phase, crystallize. This crystallization is normally complete by the time the clinker exits the rotary kiln and enters the cooler at a temperature of ~1200C. Slow cooling should be avoided as this can result in an increase in the belite content at the expense of alite and also the formation of relatively large C3A crystals which can result in unsatisfactory concrete rheology (water demand and stiffening).
Portland cement clinker, as ordinarily prepared, is not a homogeneous substance, but a rather fine-grained mixture of several solid phases. It is therefore difficult to draw any conclusions from a study of its chemical reactions alone, since these may involve more than one constituent, and the only methods capable of yielding trustworthy results are those which enable us to deal with the individual constituents separately. The conditions have a close parallel with the study of igneous rocks. It would be impossible to determine the structure of a granite by observing its gross behaviour towards reagents. The reactions with the quartz, feldspar and mica would be superimposed and confused, and the resulting action would give only a meaningless average. A number of methods are available for overcoming these difficulties, such as mineral separation after crushing, optical examination of the crushed material or of thin section or of polished and etched surfaces, the use of X-rays, and electron microprobe analysis.
A rock may be crushed to such a degree of fineness as to release the constituent crystal grains and the powder thus obtained can be suspended in liquids of suitable densities, bringing about a separation of the light and heavier minerals. In skilled hands, the method is capable of giving very accurate results. It has been applied to cements, but with less success, as the close intermixture and friable character of the constituents render a separation impossible until the whole has been reduced to a fine powder, when the particles no longer settle satisfactorily after suspension in heavy liquids. An improvement may be obtained by effecting the separation of the crushed material by centrifugal or magnetic means, but even then a clean separation has not been effected. Microscopic examination of the crushed material shows that many of the grains are composed of one constituent, but that attached to their edges there may remain fragments of a second constituent, thus rendering a perfect separation impracticable. Such partially successful separations, using chemical or physical methods, can be a useful preliminary to X-ray examination since the intensity of the reflections obtained for a given constituent will be enhanced.
Tricalcium silicate grows to a larger size than any other constituent, and separation of enough material for analysis by centrifuging a graded clinker powder in heavy liquids has been achieved. Partial hydration35 of a clinker left the slowly hydrating 2CaOSiO2 as a residue after acid extraction of the set cement. Many workers have used an alkaline solution of a dimethylamine salt to dissolve the silicate phases, leaving a concentration of other constituents. A solution of salicyclic acid in methanol has also been used to dissolve the calcium silicate phases from Portland cement.4649 The cement specimen must first be ground to a particle size less than 5 m in an agate ball mill, with cyclohexane as a grinding aid. Other organic acid solutions have been proposed.46 These methods, or alkaline ammonium citrate or acetic acid,5052 have been used to intensify the X-ray reflections from the ferrite phase in order to obtain its composition from the d-spacings. A solution of KOH containing sucrose is recommended as a dissolution medium which removes the calcium aluminate phases.49 These solubility methods depend on differences in the rate of solution of the various compounds and require, therefore, a suitable choice of the extraction conditions. A bar magnet has also been used to separate the ferrite phase.53
The sintering of Portland cement clinker is simply called twice grinding and once sintering, that is, grinding the cement with cement raw material; sintering the calcined part of the raw material into clinker; grinding the clinker with a limited amount of gypsum into Portland cement clinker. The sintering process of Portland cement clinker is shown in Figure4.1.
Different proportions of cement raw materials directly affect the proportions of the mineral components of Portland cement clinker and the main building technical performance. The process that cement raw materials is sintered in a kiln is the key to the quality of cement clinker.
In the sintering process of cement raw material, the useful components decomposed by various raw materials at 1000C are mainly: calcium oxide (CaO), silicon dioxide (SiO2), aluminum oxide (Al2O3), and ferric oxide (Fe2O3). And the solid-state reaction occurs to a small amount of oxide at about 800C which generates calcium aluminate, a small amount of dicalcium ferrite, and dicalcium silicate.
At 1300~1450C, tricalcium aluminate and tetracalcium aluminoferrite are in molten state and CaO and part of dicalcium silicate are dissolved in the generated liquid phase. In this liquid phase, dicalcium silicate synthesizes tricalcium silicate by absorbing CaO which is the key to the sintering of cement. Sufficient time should be cost to make the free CaO in the raw material be absorbed, for the quality of cement clinker.
Once the Portland cement clinker has been manufactured it is normally fed to a store to effect a measure of blending and also to allow it to cool to ambient temperature. The latter operation is desirable because most of the clinker coolers associated with kiln operation are unable to lower the temperature below 5080C, and even at this temperature the amount of heat introduced into the grinding process is unwelcome.
At most manufacturing plants the ball mill is used to grind the clinker and, since the production rate is directly related to the amount of electrical energy supplied, the power of the electric motor used to turn the mill is a first-order measure of the output achieved. Mills vary in their power input from as little as 200 kW up to 10000 kW.
An efficient mill system grinding a Portland cement (without secondary components) to a level of fineness required for a 42.5 strength class4 can be expected to consume of the order of 30 kWh/t, and on this basis the mills cited above should be capable of producing 67 and 333 t/h, respectively.
The ball mill in its simplest form consists of a tube rotating about a horizontal axis. The inside is normally divided into at least two chambers separated by slotted diaphragm(s). This division enables the mill to operate with at least two different size gradings of grinding media (usually balls). Such an arrangement is necessary because the clinker normally fed to the mill to be ground can contain lumps as large as 60 mm in diameter. These large lumps require a larger ball size (90100 mm) to break them down. At the same time, balls as small as 13 mm in diameter are needed to grind the material to the fineness required in the more rapid-hardening cements. If such a wide range of ball sizes were to be placed into one chamber, the smaller balls would move to the inlet end and the larger balls to the outlet end, thus making effective grinding impossible. By segregating the ball sizes through the use of diaphragms the larger balls can be kept at the inlet end of the mill and the smaller balls at the outlet. It is possible to achieve a similar effect in a single chamber through the use of classifying liner plates in the mill. These are plates with a wedge-type profile with the tapered part of the wedge facing the mill inlet. These are effective in dealing with ball sizes within the range 6013 mm, and it is normal practice where larger clinker particle sizes have to be dealt with to introduce a separate chamber accommodating 9060 mm diameter media.
In order to achieve efficient grinding it is necessary for the media to cascade over each other as the mill rotates. This means that the rotational speed must be kept below the critical point where the media are held against the mill shell by centrifugal force. This critical speed is defined as 42.3 divided by the square root of the internal diameter of the mill (in metres), and the rotational speed of most mills is kept within 6580 per cent of the critical speed.
The volume of grinding media used in the mill is normally established at a level which either gives the lowest specific power consumption (kWh/t of product) or gives the maximum output. In the case of the latter, it is often decided by the maximum amount of power available from the motor. However, subject to the motor power available, a good starting point would be 30 per cent of the internal volume of the mill taken up with the grinding media. A useful relationship between mill power, media loading speed and rotational speed is given by the following formula:
where D = internal diameter of the mill (m); A = a constant for a given mill system, usually about 0.245; W = the mass of the grinding media in tonnes; and N = the rotational speed of the mill (rev/min).
Only 12 per cent of the electrical energy supplied to the ball mill is used in actually fracturing the particles. This means that in the course of the grinding operation a not inconsiderable amount of heat is produced, and one of the principle problems associated with cement grinding is to remove the heat. In the case of small output mills (up to 900 kW) fed with cold clinker it is possible to achieve an adequate degree of cooling by spraying water on to the mill shell. However, with larger mills the ratio of shell surface to heat input decreases and other methods have to be adopted. One of the most satisfactory is to spray water into the outlet chamber of the mill and to use the latent heat of vaporisation to remove the heat. This requires good control facilities and also sufficient ventilation to prevent the water hydrating the cement and causing a loss in its strength-giving properties. Other methods which are used, often in conjunction with water injection, are to introduce a classifier into the milling system. This involves using the mill to produce a relatively coarse product (having, say, a Blaine specific surface area of 270 m2/kg) and to use the classifier to separate out a product of the required fineness. The power required to produce a coarse product is less and hence the heat introduced is also less. In addition, the movement of the hot cement in an elevator and through the classifier, particularly if it is air swept, can effect a considerable degree of cooling. In this type of mill system -known as a closed-circuit mill the coarse material produced in the classifier is fed back to the feed end of the ball mill. Closed-circuit grinding is necessary when large mills are required to produce finely ground cements and also in situations where there is a significant difference in the hardness of the Portland cement clinker and any secondary material incorporated into the cement. Grinding cement in a closed-circuit mill system generally produces a narrower particle grading in the cement than that produced from a mill system without a classifier (an open-circuit mill) and this in turn leads to a higher water demand in the mortar or concrete produced from that cement. The higher water demand means that, for a given cement content, lower strengths are produced.
The emphasis placed upon grinding temperatures is associated with the effect that this has upon the calcium sulfate (gypsum rock) added to the clinker, in amounts normally between 3 and 8 per cent, to retard the hydration of the tricalcium aluminate and to optimise the strength-giving properties of the calcium silicates.
If the calcium sulfate is added in the form of gypsum, it can become dehydrated at grinding temperatures of the order of 115130C or above, and can be present in a form which when mixed with water forms a supersaturated solution with respect to gypsum and from which secondary gypsum precipitates to provide a structure having some rigidity. This has the effect of stiffening the concrete or mortar and making it necessary to add additional water and thereby lower the strength-giving properties. This is known as false set and should be distinguished from flash set, which results in cements which have insufficient sulfate present effectively to stop the hydration of the tricalcium aluminate to the hydrate rather than to ettringite. Flash set is accompanied with the release of considerable amounts of heat whereas it is sometimes possible with false set (as its name implies) to break down the structure developed through vigorous mixing. Some of the problems with false setting can be alleviated by replacing some of the gypsum added to the cement by natural anhydrite. Other problems associated with the calcium sulfate addition arise when the clinker contains a relatively high SO5 content. As most cement specifications contain a requirement for a maximum SO3 level, the amount of calcium sulfate which can be added is restricted, and in that situation must be in a form where the solubility is sufficient to prevent the formation of the tricalcium aluminate hydrate rather than ettringite.
Other problems occur when the reactivity of the tricalcium aluminate is reduced through the uptake of moisture and/or carbon dioxide.107 In this case it may be unable to react sufficiently rapidly to remove enough sulfate from the aqueous phase to prevent the precipitation of secondary gypsum, although when in its original (unaerated) state this situation prevails.
The pre-eminence of the ball mill has been challenged by the vertical spindle (or roller) mill on the grounds that it is capable of a lower power consumption per unit mass of product (kWh/t). However, the particle grading produced from such a mill tends to be narrower than the ball mill and products made in this way tend to suffer from relatively high water demands when made into concrete or mortar. More work on this approach would appear to be required before any benefits can be realised.
Another development is the introduction of the roller press (otherwise known as high-pressure material-bed comminution)108 to disintegrate the clinker. This involves the use of two rollers turning at a peripheral speed of between 0.9 and 1.8 m/s and with a gap between them of 830 mm. The pressure developed on the particles exceeds 50 MPa, and claims have been made that the clinker is activated by the very intensive stressing which occurs as a result of the passage through the rollers. It has been suggested that, provided the product from the roller press is operated in closed circuit with a disintegrator and a classifier, a ball mill may become unnecessary. If this is done, power savings of 45 per cent have been claimed.109 However, despite this, the normal application of the roller press is in conjunction with a ball mill, taking advantage of the smaller feedstock size to use it in the single chamber configuration. Power savings of 30 per cent have been claimed.
In the alkali-free samples, setting time is reduced slightly with increasing silica ratio and with increasing alumina ratio. The setting times of cements made from alkali and sulfate free clinkers were reduced with increasing lime saturation factor.
This formula represents fairly well the maximum amount of lime that can be combined in Portland cement clinker. A refinement for the maximum combined lime content to allow for the small amount of MgO combined in 3CaOSiO2 has been suggested31:
The maximum value for % MgO that can be inserted in this formula is ~2%, since any excess tends to be present as free MgO (as the mineral periclase) after firing, thus rendering the formula inappropriate.
In the manufacturing of PC clinker, the raw materials are mixed and heated to temperatures up to 1450C. To identify the potential phases after heating the raw mix blend, the lime saturation factor (LSF) is often used to verify the ratio of C3S to C2S. It also shows whether the clinker is likely to contain an unacceptable proportion of free lime. Values between 0.92 and 0.98 are typical of modern clinkers, and a mix with an LSF greater than 1.0 will yield free CaO, which is liable to persist in the final product, regardless of the degree of mixing and time during which the clinkering temperature is maintained (Taylor, 1997).
The silica ratio and alumina ratio (also respectively called silica modulus and alumina modulus) are empirically used to characterise the potential mineralogical composition of the cement clinker. The silica modulus mainly governs the proportion of silicate phases in the clinker, whilst the alumina modulus governs the ratio of aluminate to ferrite phases in the clinker; for normal PC clinker, the silica and alumina moduli usually vary from 2.0 to 3.0 and from 1.0 to 4.0, respectively (Taylor, 1997).
Ali etal. (2013) studied the effect of incorporating up to 2.5% CS in the raw mix by replacing limestone, bauxite and iron ore. An analysis of the LSF and silica and alumina moduli showed that, although the LSF was not affected by the incorporation of CS, the silica and alumina moduli decreased mainly because of the greater Fe2O3 content of CS, when compared to that of the raw ingredients that were replaced. This would suggest that the amount of silicate and aluminate phases would decrease with an increase of the C4AF phase. This was also shown by Bogues method, in which the estimations for the amount of tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) phases were in the ranges of 54.758.9%, 16.319.3%, 4.66.3% and 12.915.3%, respectively, with liquid content varying between 27.4% and 28.9%.
Although the aforementioned methods suggested a decrease of the silicate phases with increasing CS content, the results of the X-ray diffraction analysis indicated that the incorporation of CS resulted in relatively rapid clinker mineral phase formations. Indeed, C3S and C2S contents in samples containing CS, heated at 1400C, were found to be within the range of 5258% and 2328%, respectively, and were comparable to those of the control PC clinker, with C3S and C2S contents of 56% and 26%, respectively, calcined at 1450C.
In the study of Medina etal. (2006), the amount of each phase produced during clinkerisation at 1350 and 1450C was quantified by means of the Rietveld method (Table 5.4). Although no significant changes were found in the C3S phase, a slight increase in the C2S phase was observed when 1.85% CS was used, which would explain the slight decrease in the free CaO content (Table 5.5) in comparison to that of the control PC clinker.
The cement clinker must be correctly burned, to minimise its free lime (CaO) content with the least expenditure of energy (Taylor, 1997). The free lime content of clinker is regarded as a practical measure of the degree of raw mix clinkerisation and is used as a means of controlling the quality of clinker produced. The typical range of free lime content in PC is 0.53%. Table 5.5 presents the free lime content in clinker manufactured with and without CS at different temperatures. As expected, the free lime content decreased with increasing clinkerisation temperatures and it decreased even further when increasing CS was incorporated in the raw mix (Figure 5.4). This trend was explained by the enhanced lime combinability at lower temperatures with the incorporation of CS containing copper oxide (CuO) as well as a decrease in the liquid phases viscosity (Kakali etal., 1996; Kolovos etal., 2005; Ma etal., 2010).
Figure 5.4. Effect of copper slag (CS) incorporation on free lime content of cement clinker subjected to increasing temperatures based on the results of (a) Ali etal. (2013) (b) Supekar (2007) (c) Sahu etal. (2011) (d) Medina etal. (2006) (e) Taeb and Faghihi (2002).
Figure 5.5, which reflects the results presented in Table 5.5, presents the relative free lime of cement clinker samples taken from several studies, in which the CS was ground with the other raw mix components and subjected to normal clinkerisation temperatures (Ali etal., 2013; Medina etal., 2006; Sahu etal., 2011; Supekar, 2007; Taeb and Faghihi, 2002). The results indicate a clear decrease in free CaO content with increasing CS content, revealing greater lime combinability at lower temperatures as observed in other studies (Kakali etal., 1996; Kolovos etal., 2005; Ma etal., 2010). The presence of CuO, which acts both as mineraliser and as flux, decreases the melting temperature by at least 50C and favours the combination of free lime, resulting in accelerated C3S formation (Kolovos etal., 2005).
The phosphate contents of Portland cement clinkers are normally low (around 0.2% as P2O5), although higher levels may be experienced where phosphate is present in significant levels in the raw materials or from alternative fuels such as sewage sludge or meat and bone meal. Fig. 3.2368 shows that up to about 0.5wt% (as P2O5) can be accommodated in the structure of C3S (giving C3S on the diagram) before it decomposes at higher phosphate levels to give a solid solution between C2S and phosphate, and free lime. These products have less satisfactory cementing characteristics. While previous work69 had indicated complete solubility between C2S and C3P at 1500C, later work showed the presence of a miscibility gap, limited by positions PSS and PSS on the figure. The phase diagram shows a number of other solid solutions in addition to the parent phases, C3S, C3P, C4P and C2S. It can be seen that although the primary crystallisation field for C3S extends to 13wt% P2O5, it becomes increasingly narrow towards its maximum limit so that the compositional constraints for the crystallisation of C3S (or C3S) become more stringent. Also, it should be noted that the nearer the bulk composition tends towards the limit of C3S stability, the less C3S can be expected in the clinker. This situation is normally avoided by careful mix proportioning. Consequently, other features such as those appearing in the C4P region of the diagram are less relevant in cement manufacture.
Fig. 3.23. Phase diagram of the system CaOC2SC3P. = phase boundary; = compatibility join at 1500C; PSS, phosphate solid solution at maximum liquidus temperature; PSS, limiting phosphate solid solution at 1500C; PSS, limiting phosphate solid solution at 1500C; C3S, C3S solid solution with Ca2+ and PO43 ions; C4P solid solution with Ca2+ and silicate ions.
The presence of fluoride in phosphatic limestones can have an important influence on the phase equilibria discussed above because coupled substitutions of phosphate and fluoride can occur in C3S. Consideration of the CaOP2O5CaF2 system would show that fluorapatite [3(C3P) CaF2] forms a compatibility with C3S with up to 2mol% of fluorapatite being dissolved in the C3S structure at 1905C. This corresponds to a C3S composition having up to 1.16% P2O5.
Imagine an online classroom that takes you to learn at your own pace, allowing more choices with your learning opportunities. The Cement Institute is dedicated to providing the most dynamic and engaging programs available, as our enhanced online experience demonstrates an interactive and hands-on knowledge applicable directly to your plant.
Cement microscopy is a valuable technique for examining clinker, cement, raw materials, raw feed, and coal. Every stage of the cement manufacturing process can be improved through the use of a microscope.
Most cement microscopy is done using a petrographic microscope. Usually, the specimen is a polished section of cement clinker examined using reflected light, although it may be a powder mount, or a thin section examined using transmitted light.
An emphasis on understanding the Factors affecting the efficiency and productivity of cement kiln operations Factors affecting clinker product quality Factors affecting safety and emissions to the environment.
A unique combination of theoretical and practical skills throughout this course will be learned, which will help you develop and execute the concepts and technical knowledge acquired in the plant process activities. The following downloadable materials are part of the course to enhance and facilitate the participants learning experiences.
Work section book: Provide learning activities and hands-on practice case study and exercises. Solutions are included after each training is completed. Certification is achieved by completing a satisfactory level of exercises, quizzes, and final exams for each module.
The course is conducted online, allowing students flexibility (within four weeks) to complete all modules. Students should expect to spend more than 6 hours per week and some additional time for private reading/study. A computer with Internet access (broadband recommended) and email will be required.
Factors of delayed expansion of periclase were summarized.The effect of MgO on the mineral phase, strength and hydration was reviewed.The occurrence of Mg2+ in clinker and hydrates were summarized.The regulation approach of the occurrence of Mg2+ clinker and cement were discussed.
MgO significantly influences clinker properties such as burnability, phase composition, mineral crystal polymorphism, cement strength, and volume stability. Moreover, it influences the activity of C3S, the rate of hydration of cement, as well as hydrates. This paper elaborates on the occurrence of MgO in cement clinker and hydration products and its influence on the properties of clinker and cement. Resultantly, optimizing the proportions of raw materials and modifying the thermal processing system improve the occurrence of Mg2+ in clinker. Furthermore, these measures reduce the expansion of periclase upon addition of supplementary cementitious material rich in silicon and aluminum to form nonexpansive hydrates bearing magnesium such as hydrated magnesium silicate.