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Characterization of Refractories



In the journey of mankind from cave to skyscraper, the Portland cement played a very important role. The modern civilization owe a lot to the contribution of cement and concrete as a building material for construction of bridges, buildings, roads, dams, tunnels, and tall structures which are being used by the people everywhere in every walk of life.

The Portland cement is manufactured by high-temperature reaction of clay or shale with calcium oxide of limestone or chalk to form cementing phases like dical-cium silicate and tricalcium silicate. This high-temperature reaction takes place inside a reactor called kiln. To contain the temperature inside the kiln and various other accessory equipments, on a continuous basis, to make the manufacturing pos-sible on industrial scale, the Refractories play a very important role. A refractory lining inside the reactor maintains the temperature range of the reactor metal struc-ture within a tolerable limit. The Refractory lining also inhibits the heat flow from inside of the reactor to outside and thus helps conserving the energy, which provides economy to the process. Without the availability of a proper Refractory, it would have not been possible to produce cement in industrial scale, economically.

Refractories are basically serving two purposes: firstly to contain the high tem-perature required for the process to produce cement clinkers and secondly to insu-late the reactors to inhibit the flow of energy out from the system. Two different types of Refractories are used to serve these two different purposes. In the first case, the dense Refractories are used to contain the temperature, and in the second case, the insulating Refractories are used to insulate the energy flow out of the system conserving valuable energy.

In the metallurgical and process industries, the Refractory practices are continu-ously getting changed, both, because of changing demand of technology at the users’ end and the availability of advanced material, which gives better perfor-mances and better economy. The cement industry is not an exception to that.

Refractory Materials

The Refractory materials are inert inorganic solid materials which are stable at high temperature in contact with corrosive solid, liquid, and gas and can retain its physi-cal shapes and structural strength at high temperature. These are mainly oxides, carbides, nitrides, and borides of aluminum, silicon, alkaline earth metals, and tran-sition metals. Table  1.1 furnishes a comprehensive chart of different refractory materials with very high melting point.

Out of all these materials, very few qualify to be used in industrial scale, because of their instability under normal atmospheric condition or because of the rare availability and high cost. For example, barium oxide or calcium carbide and aluminum carbide react very fast with atmospheric moisture. Vanadium, niobium, molybdenum, haf-nium, etc. are too expensive to be considered for Refractory application. Finally, the oxides like Al2O3, SiO2, MgO, CaO, Cr2O3, ZrO2, and carbon in different mineralogi-cal form, individually or in combination (Fig. 1.1), are used most widely to manufac-ture refractories for all metallurgical, chemical process industries and in other applications. The criteria of selection, of the abovementioned materials, are their abun-dance in nature, stability, and ease of processing to manufacture Refractory products.


The source of the raw materials can be natural or synthetic. The raw materials used for Refractory manufacturing are mainly naturally occurring minerals like bauxite, magnesite, clay, etc. which are mined and processed before being used for refractory manufacturing. Some synthetic materials like mullite (3Al2O3·2SiO2), fused alumina (Al2O3), silicon carbide (SiC), spinel (MgO·Al2O3), etc. are also being used widely in Refractories for cement industry. Properties of the naturally occurring raw materials may vary considerably from one country to another, because its geological formation and associated impurities vary.

Refractory Properties


Refractories are characterized by their chemical and physical properties and are used to correlate its behavior in actual high-temperature application.

Specific Gravity

All different Refractory minerals have different densities, and it is a fundamental property of the material. Refractory materials can be identified by their specific gravities. Specific gravity can be determined by making powder of the sample of a specific size and using a specific gravity bottle and a balance. Table 1.2 gives the specific gravity value of some Refractory bricks and Refractory minerals.

Bulk Densities

It is the mass of the material per unit volume including pores. For same kind of Refractory, the bulk density can vary. The higher is the bulk density, the lesser will be the porosity and normally more will be the mechanical strength. Bulk density is different from the true density in the way that the total volume considered in the calculation is the sum of the volume of both material and pores. Therefore the value of true density is always more than that of the bulk density.

Apparent Porosity

Refractories contain pores; some of the pores are open and connected and some are closed (Fig. 1.2). Total volume of a refractory body = volume of the matter + vol-ume of the open pores + the volume of the closed pores. The apparent porosity of refractory, expressed in %, is defined as

It is a very important property and influences the mechanical strength, corrosion resistance, and thermal conductivity of a Refractory. Porosity and bulk density of a refractory are inversely related. The lower the apparent porosity, the more will be the bulk density, mechanical strength, thermal conductivity, and corrosion resis-tance of the body. Besides total pore volume, the pore sizes are also very important to influence the corrosion resistance and thermal conductivity of the Refractory. The smaller the pore sizes, the better is the corrosion resistance and the lower is the thermal conductivity. True porosity is the total volume of open and closed pores. It is expressed in % and is defined as

The test for dense brick can be done as per the method ISO 5017.1988 of International Standard Organization and for insulating shapes the test is carried out by the method given in ISO 5016.1986.

The volume, size, and structure of the pores have close relationship with the penetration of slag and the permeation of gases inside the Refractories. The pore diameter in dense and fired refractory materials is in the range of 0.1–100 millimi-cron (μm). Larger pores are found in insulating refractories. Refractory castables have high share of pores below 1 μm.


It is the measure of flow of gases through pores within the Refractory body, and it indicates the extent of pore linkage. Permeability of Refractories gives an indication on how well the Refractory will stand up to molten slag, a melt or to a gas penetration.

Specific gas permeability is defined by the equation given below, with laminar gas flow:

The factor 2p/(p1 + p2) = 1 for small pressure differences.
The unit of gas permeability is m2. The value for the gas permeability of refractories is usually very low and is normally expressed as μm2. The previously used unit was perm or nanoperm and 1 μm2 = 10 nPm.

Gas permeability of the refractories is determined by the share of pores with diameter greater than 10 μm. Gas permeability decreases substantially with increas-ing temperature and the increasing viscosity of the gases at higher temperature. A decrease of 50% permeability can be expected at 500 °C temperature. At higher temperature the closure of microcracks also bring down the permeability.

The permeability of a Refractory to gas can be determined by ISO standard method 8841.

Mechanical Properties

At ambient temperature these properties gives an idea about the mechanical strength required to transport and handle the Refractory-shaped products at work sites.

Cold Crushing Strength (CCS)

In this test, the cube of a specific dimension cut from the brick sample is subjected to increasing load, until it gets crushed and the test result is reported as the value load per unit area. It indicates the adequacy of firing temperature, for shaped Refractory products, required for proper sintering and to develop the required microstructure and the quality of hydraulic or chemical bond in case of unshaped Refractories. In the unshaped products, the CCS does not remain same after heat treatment, and it decreases or increases with temperature of heat treatment. The good cold crushing strength of shaped Refractories protects them from damages during handling and also from mechanical abuses in service. CCS can be deter-mined following the method given in ISO standard method 10059-1 and 10059-2.

Modulus of Rupture

The test is conducted by putting the bar of a specified size cut from the Refractory body on two-point supports and applying load on the middle of two supports till the bar breaks (Fig. 1.4a). It is calculated as

Modulus of Elasticity, Poisson’s Ratio, Hardness

It defines the stress-strain relationship and is a fundamental property of material. Like any other material, the Refractories also obey Hooke’s law, and, accordingly, it exhibits a linear relationship between applied stress (σ) and the mechanical defor-mation (strain  =  ε). The proportionality constant between the two is modulus of elasticity, E, when the stress is compressive or tensile. If shear stress (G) or torsional stress (τ) is applied, the strain is γ, and the proportionality constant is called shear modulus, G. Thus the mathematical relations are


Refractories are composite brittle material at ambient temperature, and its frac-ture process is different from that of high strength single-phased ceramic material with fine grain size. Refractory lining is to withstand different mechanical and thermo- mechanical stresses developed during its use. For example, mechanical stresses generated because of ovality in the kiln shell or thermo-mechanical stresses generated during heating and cooling of refractory lining.

The brittleness of a material can be visualized from the load-displacement curve of that material under three-point bending as done in Modulus of Rupture Test (Fig. 1.4a). If the load-displacement curve shows a pattern like that in Fig. 1.4b, it will be considered as brittle material [3].

But it is to be borne in mind that Refractories are not very strong, by nature, and need not to be very strong also, because it is not used to carry any load except its own weight. The main purpose of Refractory lining is to protect the equipment steel casing or shell, from high temperature. The strength of refractory at higher temperature is of more concern to the Refractory engineers than its strength at ambient temperature. When the HMOR of Refractory is plotted against the tem-perature, it exhibits a maximum value and then decreases rapidly with increase of temperature.

The magnitude of the maximum strength obtained at a temperature between 600 °C and 1400 °C, depends upon the type of Refractory. The displacement curve of a Refractory if plotted against load at high temperature shows a curve as in Fig. 1.4c, which shows no more brittleness and plastic flow in the material. Thus, it is very important to understand that Refractories are brittle material with low strength at low temperature and becomes still weaker at elevated temperature, but it develops plasticity above 600 °C and becomes much less brittle [4, 5].

Microcracks always exist in refractory body with coarse grains. When a stress is applied on the refractories, some of the cracks may propagate to cause failure of the Refractory. The strength of the refractory body depends on the dimension of the crack. The fracture toughness KIC is proportional to the square root of the critical crack length (Eq. 1.4) [4]:

where σ is the critical stress, Y is a geometric factor, and C is the critical crack length. The higher is the fracture toughness, the more difficult it is, to initiate and propagate a crack.

For Refractories the fracture toughness remains within the value 0.5–1.2 MPa·m0.5.

Wedge-Splitting Test

Technologists have devised a test called wedge-splitting test which gives the idea about the toughness of a refractory or its resistance to crack under the influence of mechanical stresses. The schematic diagram of the devise is shown in Fig. 1.5 [5]. A sample of 100 mm × 100 mm × 75 mm is cut out of the brick and is provided with a starter notch, and two side-guided notches are taken and are put into the testing machine, and the load is applied on the wedge vertically [6, 7].

The application of the vertical force Fv develops two horizontal forces FH and causes the splitting of the sample. During the testing process, the displacement is recorded. From the load-displacement diagram, the specific fracture energy can be determined by integration:

where δult is the ultimate displacement before splitting of the test sample, A is the area of projection of the fracture surface, and GF is the specific fracture energy in N/m2.

where β = angle of the wedge in Fig. 1.5.

The higher the value of the GF, the tougher will be the Refractory for an applica-tion. The same test can be carried out in higher temperature also. This test becomes very useful to predict the behavior of basic Refractories in the application of burn-ing zone in cement rotary kiln. The typical load-displacement graph of wedge- splitting test is shown in Fig. 1.6 [8, 9].

Abrasion Resistance

This test becomes important for the application where the Refractory lining is exposed to moving gases, liquid, or solid. The higher is the velocity of the moving particles, the higher will be the abrasion. Two standard testing methods are followed to com-pare the abrasion resistance of Refractory body, namely, ASTM C704 or BS 1902.4 and EN 993-20 – the grinding method according to DIN 52 108 or DIN EN 102.

The abrasion resistance of Refractories depends on the intrinsic hardness of the grains in the bonded structure and also depends upon the microstructural features,

e., grain size, porosity, pore sizes, etc. Correlations exist between porosity, cold crushing strength, and cold modulus of rupture, which can be utilized for the rough evaluation of abrasion resistance. But the prediction of abrasion resistance, based on strength factor alone, is insufficient, because the bond phase of the refractory, the abrasive media grain size, grain morphology, and the angle of impingement of the grains have tremendous influence on abrasion resistance [10–12, 14]. Abrasion gen-erally decreases in a fired brick with increasing temperature [13, 15]. It can be zero when the brick surface attains a visco-plastic state.

In monolithic Refractories, a general relation is known to exist between abrasion resistance and the modulus of rupture; the higher is the modulus of rupture, the bet-ter is its resistance to abrasion (Fig. 1.7) [11]. Abrasion loss is highest when the impingement of the particles on the refractory surface is at right angle, but it does not hold good when the impinging particles are much smaller compared to the aggregate size of the monolithic refractory. When the size of the impinging particles are much smaller than the aggregates, then the matrix of the monolithic refractory undergoes abrasion first, although the abrasion resistance of the aggregate may be very good, and loosens the aggregates, which falls off. Therefore, in that case, to withstand the abrasion of dust laden gases, the matrix must have to be abrasion resistant, and use of abrasion resistant aggregate alone will not be effective. If the impinging grains are larger than the average aggregate size, then both the aggregate and the matrix are removed together and both need to be abrasion resistant.

Abrasion resistance of a refractory can be determined following ASTM C704-94 standard. It is a comparative method to test the abrasion resistance of two or more products under identical condition.

The abrasion resistance of a Refractory surface in service can change drastically as a result of corrosion or by a coating on the surface. Extensive abrasion and ero-sion can also occur when hot gases (even dust unladen) pass over the lining at high speed. Table 1.3 shows the variations of abrasion of some of the refractories with temperature.

The abrasion results in the wear of the refractory lining. For a preheater kiln, the normal wear rate is about 0.6 kg/ton of clinker produced.

Thermal Properties

Pyrometric Cone Equivalent (PCE)

Most of the refractories are made of the naturally occurring raw materials, which contain some inherent impurities. Sometimes the presence of the impurities brings down the softening point of the refractory. Refractory products are normally a com-bination of different raw materials and do not have sharp melting point similar to pure crystalline material. Depending upon the quality and quantity of impurities, the liquid phases are formed at elevated temperature. The quantity of the liquid and its viscosity dictate the softening behavior of refractory. The PCE test gives an idea about the softening temperature and behavior of the Refractory material. From the idea of softening point, we can roughly estimate the MST (maximum service tem-perature) which can be considered as 200 °C below the PCE. The PCE value can also be used to compare the refractoriness of two refractory products from different sources or two similar raw materials from different sources. In this test the Refractory material is ground fine and made in the form of a cone of a specific size. The cone is mounted on an alumina plate (Fig. 1.8) along with few standard cones having a definite softening temperature and put in a furnace, and the temperature is gradually raised till the test cone starts bending along with another standard cone. The softening point of the test cone is reported as the cone number of the standard cone along with which it bends. The test method in ISO528.1983 can be followed to determine the PCE.

Reversible Thermal Expansion (RTE)

Like all other materials, the Refractory also expands when heated and comes back to its original dimension on cooling. The reversible thermal expansion values of different types of Refractories are different. For construction of the furnace lining, this is an important parameter to be taken into consideration, because the provision has to be kept for expansion of the Refractories, during actual operation of the fur-nace, when the lining gets heated up.

In the Refractory industry, it is expressed as % expansion from room temperature to a certain temperature. For example, if the length of a Refractory bar of length L1, on heating to a certain temperature, becomes L2, then the thermal expansion of the body at that temperature is

If the provision for expansion is not kept in a lining, tremendous amount of stress can be generated which may lead to collapse of the lining.

The thermal expansion is measured by the instrument dilatometer.

Permanent Linear Change (PLC) on Reheating

Refractory bricks, during the course of manufacturing, when fired at high tempera-ture, generally change in dimensions. It either shrinks or expands. It is because of solid-state reaction, forming liquid phases or because of some phase transformation. The kinetics of the solid-state reactions, generally, are very slow and never attains equilibrium in the course of the firing process, adopted during manufacturing. Therefore, the Refractories, when exposed to high temperature for long duration in service, change dimension often, which is irreversible and is known as permanent linear change (PLC). It is also expressed in %, similar to RTE.  This property is determined by heating a bar, cut out from a Refractory body, at a specified tempera-ture for specified time. Initial length and the length after the firing are measured to determine the % PLC. Refractories, sometimes, are designed purposefully to have a positive or negative PLC. The PLC of a refractory product can be determined fol-lowing ISO standard method ISO2478.1987.

Thermal Conductivity

It is the fundamental property of a material and can give the estimation of heat flow through a material. Heat energy flows from high temperature, inside the furnace, to low temperature ambience, outside the furnace, through Refractory lining of the furnace. In a steady-state operation of a furnace, after a part of the thermal energy is absorbed by the furnace construction material, there is a steady outflow of energy from the system, and it is a very important factor to decide the economy of the pro-cess. Thermal conductivity of Refractory lining material is thus an important param-eter for designing the furnace lining.

The thermal conductivity is different for different material, and it is a function of temperature and atmosphere. That is why the thermal conductivity at mean tem-perature is used for design purpose. The major factors that affect the thermal con-ductivity of Refractory material are the mineral composition, the amount of amorphous material, its pore volume, pore size distribution, and temperature. Thermal conductivity also depends upon the composition of the gases surrounding the refractory. The thermal conductivity of an aluminosilicate Refractory is much higher in hydrogen atmosphere than what it is in normal atmosphere. Thermal con-ductivity is a difficult property to measure in a steady-state method, because it takes a very long time to attain steady state and to maintain it. Therefore unsteady-state method which is easier and saves time is widely used, and one of these widely used methods is hot wire method.

Thermal conductivity can be measured following the method given in ISO- 8894.2.1990 or 8894.1.1987.

Thermal conductivity of the refractories used in cement industry is important because the cement manufacturing is an energy-intensive process. The heat loss due to radiation and convection from the surface of the kiln shell is substantial, and if it cannot be controlled, the energy cost will go up. Table 1.4 shows the heat loss from the surface of the rotary kiln shell, per unit area per hour in case the ambient temperature is 21 °C and air velocity zero and 1.5 m/s, respectively [14]. The heat loss is quite substantial and can be calculated for the whole kiln of a known diameter and length.

Heat Capacity (Cp)

It is a fundamental property of the material. This gives an estimation of the heat stored in Refractory structure in a furnace. Cp value is important, when it is required to calculate the heat required to raise the temperature of the Refractory to a certain temperature.

Thermal Diffusivity (D)

The thermal diffusivity is a measure of how quickly a body can change its tempera-ture. It is defined as

Thermo-mechanical Properties

The refractory materials are used in high temperature and different types of com-pressive, tensile, and shear stresses act on the refractory body at high temperature. The refractory must be able to withstand those stresses at high temperature. The thermo-mechanical properties of the refractory give an idea about the capability of the refractory to withstand those stresses at elevated temperatures. The thermo- mechanical properties and some tests, devised to measure those properties, will be discussed here.

Hot Modulus of Rupture (HMOR)

This is similar to three-point bending test performed in cold modulus of rupture, but it is performed at higher temperature inside a furnace, and the HMOR value is calcu-lated using same formula. HMOR value indicates the tensile stress the refractory can withstand at certain temperature. It gives an idea about the bonding strength between the matrix and the grains, in the Refractory body. It is influenced by the amount of liquid generated at the test temperature, its viscosity, and the microstructure, i.e., the nature of porosity, microcracks, etc., and their distribution in the Refractory body. This test can be carried out following the method given in ISO 5013.1985.

Refractoriness Under Load (RUL)

Refractories deform when put under a compressive stress at elevated temperature. The deformation is mainly due to the formation of liquid phases formed in the matrix of the refractory body at elevated temperature. The extent of deformation depends upon the temperature, the compressive stress, and the quantity and viscos-ity of the liquid phases formed. The higher is the volume of the liquid phases formed and lower is their viscosity, the higher will be the deformation at high temperature. Higher temperature and higher compressive stresses lead to more deformation.

RUL measures the temperature at which a specific deformation of Refractory occurs and gives an idea about the load-bearing capacity of the Refractory at high temperature. RUL can be determined following the test method given in ISO1893.1989.

In this test the cylindrical sample of diameter 50 mm and height 50 mm is drilled out of the Refractory body and is put into a vertical tubular furnace, on one of its flat side. A load of 0.2 MPa is applied on the surface through a graphite rod, and tem-perature is raised at a rate of 5 °C/min. The change of length of the sample is mea-sured during heating and plotted against temperature as a curve shown in Fig. 1.9. The temperatures at which deformation of 0.2%, 2%, and 5% occurs are denoted on the temperature vs deformation curve as T0.2, T2, and T5. The deformation shown in the figure is [{ΔH(in mm)}/50] × 100% = 2ΔH %.


This property characterizes the time-dependent deformation of the refractory at high temperature. When a refractory is subjected to a compressive stress for a long time, then its deformation behavior over that period can be predicted by this prop-erty. In this test, the cylindrical sample of 50 mm diameter and 50 mm height is drilled out of the Refractory under testing. The sample is put into a cylindrical fur-nace and temperature is raised to test temperature and a load of 0.2 MPa is applied. The temperature and the load are kept constant. The deformation of the sample is measured against time over a long duration to get its creep behavior. The creep in compression can be measured following the ISO method 3187.1989.

Thermal Shock Resistance

Refractory lining faces temperature cycling during its campaign for various reasons, for example, because of start-up and shut down of the kiln, because of variation in fuel rate, etc. In most of the cases, because of exigency of production, the heating and cooling are made fast. The fast temperature variation in the kiln or furnace causes the development of thermo-mechanical stresses in the refractory lining.

Differential temperature in the lining results to differential expansion in the same body and develops mechanical stresses, either compressive or tensile in nature, which ultimately damages the refractory.

The Refractory lining develops compressive stress on fast heating and tensile stress on fast cooling of the furnace. Normally the ceramic materials are weaker under tensile stress, and therefore the fast cooling damages the Refractory more than fast heating.

The thermal shock resistance of a Refractory is related to its other properties like thermal conductivity, thermal expansion coefficient, modulus of elasticity, and rup-ture strength. The higher the thermal conductivity and rupture strength, the better is the thermal shock resistance. Lower value of modulus of elasticity and thermal expansion coefficient make the thermal shock resistance property of the body better.

Rapid temperature change on the surface of a refractory body causes large tem-perature gradient that leads to considerable stress which can be expressed by the equation below [2].

This equation says that a refractory with high strength and high thermal conduc-tivity, low modulus of elasticity and low thermal expansion the resistance to crack initiation is high.

To test the thermal shock resistance property in laboratory, normal Refractory samples of definite shape are cut out of the Refractory bricks, and the samples are heated at a definite temperature for definite time, and then it is quenched either in cold air or water for a certain time and the total process is called one cycle. The same process is continued till the refractory suffers a specified dam-age, and it is reported as the number of cycle under the specified condition of heating and cooling. The thermal shock resistance testing is carried out to compare between two or more refractory products. The PRE/R5 method of European Federation of Refractories can be used for this comparison.

Structural Spalling

Many times, during service, an altered zone is formed in the refractory lining, near the working surface. It is formed due to chemical reaction of the refractory lining, with the different materials it remains in contact, during the operation of the fur-nace. The altered layer tends to crack or fall off in the form of thin or thick layer called “peeling” or “slabbing,” even due to the formation of an internal stress caused by the small temperature fluctuation. The stress builds up, due to the mismatch of properties between the original and altered zone of the refractory as a result of min-eralogical changes and liquid phase formation. The major cause of the formation of altered zone is the chemical reaction of refractory at elevated temperature, with dif-ferent gas, liquid, and solid phases remaining in contact with refractory, forming different mineral phases. The overheating, which accelerates those reactions and the formation of the liquid phases, causes densification of the refractory body in the altered zone.

Corrosion Resistance

It is the property of the refractory which indicates its inertness to chemical reaction with the specified solid, liquid, or gas at high temperature. Due to the chemical reac-tions mentioned, the Refractory forms different products or phases which are not suitable as a Refractory material, and the Refractory stops functioning for the pur-pose it is used. The reactions are more prominent with liquid and gases, and the reaction rate increases fast with increasing temperature. The process of corrosion is very complex in nature and is a combination of chemical and physical processes. Besides the chemical reaction, the pores in the refractory allow the molten liquid or gas to permeate inside and to react further [16]. The reaction products may develop stress because of its higher volume and cause cracks in the body, which increases contact area with further enhancement of corrosion. For the physical penetration of the liquid, the rate of penetration dl/dt is governed by the equation:

where r = radius of pore, γ = surface tension of liquid, η = viscosity of the melt, and θ = wetting angle or contact angle (Fig. 1.10). Equation 1.17 shows that higher pore size of refractory, lower wetting angle of the liquid on the refractory, and lower viscosity increase the rate of penetration. The reaction rate increases fast with increas-ing temperature and the type of flow of the fluid (laminar or turbulent) in contact.

It is very difficult to simulate the conditions of the refractory and melt reaction in the laboratory. The corrosion resistance test is a qualitative test, designed to com-pare the affinity of two or more refractory products, to react with a liquid, at a cer-tain temperature. However, to get the idea of the compatibility of a refractory to a particular liquid phase at certain temperature, tests are carried out. There are two different type of tests are normally carried out, static and dynamic.

Static Test Method

In static test, a cube of 75 mm size is cut out of the refractory under testing. A drill is made of 25 or 50 mm diameter and depth of 40–50 mm in one face of the cube. The cup made is filled with the solid with which the reaction of the refractory is to be studied, when it is in molten condition. The cube is then put in the furnace and fired till the desired temperature is reached. The sample is allowed to reside inside the furnace at that temperature for a specific time, allowing the reaction to happen, and then the furnace is put off and cooled. The refractory cube is taken out, and the cup is cut vertically to observe the extent of the reaction. It is a comparative test that is difficult to quantify. The judgment of corrosion resistance is made by visual observation only, after a refractory is allowed to undergo through a corrosion reac-tion at a specified temperature for a specified time.

Dynamic Test Method

In dynamic slag test, the melt remains in the furnace in a crucible, and an attachment holds a small bar (cut out of refractory) of a specified dimension which rotates on its axis in the melt at a desired test temperature for a specified time. Then the sample is taken out of the melt, cooled, and is visually observed for the reaction happens.

Alkali Resistance Test

It is qualitative test and compares the alkali resistance of a Refractory with others. In this test a 40 mm hole is drilled in a Refractory brick to form a cup. The cup is filled with potassium carbonate, covered with a refractory lid and the test piece is put into a furnace, and the temperature is raised to 1100 °C and kept for 2 hours. The furnace is put off and cooled. The test piece is observed. The test piece develops crack if it is not resistant to alkali attack. The extent of crack determines its resis-tance towards alkali attack. Figure 1.11 shows the test samples after alkali resis-tance test. Different test methods are available to compare the corrosion resistance of two or more Refractory products under identical condition. It can be tested by C454-83 of ASTM standard.

Microstructure Study

Refractories are normally made of small grains bonded by a matrix of fines and glassy phase. The key elements of the microstructure are the individual grain bound-aries, pores, and microcracks. Microstructure tells about the grain sizes and mor-phology, pore sizes and morphology, orientation and distribution of the grains, matrix and pores, presence of different mineral phases, presence of microcracks, etc. All the microstructural features influence the physical properties of the Refractory. The microstructure of the Refractory cannot be seen in the naked eye and is revealed through different instrumental methods as mentioned here.

Optical Microscopy

The polished section of the Refractory or thin section of the Refractory can be seen under high-resolution microscope by either reflected light or by transmitting light. The photo can be taken to record the microstructural features. It shows the grain sizes, pore sizes, and their distribution along with that of matrix, the morphology of grains and pores, and different mineralogical phases (Fig. 1.12).


In this instrument electron beam is used in place of ordinary light. Magnetic field does the function of lens in electron microscope. The resolution in this microscope is very high and 1000 times more than the optical microscope. It enables us to see the microstructure with much larger magnification (Fig. 1.13). The different meth-ods of testing, using electron beam, are scanning electron microscopy (SEM), elec-tron probe microanalyzer (EPMA), and transmission electron microscopy (TEM).By EPMA, any point on the section of the refractory can be chemically analyzed, and chemical map of a selected area on the sample under microscope can be obtained which is very useful to have an idea of mineral phases present and their distribution (Fig. 1.14).

X-Ray Diffraction

XRD pattern is unique for any mineral phase.

By exposing the powder of a Refractory sample to X-ray, a pattern is obtained. When that X-ray pattern is analyzed, then different mineral phases present in the Refractory body can be known by the presence of their characteristic peaks (Fig. 1.15). The presence of the peaks indicated the presence of the particular min-eral, and the intensity of the peak gives an idea about its quantity present.


It is used for the chemical analysis of any Refractory material. In this method the powder of the test sample is used to form a pallet which is fed to the XRF machine, and on running the machine, it produces a diffraction pattern (Fig. 1.14). The stan-dard X-ray diffraction pattern is already there for different mineral. The produced X-ray diffraction pattern when matched with the diffraction pattern of the known minerals and then the mineral phases present in the sample under investigation can be known.

Classification and Features of Different Types of Refractories

Classification of Refractories

There is no rigid norm for classification of Refractories, and it can be classified in various ways. The basis of classification of refractories in different ways is dis-cussed here.

Basis of Classification

Refractories are generally classified based on different parameters, e.g.:

(a) Basicity of oxides
(b) Form
(c) Manufacturing process

(d) Method of application

(e) Insulation property
(f) Special chemistry

Let us see the classification in details.

By Basicity

The oxide Refractories can be classified according to the basicity of the oxides. Refractory oxide can be characterized by the electropositive character of the central atom in that oxide. The more electropositive is the central atom, the more basic is the oxide, and the more electronegative is the central atom, the more acidic is the oxide. Electropositive character increases from right to left across the periodic table and increases down the column.

According to the position of the element in the periodic table, its oxide can be classified as acidic, basic, or amphoteric in nature. Accordingly, the Refractories can be classified as acidic, basic, or amphoteric as shown in Table 2.1. This classifi-cation has significance in selecting Refractories to be used in contact with molten slag with a known basicity. For example, basic Refractories should be selected where the basicity of the slag, in contact with Refractory, is more than 1.0 to get a reduced corrosion rate, although many other factors should be taken in consider-ation in selecting the right kind of Refractories for a certain application.

By Form

The other classification of Refractories has been made on the basis of its form in which it is produced, manufacturing process and application. According to form Refractories are classified as shaped Refractories, commonly known as refractory bricks, and the unshaped refractories as monolithic refractories. The shaped refrac-tories are delivered in the form of definite geometrical shape, and unshaped Refractories are supplied as loose powder, which is to be mixed with required amount of binder/water and applied at the site to take the shape of the area, where it is used. Figure 2.1 shows the scheme of the classification of refractories based on form.

Shaped Refractories

Shaped Refractories are made in different geometrical shapes according to the requirement of the user. The shapes are designed depending on the geometry of the furnace or equipment which is to be lined with refractory. Shaped Refractories are classified by the process of manufacturing adopted and are termed accordingly.

Pressed Shape

Different size fractions of different Refractory raw materials are mixed with a liquid binder. The mixture is pressed in a mold at high pressure in mechanical press, fol-lowed by drying and firing to impart mechanical strength and volume stability at

Fig. 2.1 Classification of Refractories

high temperature to the body. These products are called pressed bricks. The bigger is the dimension of the brick, the higher is the capacity of the press needed. Product with complicated shape and geometry cannot be made by this process.

Hand Molded

Hand-molded shapes are made by mixing different size fraction of different Refractory raw materials with a liquid binder and to give shape by pneumatic ram-ming in a mold followed by drying and firing to impart mechanical strength and volume stability at high temperature. These products are having lower density and strength than pressed products. This method of production is much slower com-pared to production by pressing. Only very critical shaped items are made in this method.


In this method different size fraction of refractory raw materials are mixed along with high alumina cement and other additives followed by mixing with required quantity of water. The mixture is then given shape in a mold by vibration casting or pouring. The cast body is then allowed to set and cured followed by demolding and drying to form the precast Refractory. Sometimes the cast body is fired at high tem-perature to meet the desired properties. This is also a very slow process of produc-tion compared to pressing and used to produce complicated and bigger shapes which cannot be made by pressing in mechanical press.

Fused Cast

The mixture of raw materials is melted in a high-temperature furnace and poured in a graphite mold. The mold is cooled very slowly when the melt solidifies and takes the shape of the mold. These are called fused cast Refractory shapes. Specialty of these products, are zero porosity and these are used mainly where the corrosion is the main factor to destroy the refractory, e.g., lining of the glass tank where the refractory remains in continuous contact with highly corrosive molten glass. This kind of product is never used in the cement industries.


This method is mostly used for making ceramic fiber insulation boards or similar products with other shapes. In this process the ceramic fiber along with the organic/inorganic binders are stirred in water at high speed to form a slurry, which is spread over a mold, fitted with fine wire mesh, and the water is sucked out under high vacuum pressure to give the shape of the body. The formed body can then be demolded and dried and heat cured to develop strength in it.

Advantage and Disadvantage of Shaped Products
The advantages and disadvantages of using shaped Refractories are:

1-The bricks made under high pressure attain very good properties like high den-sity and low porosity.

2-The laying of shaped Refractories does not demand very high skill of masons.

3-The laying process is normally manual and does not involve the use of expensive machines, although in case of rotary kiln lining, the brick laying machines are being used very successfully.

4-The total time of Refractory lining and preheating, i.e., starting of the installation to furnace ready-to-use duration, is less compared to lining with unshaped Refractory.

5-Many different shapes are required in combination with line the equipment, and inventory cost is higher compared to unshaped refractories. On nonavailability of even one shape, the total lining process may come to a halt.

Unshaped Refractories

Unshaped Refractories are normally manufactured and supplied in bags as loose powder. The manufacturing of unshaped Refractories is simpler than the shaped Refractories. In its manufacturing the different ingredients in different size fractions are mixed thoroughly in a mixer with required additives and bagged. During use, the bags are opened at the site and the powder is mixed with required quality and quan-tity of water or supplied liquid binder, in a mechanical mixer, and is rammed or cast or sprayed at desired place of installation. The unshaped Refractories can be further classified based on its application method.


Mortars are powder materials which are used for the purpose of joining the Refractory bricks. The powder material is mixed with water or a liquid binder sup-plied along with the mortar and applied as glue on the brick surface to join two bricks. The mortar can set and harden either by chemical reaction (chemical-setting mortar) at ambient temperature or by heating (heat-setting mortar) the joined brick. Mortar can be both of basic or aluminosilicate type, used to join either basic bricks or aluminosilicate bricks. Mortar joints in the brick lining acts as a cushion to reduce the thermo-mechanical stresses generated during operation of the furnace.

Ramming Masses

Ramming masses are used in very less quantity and for very special purposes only, in cement industries. It is a mixture of coarse aggregates and fine powder with some additives, and it is mixed with water or liquid binder and rammed by hand or by pneumatic rammer at the desired area. It is normally applied to fix up the gaps which are difficult to be filled otherwise, for example, a small gap between the bricklayers and retainer plate. Ramming masses can be made both in basic and non-basic composition, but in the cement industry, the nonbasic ramming masses are used normally.

Plastic Refractories

Plastic Refractories are ready-made unshaped Refractories. These are supplied, in ready-to-use form, of small blocks, wrapped in polythene sheet. The material is unwrapped at site and put at the place where it is to be used and then stamped with wooden mallet or rammer with flat-bottomed tool. Plastic Refractories are made with aluminosilicate material only. This material is hardened fast after installation and can be dried and preheated faster, compared to castable. Because it is a ready- mixed material and binder is mixed already, it has got a low storage life, especially in hot climate of tropical countries. It has the advantage that it is a ready-mixed material, so the chance of mistakes due to wrong addition of binder or improper mixing of binder can be eliminated. It is a good material to use for lining the roof of a furnace. Ceramic anchors are used, similar to castable installation, to hold the material. Its installation is a slow process, but it can withstand fast heat up and does not develop crack.


Castables actually are the major products in the unshaped Refractory category and are made up of aluminosilicate aggregate and calcium aluminate cement (also called high alumina cement) as binder, along with special additives to impart or modify certain special properties in it. The calcium aluminate cement, like Portland cement, has hydraulic property and sets to a hard mass in contact with water. In case of no- cement castables, the liquid binders are supplied separately along with dry powder, and both are to be mixed during application. Of course, recently the no-cement cast-able has also been developed in which the water can be used for casting instead of any separate liquid binder.

Castables are the material which can be cast after mixing with water or other liquid binder (in no-cement castable), and it set hard at ambient temperature. Castables can be installed by different methods, e.g., by pouring, by vibration cast-ing, by gunning, and by shotcreting. Two major steps in the installation processes are mixing of castables with water and the method of consolidation of the castable. Mixing quality is very important, and the aim should be to get maximum flow using the prescribed water or binder content. That is possible only when the mixing is very thorough and intimate, done in high-shear mix.

The consolidation of the castable can be done by the following methods:

1-Pouring it as slurry or semi-slurry and to consolidate it with light vibration, called casting

2-Vibrating strongly a stiff thixotropic material when it starts flowing, called vibro-casting

3-Transporting and spraying of water-mixed slurry, called shotcreting

4-Spraying a dry material which is mixed with water at the spray nozzle tip, called gunning

Castables can have further classification depending upon its composition, method of application, and flow behavior. The purpose of this classification is to distinguish the difference between the products which are made out of same basic ingredients but differ substantially in their rheological (flow behavior) character, drying behav-ior, etc. The relationship of different categories of castables is shown in Fig. 2.2.

Conventional Castable

Conventional castable with higher content of calcium aluminate cement requires high percentage of water to attain required flow character, and the mixing with water can be done even on the floor, although not recommended, with the help of a shovel. The setting can be controlled and modified with the addition of special addi-tives. The heat-up curve to remove water is not very critical unless it is cured at a temperature below 10 °C.

In many occasions when a shutdown is taken at any plant, the refractory lining is first inspected to check for any damage in the refractory lining. Then small patching work is usually done to repair small areas of worn out brick or castable lining by troweling or plastering the castable, mixed with water. Conventional castables are suitable for this purpose.

Low and Ultra-low Cement Castable

The low and ultra-low cement castable, have almost the same ingredients as conven-tional castable, but with lower content of calcium aluminate cement. It can attain the desired flow character necessary for vibration casting, with almost half of the water

as required in conventional castable. But it is to be borne in mind that very intense mixing is required to develop the desired flow property in the low cement castable and it is to be mixed in a high-shear mixer. Mixing of low cement castable and water in ordinary mixer or with shovels do not yield the desired flow property required for casting. This enhancement in the flow property, made by engineering the particle size distribution and addition of certain special additives, actually revolutionize the whole castable technology. Lower content of the calcium aluminate cement, as sug-gested by the name, imparts better properties to this type of castables at elevated temperature over the conventional castables because of lower content of CaO in the mix. Low cement and ultra-low cement castables develop a thixotropic property on mixing with water in high-shear mixer. The mix starts flowing only when a shear force is applied on the mix and requires strong vibration either by means of needle vibrator or form vibrator. This kind of castables forms a much denser body on cast-ing and has lower permeability compared to conventional castable, and therefore it’s drying and preheating are very critical. The casting may crack on preheating unless proper care is taken.

A further development of low cement castable technology has made it self-flow type, which, when mixed with low percentage of water, can flow very easily and requires very mild or almost no shear force or vibration for its placement. It can be used at a place where the vibration casting cannot be done and at the same time properties of low cement castables are required. This is actually an extension of the technology, developed for low and ultralow cement castable.

No-Cement Castable

No-cement castables either contains no calcium aluminate cement or a very low amount. Calcium oxide, in the cement, acts as a flux towards aluminosilicate raw materials and brings down the refractoriness of castable. Low and ultra-low cement castables were developed to reduce the content of the cement and to overcome that drawback.

No-cement castables were developed to completely eliminate the calcium alumi-nate cement and to replace it by alumina or silica sol as the binder. It is normally two-pack system, and the powder component and the binder component (liquid) are supplied in two separate packets. The user is to mix both powder and liquid at the site and apply. It can be poured or cast similarly as calcium cement-based castable. It has an advantage over calcium aluminate cement-based castable that it can be preheated at a much faster rate than cement-based castable. A comparison of usual cement-bonded and no-cement castables is given below in Table 2.2. Now further development has made possible, the availability of a one-pack no-cement castable, in which the binder is used in the solid form and is available commercially.


Further progresses were made in castable technology to make it (pumpable cast-able) amenable for pumping as a water-mixed slurry, for easy transportation. By this, the mixing activity of castable with water can be kept far off from the actual site, for ease of installation job. Pumpable can be used for casting a horizontal section because the slurry can rest on its own and set.


In the shotcreting process, the castable in similar way is pumped and transported through rubber hose and is sprayed through a nozzle with the help of compressed air on the target wall. It is very useful for fast installation of a large volume of refractory at a difficultly accessible site, like preheater cyclone, situated at a consid-erable height. Moreover, the fixing and dismantling of steel former, used for cast-ing, involve manpower and take time for fixing. The shotcreting method does not require any former and eliminates both time and cost. To enhance the setting of the slurry on the target wall and to convert it to a gel, so that it does not flow by its own weight, a gelling additive is added to the shotcrete material at spraying nozzle. This helps to install a thicker lining, and because a thick layer can be built up on a verti-cal wall, the requirement of former, as used in traditional casting process, can be eliminated.


Gunning material can be made both in aluminosilicate base and in magnesia base (basic) composition. Basic gunning materials are used generally for the hot repair-ing of the refractory lining in steel melting furnaces. There is no scope of use of basic gunning material, and they are never used in the cement industries. For cement industry the gunning materials are made using formulation similar to castable but modified by adding special additives to suit the gunning process. In the process of gunning, dry material is conveyed pneumatically through the rubber hose of the gunning machine, to the nozzle. At the nozzle tip, water is added through a separate rubber pipe at high pressure and sprayed on the target area. It must set fast, must get hard quickly, and must not crack on fast heating. Gunning materials of varied speci-fications, based on aluminosilicate raw material and calcium aluminate cement mix, are used in cement industries. This method can be very effectively used for cold or hot repairing of an area inside the furnace without taking major shut down. This method is not very effective for installation of large volume of material in a short span of time. In gunning process there is always some loss of material due to rebound of the material when the gunning material hits the wall. The rebound per-centage may vary depending upon the material or the operator, but it is always there.

Special Features of Unshaped Products

The special features of the unshaped Refractories are as follows:

a)Although its manufacturing is easy, its installation requires very skilled and experienced hands with much attention. Installation of monolithic refractory requires less labor and can be automated to a large extent.

b)The unshaped Refractories are more environment-friendly materials because they do not require any high-temperature operation for its production.

C)It is a semifinished material. It is partly finished at the premises of supplier, and the final finishing is made at customer’s premises. Therefore, its quality of installation, including consolidation, drying, and preheating, is very important, unlike shaped product.

d)Normally, from the beginning of the installation to ‘ready for use’, duration is much longer in case of castable Refractories, because of longer time needed for (1) preparation (welding of steel anchors to hold the Refractory) of the surface, over which the unshaped refractory will be applied, (2) preparation of the mix and consolidation of the Refractory mass, (3) setting of the Refractory to become hard, and (4) preheating the cast lining. The unshaped Refractories comprise of large group of products with varied application method and pur- pose to satisfy the particular technical requirement of a refractory lining.

Insulating Property

Refractories can be broadly classified by its purpose of use. As was discussed ear-lier, two main purposes of use of Refractories are:

  1. To contain the temperature and to withstand the abusive environment inside the furnace or the process equipment
  2. To conserve the thermal energy inside the furnace and not to allow it to flow out of the system

Most refractory linings are composite in nature and made of the combination of dense Refractories, in front, to contain the high temperature and to withstand the harsh operating condition and insulating refractories, at back, to contain the heat and protect the energy loss from the system.

Insulating refractories, like dense refractories, are available in shaped and unshaped form. The main features of insulating Refractories are the high porosity and low bulk density which leads to low thermal conductivity and low mechanical strength of the insulating refractory. The main purpose of use of insulating refrac- tory is to preserve thermal energy. But the insulating refractories cannot withstand abusive environment, and they are used mostly as the backup layer along with the steel shell unless the furnace environment is very friendly to it.

Insulating Refractories are made out of aluminosilicate range of materials because of their inherent lower thermal conductivity compared to many other mate- rials. The insulation product can be designed for continuous use up to a very high temperature (1600 °C). The use of insulating products is gradually increasing because of the demand of conservation of energy.

Special Chemistry

The refractories, which have very special chemistry, like carbides, nitrides, and borides (Table 1.1) and are used for very special applications and not in bulk volume.

Manufacturing and Properties of Refractories


Study of the properties of Refractory is very important for the purpose of selection of Refractories for an application. The properties of the Refractory both at the ambi-ent as well as high temperature are important to design the lining details of a furnace or kiln. Here we shall discuss the general manufacturing flow sheet and properties of different shaped and unshaped Refractories.

Shaped Refractories

The manufacturing process of all shaped refractories is more or less similar. Refractory raw materials are normally available as hard lumpy materials, which are crushed first in primary crusher like jaw crusher (if the size of the lumps are above 50 mm) and then in the secondary crusher like hammer mill. The crushed materials are screened to several size fractions like (3–5) mm, (1–3) mm, and (0–1) mm and fines. The dif-ferent size fractions are mixed together along with a binder (like molasses water mixture) and pressed in a mold of desired size in high capacity press to form the brick. The brick is then dried to remove the moisture inside and fired at high tempera-ture (1150–1750 °C) to impart the strength and desired properties to the refractory.

Aluminosilicate Refractories

The main chemical constituents of this category of Refractories are alumina (Al2O3) and silica (SiO2). If the Al2O3% is less than 50%, it is normally categorized as fire-clay Refractory, and Refractory with more than 50% Al2O3 is called high alumina refractory, although this categorization has got no authentic base.

Fireclay Refractories

The main raw material used for fireclay Refractories is calcined fireclay or cham-otte, along with plastic fireclay. The flowchart given (Fig. 3.1) shows the manufac-turing process of fireclay Refractories.

Fireclay refractories have alumina and silica as main constituents. The other oxides that remain present as impurities are Fe2O3, CaO, MgO, K2O, and Na2O. All these oxides are acting as flux and bring down the softening temperature of the Refractory. Although the presence of these oxides is undesirable, they cannot be avoided because of their inherent presence in the naturally occurring raw materials. Microstructure analysis of the fireclay refractory reveals that it can contain up to 50% of glassy phase with embedded cristobalite and mullite crystals. The percent-age of glassy phase increases with more fluxes and higher firing temperature. In a fireclay brick with 40% Al2O3, using raw materials with low amount of alkali and fluxes, the share of glassy phase can be reduced to 20% with mullite 55% and cris-tobalite 20–25%. The presence of glassy phase and mullite crystals and their sizes influence the thermo-mechanical behavior of the Refractory. Fireclay Refractories have good volume stability at high temperature, fairly good abrasion resistance, good thermal shock resistance, and very good resistance towards alkali attack at high temperature. Mullite is always a highly desirable phase in fireclay and high alumina refractories.

Addition of silicon carbide (SiC) in the fireclay Refractory improves all those properties, to a great extent. In reducing atmosphere, e.g., in CO gas atmosphere, the presence of Fe2O3 catalyzes the decomposition of CO to deposit C in the brick structure, and it becomes fragile because of the expansion associated with the pro-cess. Fireclay Refractories find a wide use in the cement industry in various equipments, e.g., tertiary air duct, cooler, kiln calcining zone, preheater, etc. Typical properties of some fireclay-shaped Refractories used in cement industry are shown in Table 3.1.

High Alumina Refractories

These are categorized as products containing 50–95% Al2O3. Different kinds of raw materials are used in high alumina refractories to achieve the target properties. The major raw materials used are sillimanite, andalusite, raw and calcined kyanite, cal-cined diaspore, calcined bauxite, mullite, brown and white fused alumina, tabular alumina, etc. The Al2O3 content of high alumina Refractories, normally used in cement industry, hardly exceeds 85%. The manufacturing flow sheet is same as that of fireclay Refractory. But there are some qualities which are chemically bonded and do not require any high-temperature firing. The high alumina Refractories can have varied properties depending upon their composition and raw material used. The Refractory composition can be designed to attain particular properties required to perform well in a particular application. Refractories for certain application may demand high thermal shock resistance, and the other application may look for very high abrasion resistance or resistance to alkali attack at high temperature. In all those cases, the Refractory composition and properties will be different. Refractory formulation is done looking to the operational conditions, where the Refractory will be put in.

Mullite is one of the most important phases present in the high alumina Refractories, and it is formed by the reaction of alumina and silica at high tempera-ture (above 1100 °C). Mullite is chemically 3Al2O3·2SiO2. It contains 72% Al2O3. The presence of Mullite improves the thermo-mechanical property and the thermal shock resistance of high alumina refractories, and the higher is the Mullite content, the better are those properties. Mullite is also available as a raw material which is made synthetically. The presence of higher percentage of corundum phase increases the abrasion resistance of the high alumina Refractories. The corundum phase is contributed by calcined bauxite, fused alumina, or tabular alumina. High alumina refractories have poor alkali resistance at high temperature. It is found that higher is the alumina content, lesser is the alkali resistance. At high temperature it reacts with alkali oxides to form some felspathic compounds like leucite, kaliophillite, etc. asso-ciated with high volume expansion which completely destroys the Refractory [1–4].

The properties of high alumina Refractories depend upon the raw material base. The andalusite, sillimanite, and kyanite are converted to mullite easily at high tem-perature, and the bricks made out of those raw materials show high thermal shock resistance, high creep resistance at high temperature, and also good alkali resis-tance. Raw kyanite has got the property to expand at high temperature and used to impart a positive PLC in the refractory. The Refractory made out of bauxite shows pyroplasticity (plasticity at higher temperature). By virtue of this property, bauxite- based high alumina Refractories can deform under an applied stress at high tem-perature and can mitigate the effect of the stress. That is how these Refractories show good performance in burning zone of rotary kiln with small to medium diam-eter. High alumina Refractories in general and specially the bauxite-based Refractories show a permanent linear expansion at high temperature, and this helps to keep the lining tight at high temperature during the service. High alumina refrac-tories also are found to take up a stable coating in the burning zone in the cement rotary kiln.

Phosphate-bonded high alumina Refractories show increased resistance to attack by liquid phase in cement clinker and alkali and also have higher abrasion resis-tance. These Refractories are normally fired at lower temperature during manufacturing.

The high alumina Refractories are used for lining various equipments in cement industry. Typical properties of some high alumina-shaped Refractories, used in cement industry, are shown in Table  3.2. In some quality of aluminosilicate Refractories, special ingredients, e.g., SiC (silicon carbide) and ZrSiO3 (zircon), are added to impart special properties like anti-buildup behavior, thermal shock resis-tance, alkali resistance, etc.

Basic Refractory

In earlier days the burning zone of cement rotary kilns was lined with high alumina Refractories. But as the technology has changed over time, from wet to dry process, and the kiln diameter got bigger and bigger, the thermal load on the kiln burning zone increases, and as a result the lining is changed from high alumina to basic.

The main raw material, for all the basic refractories, is dead burnt magnesia (MgO). It can be obtained by high-temperature calcinations of naturally occurring magnesite (MgCO3) above 1700 °C. The processed material is called dead burnt magnesite (DBM), because of its inertness towards hydration. MgO can also be synthesized from seawater [5]. Seawater contains magnesium salts like MgSO4 and MgCl2, which when treated with lime, Mg(OH)2 is precipitated. Mg(OH)2 is then filtered, briquetted, and calcined above 1700 °C to obtain DBM. In another method the magnesite rock is fused by electric arc at very high temperature at about 3000 °C and allowed to cool very slowly when MgO gets crystallized from the melt and forms fused magnesite with large crystals of periclase. It has been found that the properties of basic bricks depend on the properties of the magnesite raw material and source.

The important properties to characterize the magnesia raw material are:

1-Bulk density and grain porosity

2-The impurities and their distribution

3-CaO/SiO2 ratio

4-The boron content

5-The periclase crystal size

Table 3.3 shows the relation between the properties of magnesite raw material and their influence on the brick properties. The purity of magnesia raw material acts in a different way in the case of natural magnesia and synthetic magnesia [6–8], and a judicious blend of both will be the best. The higher the periclase (MgO) crystal size, the better is the corrosion resistance but lower is the coating formation. The magnesite with higher Fe2O3 is found to have better plasticity etc [9].

Magnesia-Chrome Refractories

Magnesia-chrome refractory is used mostly in the burning zone of cement rotary kiln. The raw materials used in this product are high purity dead burnt magnesite (DBM) or fused magnesite or their mixture and high-grade chrome ore. The flow sheet of its manufacturing process is shown in Fig. 3.2. The chrome ore contains MgO.Cr2O3 and FeO.Cr2O3 spinels.

Addition of chrome ore reduces the modulus of elasticity [10] and improves the thermal shock resistance of magnesia Refractory and its resistance to acidic slag. Magnesia-chrome Refractories are found to form a good coating of clinker on the brick-lining surface in cement rotary kiln, which protects the brick lining from high temperature and thermal shock and enhances the campaign life. But many countries have put a ban on the use of magnesia-chrome Refractories because of environmen-tal pollution, caused by the disposal of the used magnesia chromite brick lining. The Cr2O3 in the refractory reacts with alkalis at high temperature, during its use, to form water-soluble hexavalent chrome containing alkali chromate salts, which are carcinogenic in nature. The rainwater can leach out the Cr+6 and can pollute the groundwater reserve causing health hazards. The typical properties of this product are shown in Table 3.5.

Magnesia-Alumina Spinel

Magnesia-alumina spinel Refractory is used in both transition zones and burning zone of cement rotary kiln [11]. The main raw materials for this product are high purity dead burnt magnesite and spinel. Spinel is a mineral phase having a chemical formula MgO·Al2O3. Spinel is not available in nature and is made synthetically by high-tem-perature reaction between magnesia and alumina. The spinel has got a general formula AB2O4, where A = divalent atom and B = trivalent atom. Other mineral phases, having similar crystal structure as spinel, used in basic refractories for rotary kiln lining, are hercynite (FeAl2O4), galaxite (MnAl2O4), and pleonaste (Mg-Fe-Al spinel) [12].

Addition of spinel reduces the modulus of elasticity of magnesia Refractories and makes it more flexible to withstand thermo-mechanical stresses. The flow sheet for manufacturing magnesia-spinel Refractory product is given in Fig. 3.3. The spinel can be added as preformed spinel, or it can be formed in situ or as a combination.

TiO2 is added as catalyst for spinel formation at lower temperature [13]. TiO2 also improves the coating forming ability of Spinel containing refractory. Addition of SnO2 is found to improve the thermodynamic stability of Al2O3- MgO Spinel [13].

This product has got very high thermal shock resistance and performs very well in transition zone lining where the coating formation is less and the thermal shock is very high. It has also got a very high resistance to alkali attack and corrosion towards the cement liquid phase at high temperature. The typical properties of this product are given in Table 3.5.

Magnesia Hercynite

Magnesia hercynite Refractory is made from DBM and hercynite. Hercynite is a synthetic mineral with spinel structure having a formula of FeO·Al2O3. Addition of hercynite to magnesia refractories reduces its modulus of elasticity and enhances flexibility. It also reduces brittleness of magnesia Refractories towards thermo- mechanical stresses. The magnesia hercynite Refractory has good ability to take up the clinker coating, and all the said properties have made it a suitable candidate for burning zone lining of cement rotary kiln. It has also got a very high resistance to alkali attack and corrosion towards the cement liquid phase. It has the added advan-tage of being free from Cr2O3 and therefore more environment friendly by nature. The flow sheet of manufacturing of this Refractory is similar to magnesia spinel Refractory. The typical property of such Refractory is given in Table 3.5.

The spinels have lower thermal expansion than the periclase (MgO). As a part of the manufacturing process, these brick are fired at high temperature (1580–1650 °C). Due to mismatch of thermal expansion behavior, microcracks are developed in the refractory microstructure, surrounding the spinel grains, which makes the Refractory more flexible and improves its capability to withstand more thermo-mechanical stresses.

Figure 3.4 shows the typical thermo-mechanical behavior of a magnesia brick without spinel and a magnesia brick containing a spinel [14, 15]. A pure magnesia brick shows an almost constant Young’s moduli at a high level, but with the inclu-sion of spinel, the Young’s modulus starts at a low level, increases slowly during heating, and shows rapid increase in the first stage of cooling to 1000 °C. The flexi-bilization effect occurs during cooling at temperature below 1000  °C, and the Young’s modulus is lowered significantly. The Young’s modulus level provides an indication of the ability of an additive to flexibilize or elastify a magnesia brick. Spinels are also called as elastifiers, when added to basic refractories [16–21].

The effect of addition of different kinds of spinels on the basic Refractory [14] is shown in Table 3.4, and it is clearly evident that the fracture toughness has been increased significantly by addition of the spinels. It has also been found that forster-ite (MgO·SiO2) phase, developed in the basic bricks, can also act as an elastifier [22]. But the chemistry and the microstructure of the brick are to be engineered properly to get the best effect.

Magnesia Zirconia

Magnesia-zirconia Refractory is made out of high purity dead-burnt magnesia or fused magnesia and monoclinic or stabilized zirconia [23–25]. This Refractory body contains periclase and stabilized zirconium oxide (ZrO2) or calcium zirconate (CaZrO3). Uniform distribution of ZrO2 in the brick structure improves thermal shock resistance due to microcrack formation. During the partial or total stabiliza-tion of ZrO2, diffusion takes place between MgO and ZrO2, and this helps to form direct bond and increases the hot strength which helps to withstand the mechanical stress at high temperature. Magnesia-zirconia bricks have high corrosion resistance against alkali and against basic slag at high temperature. The addition of ZrO2 in Magnesia Spinel bricks has been found to improve both coating formation and cor-rosion resistance [26]. The simultaneous addition of MgAl2O4 and CaZrO3 to small volume percentage (4%) is reported to improve its corrosion resistance and its coat-ing property [27]. The flow sheet of manufacturing is the same as magnesia spinel brick. Typical property of magnesia-zirconia brick is shown in Table 3.5.


Dolomite Refractory is made out of naturally occurring dolomite which is a solid solution of MgCO3 and CaCO3. The pure grade of dolomite is calcined at very high temperature to reduce its affinity to react with atmospheric moisture and to make itstable. The dead burnt dolomite is the basic raw material for manufacturing the dolomite Refractories. There are a number of unique properties in dolomite brick that make it well suited for use as a Refractory lining for burning zone of a cement rotary kiln [28, 29]. A high degree of refractoriness allows the dolomite brick to withstand the temperature and involved stress in the burning zone. The corrosion resistance to alkalis and a reducing atmosphere, as well as excellent ability for coat-ing formation, make it a good candidate for the lining of burning zone of cement rotary kiln. A good coating formation is the most important property of a basic brick. In practice, a coating will form if the conditions in the kiln are appropriate. The biggest difference between different types of bricks used in this context is how well they will hold onto the coating, once it is formed.

In practice, the clinker coating on the refractory lining is not always present dur-ing campaign of the lining. Some or all coating loss may be caused by thermal shock. When the coating on the lining is destabilized, the brick underneath is sub-ject to a sudden temperature change, which will expose the brick to severe thermal shock. For a long period of time, standard dolomite was limited to the area where stable coating existed over the refractory lining, and shutdowns were minimal.

However, the thermal shock resistance of the dolomite brick has been improved significantly by the development of zirconia-enriched dolomite refractories [25, 29]. Zirconia was chosen because it is relatively nonreactive with dolomite, and addition of special grade of Zirconia imparts high thermal shock resistance. Finally, the environmental safety of the used dolomite bricks is outstanding. One of the greatest disadvantages of dolomite Refractory is its susceptibility to react with atmospheric moisture. The process is called hydration, in which the CaO of dolo-mite reacts with water vapor to form Ca(OH)2 and causes destruction of the brick lining. In coating-free area, dolomite reacts with kiln gases to form CaCO3, CaSO4, or CaS [30, 31].

The phenomenon is more prominent in tropical countries with high level of humidity. Table 3.5 furnishes the typical properties and specification of dolomite Refractories.

Unshaped Refractories

Various types of unshaped Refractories are available for different applications in different industries. These includes castables, ramming masses, plastics, and mor-tars. Out of all these, castables and mortars are the main types of unshaped Refractories used in the different applications across the cement industry. Castables can be of different types, meant for different applications. Castables are basically a dry mixture of Refractory aggregates, calcium aluminate cement [32, 33], and some small but very important additives. The Refractory aggregates can be chamotte, andalusite, sillimanite, calcined diaspore, calcined bauxite, brown fused alumina, white fused alumina, tabular alumina, silicon carbide, etc. of different sizes, varying normally from 10 mm to superfine material. The different ingredients in different sizes and in different proportion as per the design of the formulation are mixed together along with high alumina cement as binder to make the castable ready for packing and dispatch.


The castables can be applied in different ways like pouring, vibration casting, gun-ning, shotcreting, ramming, troweling, etc [34]. Depending upon the place of appli-cation and method of application of the castable, the aggregate quality and the percentages and quality of calcium aluminate cement and additives are decided to engineer its rheology or flow property, to make it suitable for a particular applica-tion method mentioned before [35].

The most important properties of castable, which are ignored sometimes by the manufacturer, are its properties related to installation, e.g., flow property, duration it remains workable, setting time, scope of installation method and preheating arrange-ments available. If the installation is sound, half of the battle is won. Many a times desired campaign life is not obtained, because of improper installation. If the prop-erties mentioned before, necessary for a good quality of installation, do not match with the actual properties of the castable, the installation quality will not be sound, and it will not perform well. The supplier must design the castable recipe taking into consideration the installation method to be employed and the operational condition that prevails (e.g., ambient temperature, etc.). Similarly, during the installation of the castable, the installer should have prior knowledge of the earlier mentioned properties of the castable required for sound installation. The same castable, when installed in a country in Europe in winter and in a country in the Middle East in summer, will not behave similarly because of different ambient conditions. Both the manufacturer of the castable and the user must bear this in mind during manufactur-ing and installation of castable refractories, and proper attention should be paid.

Castables can be installed by different methods, e.g., by pouring, by vibration casting, by gunning, and by shotcreting. Two major steps in the installation pro-cesses are mixing of castables with water and the method of consolidation of the castable. Mixing quality is very important, and the aim should be to get maximum flow using the prescribed water or binder content. That is possible only when the mixing is very thorough and intimate, done in high shear mix.

The general flow sheet for manufacturing of castable is given in Fig. 3.5. The characteristics of different types of castables are discussed below.

Conventional Castable

This is a mixture of Refractory aggregates with 15–25% high alumina cement. These castables are required to mix with prescribed quantity of water and either cast, poured, or rammed. In castable the aggregates used are normally aluminosili-cate type, but in some special cases, SiC or zircon is also used. Conventional casta-bles can be rammed, vibrocast, or poured. These are also good materials for patching by troweling.

The special feature of these castables is the loss of strength, with rise of tempera-ture, till it reaches a minimum value at a temperature range 800–1000 °C and then there is a rise of strength with increased temperature. The initial loss of strength with rise of temperature is because of the disintegration of the hydraulic bond formed, and later the increase of strength is due to sintering process. Typical speci-fication of this type of castables used in cement plant is given in Table 3.6.

Low Cement Castable (LCC)

This is a mixture of Refractory aggregates and 5–8% high alumina cement with other minor, but very important, additives which helps to get the desired flow prop-erty and enables installation by vibration casting with much lower water percent. Good flow property of the castable mix with low water content ensures better pack-ing lower porosity higher mechanical strength and high volume stability at elevated temperature. The low cement castable should always be mixed with water in mechanical mixer and is to be installed by vibration casting. Ordinary mixing with water does not develop the desired flow property in the mix, and vibration casting with low water content is not possible. Addition of higher percentage of water will be disastrous and will impair setting and strength of the castable.

Ultra-low Cement Castable (ULCC)

These products are very similar to the low cement castables except further lower content (2–4%) of high alumina cement. Its features are similar to that of the LCC, but the high-temperature tensile strength is better than that of LCC with similar Al2O3 content because of lower flux content (CaO) due to the lower content of high alumina cement.

The very important differences in the behavior of the low cement and ultra-low cement castables from that of conventional castables are:

Obtaining required rheological property with addition of much lower amount of water, compared to conventional castable.
Increased strength with increasing temperature (Fig. 3.6).
Having higher density, lower permeability, and high sensitivity to method of pre-heating because of the low water requirement for casting. Faster preheating may lead to explosive spalling and crack in the body, although the addition of special additives reduces its sensitivity towards heating rate.

No Cement Castable (NCC)

In these products silica or alumina sols are used as binders, and a gelling agent (high alumina cement up to 1%) is used to set the castable.

These products are mostly two-pack products. Refractory aggregates are supplied loose in bags, and the binder liquid (silica sol) is supplied in drum. The two compo-nents are mixed at site and are cast. But recently the solid powder is developed which forms sol when added to water, and one pack product of no cement castable can be produced using that binder. The special features of these products are:

  1. These products are to be applied by vibration casting.
  2. The strength increases with temperature.
  3. These can be preheated at a much faster rate compared to the cement-bonded castables (Fig. 7), and therefore total preheating time is much less which is a major advantage of chemically bonded castable [36].Recommendations for mixer type, mixing time, and application methods for dif- ferent types of castables are given in Table 3.7.


Gunning Mass

Gunning mass is supplied in powder form, and it is made out of aluminosilicate raw material and calcium aluminate cement mixed with other additives [37]. Gunning machine is used to spray the material on the place of installation. After charging the material in the hopper of the gunning machine, the material is transported through a long rubber hose, connected to a steel nozzle at the other end, by pneumatic pres-sure. The gunning material is mixed with water at the junction of rubber hose and steel nozzle, through a separately connected pipe, and ultimately sprayed on the application area.

It is used for both installation of new lining and for repairing a worn-out lining. Gunning installation is a very specialized job, and the quality of the installation depends largely on the skill of the operator (gunman).

Portion of the gunning material sprayed, rebounds from the target wall and falls down and is called rebound loss. The lesser is the rebound loss, the better is the quality of gunning. The rebound loss depends upon the material quality, the water percentage used for gunning (controlled by operator), distance of the Gunning noz-zle from the target surface, and the angle of nozzle with the wall to be gunned.

Shotcrete and Pumpable Material

Shotcreting is normally done in an installation where large quantity of Refractory needs to be installed. It is very convenient to use this process when the installation is to be done at a height, for example, in preheater cyclone. In shotcrete installation, the shotcrete machine is kept at ground level. The castable material is mixed with recommended amount of water in the shotcrete machine to make slurry, and the slurry is then pumped through the rubber hose to the installation point and is sprayed on the target surface by compressed air. Before the material comes out through the nozzle, a setting agent is mixed with the slurry to accelerate the setting. The rheological property of shotcrete is very important towards its successful use. The differences in the installation process of castables by gunning and shotcreting are given in Table 3.8.

As it was discussed in Sect. 3.3.1, basically, the castable with the same chemical specification can be modified to install by different methods, e.g., by pumping the slurry, by self-flow technique, by gunning or shotcreting, depending upon the demand of the user. The basic ingredients of the castable remain the same, but the rheological or flow property is modified by addition of small amounts of additives [38] and control of particle size to achieve the desired flow property as the selected installation method demands. The properties of 50% alumina castable installed by different methods described are given in Table 3.9.

Ramming Mass

This is a combination of Refractory aggregates and some additives as binders. It is supplied in powder form. The powder mass is mixed with prescribed quantity of water or any other specified liquid binder to get a semidry consistency and then installed by pneumatic ramming or by hand ramming. These products are recom-mended for small repair jobs or to fill up some cavity or gaps, in the new brick-lined construction in cement plant application. Typical property of a Ramming mass is shown in Table 3.10.


This is manufactured and supplied in ready to use form, in small blocks, wrapped in polythene packet with stiff mud consistency. It is to be unpacked and to be installed either by ramming or tamping in the place of application. This is normally available in aluminosilicate range. The advantages of this product are it is available in ready condi-tion saves time. Quality consistency of the installed refractory lining is ensured because the water or the binder need not to be added at site. It gets set and hardened when it gets temperature. The preheating of this product can be done at a much faster rate compared to castables. Typical property of a plastic material is shown in Table 3.10.


Mortars are used to join the bricks in the refractory lining. Mortars used to have almost similar chemical composition as the bricks to be joined. Mortars are gener-ally supplied as dry powder which is to be mixed with water or other specified liquid binders to form a paste which is trowelable and can be spread over the brick surface to act as glue to fix another brick. The mortars can be heat set, air set, or chemical set, depending upon the development of strength of the mortar on heating or in the presence of air or by its own. The heat-set mortars develop their strength after heat-ing the lining, whereas the chemically set or air-set mortars develop their strength at ambient temperature itself, by chemical reaction. The mortar should have the prop-erty of good workability when mixed with water and should not get dried up fast, because of absorption of water in the brick pores, when applied on a brick surface.

Insulation Refractory

Cement industry is an energy-intensive industry. Consumption of more energy is uneconomic, and it has got a negative impact on the environment for liberation of more greenhouse gases. Because of this reason, the use of insulation refractory in cement industry is becoming more and more important. The thermal insulation can be provided by both shaped and unshaped refractories. The insulation refractories have high porosity, low strength, and low abrasion resistance, and due to this reason, the insulation refractories are used always as backup lining. The different insulation materials have different properties and have different applications. The different types of insulation refractories are classified in different groups according to the ASTM standard as given in Table 3.11. The classification is based on the bulk den-sity and the reheat change of the brick.

The main features of the insulating refractories are high porosity and low bulk density. The pores are deliberately created in insulating refractories to entrap air, which provide very good thermal insulation and impart low thermal conductivity to the body.

All the thermal-insulating materials are basically oxides, because compounds other than oxides have innate higher thermal conductivity. The service temperature of insulating refractory and its volume stability at that temperature is also important to design a refractory lining.

Insulation Shapes

Insulation refractory shapes can be manufactured from porous raw material, e.g., molar clay or exfoliated vermiculite, or from fireclay by introducing some organic combustibles in the mix which leave behind the pores on burnout. The properties of the fireclay insulation bricks are given in Table 3.12.

Calcium Silicate

This is available as boards of 25, 50, and 100  mm thickness and with different length and breadth. Calcium silicates are very light and efficient thermal-insulating material and are used in large quantity in cement industries. They are produced synthetically by hydrothermal reaction between lime and silica. The calcium sili-cate blocks are available in two different grades suitable for application tempera-tures up to 800 and 1100 °C. The thermal stability is based on the amount of calcium

silicate hydrate shares (tobermorite 5CaO·6SiO2·5H2O and xonotlite 6CaO·6SiO2·H2O), due to their different shrinkage behavior and removal of bonded water. The calcium silicate blocks can be sawed and drilled as per the need. The properties of these materials are given in Table  3.13. Calcium silicate insulation blocks can be used as backup lining for both brick lining and for castable lining.

Ceramic Fiber Products

Ceramic blanket and ceramic papers are used widely as insulation material in dif-ferent equipments during the refractory lining. These products are made from ceramic fibers.

Ceramic fibers are manufactured by melting aluminosilicate material at very high temperature in electric arc furnace, and the pouring stream of the melt is blown at, by jet of high pressure steam, when the molten aluminosilicate material will form fluffy fibrous material. These fibers are woven or processed to form ceramic blan-ket, ceramic papers, and ropes. The products are available with different grades according to the service temperature, density, and shrinkage value. The character and properties of these products are shown in Table 3.14.

Insulation Castable

Insulating castables are made from porous refractory aggregates and mixed with calcium aluminate cement as binder. The content of high alumina cement used in insulating castables is normally much higher than dense castables to get the desired

mechanical strength. The insulating property of the castable depends upon the type of aggregate used and the composition of the castable. The properties of the insulat-ing castables are shown in Table  3.15. Insulating castables require much higher percentage of water to attain the consistency required for its placement.

Cement Manufacturing and Process Control

Cement Manufacturing: Basic Process and Operation

To design the Refractory for the lining of different equipments in a cement plant, it is necessary to understand the total operational process of a cement plant in depth. It is necessary to know the conditions prevailing in different equipments and the physical and chemical parameters the Refractories will be exposed to during opera-tion. The physical parameters are temperature, pressure, air velocity, abrasion, mechanical stress, content of liquid phase generated at the operating temperature, liquid viscosity, surface tension, etc., and the chemical factors include chemistry of solid, liquid, and gas in contact with the Refractory. Once these information are available, the right kind of Refractories, which can withstand best under those con-ditions, can be designed.

The common raw materials used for manufacturing of cement include limestone, shell, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore. These ingredients, when heated at high temperatures, form a rock-like substance that is ground into the fine powder that we commonly call as cement.

The early manufacturing process of cement was through wet process, in which slurry, made of ground raw material and water, used to feed the rotary kiln. But this process requires a high amount of energy, mainly to evaporate the water in the slurry and become uneconomic and largely abandoned. The energy requirement to pro-duce cement in dry route is approximately half of the energy required to produce it through the wet process.

The most common way to manufacture Portland cement is through a dry process. The first step is to quarry the principal raw materials, mainly limestone, clay, and other materials. After quarrying, the rock is crushed. This involves several stages. First, crushing reduces the rocks to a maximum size of about 6 inches. The rocks then go to secondary crushers or hammer mills for reduction to about 3 inches or smaller. The crushed rocks are combined with other ingredients, such as iron ore or fly ash, and are ground, mixed, and fed to the cement kiln.

In the cement kiln, all the ingredients are heated to about 1400–1500 °C in huge horizontal cylindrical steel tubular vessel called rotary kiln, lined inside with special Refractory material. Kilns are as much as 3–7 m in diameter and about 60–75 m in length. The kilns are mounted on two or three supports with the axis, inclined slightly from the horizontal. The finely ground raw material is fed into the higher end. The lower end is fitted with a concentric burner producing a strong flame, by controlled burning of powdered coal, oil, alternative fuels, or gas under forced draft. As the material moves through the kiln, the materials react among themselves at high temperature and form a new substance called clinker. Clinker comes out of the kiln as dark gray balls, about the size of 0–50 mm.

Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in various types of coolers. The heated air from the coolers is returned to the kilns, a process that saves fuel and increases burn-ing efficiency.

After the clinker is cooled, it is ground and mixed with small amounts of gypsum and limestone. The cement is now ready for use in construction projects. The flow sheet of cement manufacturing is given in Fig. 4.1. Kiln feed is subjected to succes-sive physical and chemical reactions as its temperature rises gradually as shown in Table 4.1.

Raw Material

The major raw materials used as a source of CaO for cement manufacturing are limestone, chalk, marl, etc. These raw materials by virtue of their natural occurrence contains other mineral impurities e.g, MgO, SiO2, Al2O3, Fe2O3, alkali compounds, and sulfides. These impurities play an important role to influence the cement manu-facturing process as well as Refractory performance. Similarly, as a source of SiO2, clay minerals and shale are used, which also contain similar kind of impurities and affect the performance of Refractory.


Different fuels like coal, oil, petcoke, gas, etc. are used as the source of heat energy to carry out the reactions inside the cement rotary kiln. The fuel cost is about 50% of the total energy cost and 10% of the total cost for cement production. Therefore a continuous effort is going around the globe in search of cheaper source of fuels. As a result, a lot of other fuels are being found and used more and more to replace the conventional fuels, and these are called alternate fuels. The cement kilns are being used in developed countries as an incinerator, and all kinds of industrial and domestic wastes are used as source of energy. The common alternate fuels used are petroleum coke, shredded tires, plastics, waste oil and solvents, animal meat, bones and fats, municipal wastes, agricultural wastes, etc. Some of these materials contain high percent of alkali, sulfur and chlorine, for which some special cares are to be taken in the manufacturing processes to maintain smooth operation and these impu-rities again affect the Refractory performances to a great extent. The acidic gases, formed because of burning of alternate fuels, permeate through the Refractory and corrode the kiln shell and anchors which hold the monolithic lining.

Different Plant Design

Energy is the major cost in clinker manufacturing process. The energy means the combination of thermal energy from the fuel and the electrical energy required to rotate the kiln and to run the cooler and the number of ID fans.

In the cement plant, the preheater system functions as heat exchanger. The hot gas generated in the kiln to burn the clinker leaves the kiln and moves up the pre-heater to heat up the raw meal falling vertically through the preheater system. The thermal energy cost of the cement manufacturing depends upon the heat exchange efficiency of the system. The electrical energy requirement depends on the kiln length, the electrical power of the ID fans, and the electrical power used to run the cooler.

The different kiln system designs are developed over the years to improve the energy efficiency and to take care of ever-increasing capacity of the cement plants. The feature and advantages of different designs are discussed here [1–3].

Suspension Preheater (SP) Kiln

This system is suitable for plant with capacity up to 4500 TPD of clinker. It has low specific power consumption and simple operation and is well suited for manual control. The extent of calcination at the kiln inlet is 30–40%. This process can accept higher input of chlorides than pre-calcining system with tertiary air duct (without bypass). The ratio of firing in riser duct is 0–15%.

Line Calciner Using Excess Air

It is suitable for plant with capacity up to 5500 TPD clinker. Ratio of firing in cal-ciner is 10–25%, and the calcination at the kiln inlet is 50–70%. The advantage of this system is low coating tendency in kiln inlet and riser duct. The coating in burn-ing zone is stable, and it is less sensitive to chloride and sulfur. The kiln dimension is smaller than SP system.

In-Line Calciner (ILC)

In this system the capacity can be made up to 6000 TPD and with multiple strings up to 10,000 TPD clinker. High material and gas retention time is obtained in cal-ciner due to its large volume and moderate swirl. Ratio of firing in calciner is 55–65%, and calcination at kiln inlet is 90–95%. Bypass of kiln gas is 0–100%. This system is well suited for low-grade fuels. Refractory life is long because of low thermal loading and stable coating.

Separate Line Calciner Downdraft (SLC-D)

Normal capacity is up to 6000 TPD and up to 10,000 TPD with multiple strings. Firing in calciner is 55–65% and calcination at the kiln inlet is 90–95%. The pro-cess is very well suited for all types of fuel, especially low volatile fuels. The combustion in the calciner takes place in hot atmospheric air, and the combustion temperature can be controlled independently of the temperature of the calcined material which is fed to the kiln. Low NOX operation is possible in this design.

The tower dimension can be made smaller as it can be installed separately from the cyclone tower.

Separate Line Calciner (SLC)

With this system the normal capacity can be up to 7000 TPD clinker, and with two strings it can go up to 12,000 TPD. The firing in calciner is 55–60%, and the calcination at the kiln outlet is 90–95%. The other features are very similar to SLC-D system.

Separate Line Calciner with In-Line Calciner (SLC-I)

With this system normal capacity can be up to 11,000 TPD clinker. Firing in kiln string ILC is 10–15% and in calciner string SLC is 40–50%. Calcination at the kiln inlet is 90–95%. Long refractory life is obtained due to low thermal kiln load and stable coating. Other features are same as SLC system.

The ILC-I and SLC kiln systems allow bypassing up to 100% of the kiln gas, while the other kiln systems can be equipped with a bypass carrying a maximum 25–60% of the kiln gas, which in most cases will suffice to ensure smooth opera-tion, even with low-grade raw materials.

Figure 4.2 shows the typical temperature and pressure profile of the cyclone preheater system.

In the SLC configuration, the hot tertiary air from the cooler enters the calciner through the central inlet in the bottom cone and leaves through either a side outlet or through an outlet cone connected to a central outlet pipe. In the SLC-I configura-tion, a SLC calciner is built parallel to the kiln riser duct, and a second ILC calciner is built into the kiln riser duct. A unique feature of the high-temperature calciner system is the fact that the temperature inside the calciner is independently con-trolled by the position of the gate that divides the raw meal flow to the calciner and to the kiln riser duct, respectively. So, by feeding a relatively larger amount of raw meal to the kiln riser duct and keeping the fuel input to the calciner constant, the mean temperature in the calciner vessel can be brought up to 950–1050 °C. The temperature of the exit gas and the degree of calcination of the raw meal leaving the calciner will increase accordingly. However, when mixing with the kiln exhaust gas that contains uncalcined raw meal, the temperature of the gas/particle suspension falls to approximately 900 °C. So a normal temperature level is maintained in the calciner cyclone. Similarly, a normal degree of calcination of 90–95% is maintained for the raw meal supplied to the kiln. The higher temperature in the calciner ensures effective combustion of even low reactive fuels and helps to greatly minimize fuel NOx formation. The calciners of the SLC and SLC-I systems are characterized by a vortex formation in the bottom cone, ensuring effective mixing of raw meal, fuel, and tertiary air for a high particle-to-gas retention time ratio.

Rotary Kiln

The rotary kiln is the heart of the cement plant. The raw meal, which is a mixture of finely ground limestone and clay or shale, is fed to the system at the top of the pre-heater tower. The material flows down to kiln. In the kiln the solid-state reaction occurs between the CaO, SiO2, Al2O3, and Fe2O3 to produce the liquid phase required for nodulization of clinker and formation of different mineral phases like C3S, C2S, etc. The kiln operation is monitored by several physical parameters as given below:

(a) Production rate, tons per hour clinker

(b) Operating hours
(c) Involuntary downtime hours

(d) Total fuel rate, tons per hour
(e) Proportion of fuel in pre-calciner/riser

(f) Specific heat consumption, kcal/kg
(g) Secondary air temperature
(h) Kiln feed end temperature

(i) Preheater gas exhaust temperature

(j) ID fan draft in mm of water column

(k) Kiln feed end oxygen

(l) Downcomer oxygen

(m) Kiln feed end material, loss on Ignition (LOI), and SO3

(n) Kiln drive power
(o) Specific kiln volume loading %
(p) Specific heat loading of burning zone, kcal/h/m2
(q) Temperature, pressure, and oxygen profile of preheater

Kiln Control Parameters

The measured values of those parameters that are the most important for kiln opera-tion are herein called the control parameters. To keep the control parameters close to a certain target, control variables (or actuators) are manipulated. These parame-ters have immense effect on the refractory performance. During any prematured refractory life, these parameters help to analyze what kind of stresses the refracto-ries were loaded to. For example, if the burning zone temperature exceeds the norm, the Refractory in burning zone is definitely thermally overstressed. The measure-ments of parameters like increase in NOx or the change in torque of the kiln give an indirect indication of increased burning zone temperature (BZT).

The most important control parameters that are looked at during operation of any type of kiln system are:


  • Burning  zone  temperature  (BZT)  as  an  indicator  for  the  produced  clinker quality.
  • Back-end temperature (BET), as an indicator for the raw meal preparation, which is mainly the degree of drying, heat up, and calcinations
  • Oxygen concentration of the kiln exit gas (as an indicator for the efficiency of combustion)
  • In case of pre-calciner systems: temperature and oxygen concentration at pre- calciner outlet
  • In case of grate cooler: bed height of the clinker layer on the grate

The location where the different control parameters are measured and some typical target values are given below. The control variables, which are adjusted, to keep the parameter close to target, are shown for preheater and pre-calciner kiln design (Figs. 4.3 and 4.4). The most important control parameters that are looked at during kiln operation are:

Burning Zone Temperature (BZT)

The BZT determines how well the clinker is burned and how complete the transfor-mation from C2S to C3S is. When the clinker is leaving the burning zone, it should not have more than ~1.5% and not less than ~0.5% of uncombined CaO or free lime (which roughly corresponds to a liter weight of about 1250–1350 g/l). The periodi-cally measured free CaO is the main indicator to state, if the actual BZT target is adequate or it needs to be made higher or lower. Due to changes of the raw meal composition, an alteration of the BZT target can be required often, since burnability of the raw material changes with fluctuations of the LSF, SR, magnesia, and alkali content among other factors. A change in the flame characteristics, which can result from the use of multiple fuels or varying combustion conditions, has a similar impact and therefore may also require an alteration of the BZT target.

Variations of the BZT target are made only over a long-term period, and the actual BZT must be measured to determine, if a deviation between the temporary target and the actual BZT exists. Since kiln operation is a dynamic process, devia-tions are quite frequent and of a short-term character. Several indicators are used to determine the BZT, and they are listed next. Most of them do not explicitly express an absolute temperature, but they give sufficient information to establish, if the BZT is increased or decreased compared to an earlier defined target. This means they give the short-term trend of the control parameter BZT.


Any hot body emits a radiation which is related to its temperature. The color and the intensity of the radiation depend on its temperature (Table 4.2). An optical tempera-ture can be used to measure the temperature by matching the color of a glowing filament against the radiation emitted by a hot body.


The NOx produced during combustion varies according to the temperature of the flame and of the gases of the burning zone. The hotter is the flame more NOx is produced during combustion and vice versa.

When BZT is deduced from the NOx content of the exhaust gases, the oxygen and CO concentration in the exhaust gases have to be considered also, since CO reduces and oxygen promotes the formation of NOx. A further factor influencing NOx formation is the type of fuel burned. If the fuel mix changes, the quantity of NOx produced may change too (especially when burning alternative fuels).

Kiln Torque

A higher burning zone temperature produces more liquid phase in the clinker, and if BZT rises, the burning zone gets longer and shifts upwards. The diameter of the clinker balls increases, and the granulation of the raw meal in the kiln starts earlier. Therefore more and bigger clinker balls have to be moved inside the kiln when the BZT is increasing. At the same time, the material bed in the kiln is lifted up more, which moves the center of gravity away from the kiln axis (Fig. 4.5). The different granulation of the clinker and the changing angle of repose increase the required torque to turn the kiln, if BZT rises.

A changing coating situation in the burning zone strongly influences the kiln torque (as more as lower the L/D ratio of the kiln). As a result the kiln torque can change significantly on a long-term basis without any change of the BZT. Therefore the kiln torque signal in most cases only serves as a short-term indication of the BZT. Short term is defined as up to 12 h. Torque is a very useful indicator of kiln conditions relative to its trend.

Secondary Air Temperature (SAT)

Kilns with grate cooler, which are equipped with a reliable SAT or TAT measure-ment device, often use SAT or TAT as an indicator for the BZT.

The higher the BZT, the hotter is the released clinker and the higher is SAT or TAT (and vice versa). Nevertheless SAT and TAT are influenced by a number of other effects (clinker granulometry, cooler operation, etc.); therefore not in all kilns can a clear correlation between BZT and SAT or TAT be established.

Back-End Temperature (BET)

The term BET refers to the temperature of the exhaust gas at the kiln inlet in case of kilns with a preheater and to the intermediate gas temperature (before the chain sec-tion), if the kiln is equipped with chains. The BET depends on the type of system and the L/D ratio of the kiln. It ranges from ~800 °C in wet kilns (before the chain section) up to ~1200 °C in air separator pre-calciner kilns.

If a reliable temperature measurement at the kiln inlet (kilns with preheaters) or before the chain section (kilns with chains) is not available, frequently the preheater exit temperature (kilns with preheaters) or the kiln inlet temperature (kilns with chains) is used.

The BET is an indicator of how well the hot meal is prepared, when entering the kiln.

Before the meal is finally clinkerized in the burning zone, it has to undergo dry out, calcination (release of CO2), structural modifications and formation of interme-diate products like C12A7.

The higher the energy demand for intermediate reactions in the kiln, (which depends mainly from the type of process), the more important is a proper control of the BET. Hence, the BET in a wet kiln is more critical (90–100% calcination miss-ing) than in an air separate pre-calciner kiln.

The BET reacts slower to any control variable adjustment as the kiln is longer (higher L/D ratio) and as more energy is consumed in the kiln for the intermediate reactions before clinkerization. Ideally the material should always have the same degree of preparation, but due to the kiln operation itself and normal fluctuations of the raw material properties, fuels, ambient conditions, and others, this is not the case.

Variations in the raw material are first observed through the BET, thereby indi-cating early the future impact on the BZT.

If all other parameters remain constant and the BET drops, then it is an indication that all reaction zones shift downwards and vice versa.

If the zones within the kiln shift either down or up, the BZT will decrease or increase consequently afterwards also, if no counter measures are taken earlier.

Oxygen Control

For a good combustion of the fuels, a certain amount of excess air is required. The quantity of excess air is determined by measuring the oxygen content of the exhaust gases. Proper control of the oxygen level is crucial for good combustion and a high thermal efficiency.

Typical oxygen ranges (at the kiln inlet) for the most common kiln systems are given in Table 4.3. The required oxygen level depends on the type of kiln system but also strongly on the type of fuel being used (e.g., gas needs less excess air than coal or solid AFR).

In most kiln systems, the required amount of excess air is slightly above the point where the flame temperature would be highest due to sulfur, chlorine, or alkali situation.

Therefore a slight reduction of the amount of excess air increases the flame tem-perature a little. Extremely low excess air (or oxygen) reduces the flame tempera-ture, which lengthens the flame (post combustion) and lowers the fuel efficiency. Additionally sulfur and alkali volatilization is enhanced, which may lead to buildup and incrustation problems within the kiln system.

Too much excess air (or oxygen) cools the flame too and leads also to a poor thermal and fuel efficiency, due to the deteriorating heat transfer. Since the total gas flow is increased, the kiln internal and external dust cycles also increase.

Hence lower oxygen levels are chosen, if sulfur volatilization is not problematic.

A general rule applies that at the minimum permissible oxygen level, no notable CO is formed (i.e., <0.05% at kiln inlet).

Control Variables

In order to run the kiln in a smooth and efficient manner, the control parameters must be kept in a narrow range close to target. To keep the control parameters close to target, it is generally necessary to make adjustments to the main control variables which are:

  • Fuel rate to the kiln
  • Material feed rate to the kiln Kiln draft
  • Kiln speed
  • In case of pre-calciner systems: fuel rate and draft in the pre-calciner
  • In case of grate coolers: grate speed

The effect of the control variables, on the operational parameters, mentioned before, is as follows.

Fuel Rate

When the fuel rate into the kiln is increased:

  • Burning zone temperature increases
  • Back-end temperature increases
  • Oxygen concentration decreases

A decrease of the fuel rate has the inverse effect.

If more energy is introduced into the system, the BZT and the BET rise and oxy-gen level drops, since part of the excess air is consumed for the combustion of the extra fuel if the combustion does not take place under reducing conditions.

Feed Rate

When increasing the material feed rate into the kiln:

  • BZT decreases
  • BET decreases
  • Oxygen concentration decreases

A decrease of the feed rate has an inverse effect.

When more feed is introduced into the kiln system, the temperature starts to drop first at the back end. After the higher amount of feed has reached the burning zone, the BZT starts to decrease also.

The extra feed releases additional H2O and CO2. The additional amount of gas from the meal reduces the quantity of combustion air (and thus of excess air), since the amount of exhaust gas remains (almost) constant. This leads to a decrease of oxygen in the exhaust gas.

Kiln Draft

When increasing the draft in the kiln:

  • BZT decreases
  • BET increases
  • Oxygen concentration increases

A slight decrease of the draft has an inverse effect. If combustion takes place under reducing conditions (too little excess air), the BZT drops again and BET rises.

When the kiln draft rises, more (slightly colder) secondary air is pulled into the kiln through the burning zone. This decreases the flame temperature, and as a result the BZT drops. Although the amount of heat supplied to the kiln system is somewhat higher (more secondary air), less heat is released in the burning zone (lower flame tempera- ture results in reduced heat transfer); thus the BET rises. The higher kiln draft increases both the oxygen concentration and the volume of the exhaust gases.

Kiln Speed

When the speed of the kiln increased:

  • BZT decreases (temporarily)
  • BET decreases (temporarily)
  • Oxygen decreases (temporarily)

A decrease of the kiln speed has the inverse effect.
When the kiln speed is increased, the meal advancement is faster. All reaction zones shift temporarily downwards, which cause the BET and the BZT to drop.

Since more meal enters temporarily in the drying and the calcining zone (same effect as an increased feed rate), a higher amount of H2O and CO2 is released, which leads to a drop in the oxygen concentration.

Those effects are only temporary, since a higher kiln speed at a fixed feed rate reduces the filling degree in the kiln. Once the kiln filling degree has been lowered in the entire kiln BZT, BET, and oxygen concentration gets recovered. In almost all kiln operation the kiln speed depends on the kiln feed rate. The objective is to keep the filling degree in the kiln constant. Thus the kiln speed which under normal oper-ation conditions is not an independent variable, is adjusted in tandem with the kiln feed rate.

Chemical Parameters

The quality of cement clinker produced is controlled by many chemical parameters, developed through the long year of experiences of the cement technologists. Some of these factors are discussed here [4, 5]. On raw mix basis

based on the raw meal chemistry. If the value is 30%, the coating will be dense and firm, and if it is 20%, the coating is loose and porous.

Start-Up and Shutdown

The start-up and the shutdown of a kiln system are the most critical phases during kiln operation. Most serious equipment failures and accidents, which are caused by the operation itself, happen usually during these two periods. Therefore special cau-tion and attention have to be paid to the kiln system before and during start-up and shutdown.

Heat Up

The heat up of the kiln is done following a preset time – temperature schedule, which is determined mainly by the kiln dimension and the type of refractory lining. The other factor to be taken into consideration is how much of the old lining is replaced. Heating rate is slower in case of complete new lining and slower if it is a part lining. The kiln is periodically turned 90 degrees during preheating, in order to avoid unbalanced heating of the kiln shell. The rotation schedule should be aligned with the preheating curve. The kiln should be turned continuously once the BET has reached 750 °C and should be done earlier in case the kiln is exposed during rain. The time-temperature curve for heat up is shown in Fig. 4.6. Occasionally introduc-ing small amounts of kiln feed during preheating will help the bricks to set during heat up (especially, if the bricks are just installed). This will also help to prevent brick migration during periods of continuous kiln rotation.


In case of short shutdown, all fuel input to the system is to be shut off and the draft in the kiln to be reduced to a minimum (to avoid overheating of the back and to maintain the burning zone hot). However in some plants, a maintenance flame is re-established in order to help maintain kiln temperature.

In case of a long-term shutdown, a certain draft in the kiln is to be maintained, but overheating of the back (cool the system as fast as possible) is to be avoided. In case of heavy rain, the kiln with the auxiliary drive is to be turned on for minimum 30 min. Later the kiln should be rotated gradually by 90 degrees, in order to avoid unbalanced cooling of the kiln shell. Unless requested for a kiln relining, the kiln should never be run empty.

Unnecessary haste during preheating and shutdown of the kiln may lead to severe refractory problems and may outweigh the time and production saved because of fast preheating and cooling down of the kiln.

Selection of Refractory


Refractories should always be designed and installed to provide, wherever possible, a balanced and predictable economic life. There are many reasons for the shutdown of the kiln, but the reparation, if due to Refractory problem, needs the total cool down of the kiln which is very problematic and expensive. It is therefore important to get a Refractory life which coincides with the scheduled maintenance of the plant, and normally it is 12 months or 24 months, rather than 14 months or 26 months [1].

Because of wide variation in the size, capacity, raw materials, fuels used, and the operational practices, it is not possible to provide a standard recommendation for Refractory use, which any plant can adhere to. But it is desirable to follow a general guideline which will improve the Refractory performance in each zone of the kiln and in all other ancillary equipments with minimum cost and minimum failure of Refractory lining [2].

Refractories are mostly tailor-made products. A Refractory, performing well in any particular equipment, in a particular plant, does not always guarantee the simi-lar performance in the same equipment in another plant. Therefore, it demands a careful and thorough study of the conditions prevailed in individual plant before selecting the suitable Refractory lining.

Although qualitative rather than quantitative, a hypothetical relationship can be proposed (Table 5.1), indicating the physical and chemical properties that exert an influence on the rate of wear by common mechanical, thermal, and chemical factors [3]. This table can be utilized to compare the suitability of a refractory compared to others for a certain application where the factors responsible for the damage of the Refractories are identified to a certain extent.

The overall use and consumption pattern of shaped, unshaped, and insulation Refractory in different areas of application in a cement plant is shown in Fig. 5.1.

Refractory Selection

Refractory selection in different equipments is done taking into consideration the operational condition prevailing in that equipment and the target campaign life. It is also essential and important to understand the flow of solids and gases through the entire cement kiln system as shown in Fig. 5.2.

Cyclones and Ducts

The cyclone system is a very important element in the cement manufacturing pro-cess. Each cyclone system is unique in its design and operating characteristics and is decided by the factors such as raw meal composition, the nominal and peak throughput, and the type and quality of the fuel burnt in calciner or smoke chamber. The cyclones are designed always to meet the objective to achieve higher thermal efficiency and greater kiln capacity which place high demand on Refractory lining and anchor system.

The criteria for a good Refractory for lining in the cyclone and ducts are:

  • It must be able to withstand high temperature.
  • It must be able to withstand the chemical attack of alkali, chlorides, sulfur oxides, and transition metal oxides at elevated temperature.
  • The Refractory must meet the construction requirements of increasing size of cyclones to form a stable lining.
  • The Refractory surface must not support the buildups on its surface.
  • The Refractory surface must be smooth to reduce the resistance to gas flow inside the cyclone.

Higher operating temperatures, particularly in the lower cyclones, require use of refractories with high refractoriness and high strength. With higher temperature, corrosive vapors can penetrate high into the cyclone Refractory system, so alkali- and chloride-resistant refractories should be used [4].

Buildups are usually caused by chemical attack, in combination with Venturi effect in the cyclone’s complex design, and can also be caused by rapid temperature drops. Buildups reduce cyclone efficiency, and their removal ultimately requires a production stop. Regardless of the method used, removal is a risky job. In the most severe cases, explosives are used. Several mechanical alternatives exist, none of which is particularly satisfactory.

Chemical attack in the form of alkali penetration is unavoidable in cement pro-duction. The worst damage occurs in lower cyclone stages and riser ducts, kiln inlets, and even recliners. Alkali and acid vapors infiltrate the refractory linings and attack the binding phase at temperatures as low as 600–700 °C, thus endangering the lining. When these gases penetrate behind the refractories, the effects are even worse because Cl2 and SO2 combine with condensing steam to form acids that cor-rode anchors and kiln shell. If proper attention is not paid, this can lead to lining collapse. The safest, most cost-effective method to deal with this is to reduce the amount of insulation used, in order to move the vapor’s “dew point” outside the shell of the cyclone. If refractory and cyclone external shell temperatures remain above 100 °C, ambient moisture and chemical vapors will not condense, and acid formation will be prevented.

The temperature varies from 300 °C to 900 °C from cyclone 1 to cyclone 5. The same quality of refractory can be used in all the cyclones with different thickness of working lining and insulation lining to achieve the skin temperature as per the design.

Some properties of the bricks and castable suitable for the lining of the cyclones are given in Table 5.2.

The prevailing operating temperature and atmosphere inside the cyclones sug-gest the suitability of dense aluminosilicate Refractories with 40–50% alumina, low porosity, and high alkali resistance and do not allow high buildup. As an insulation material, calcium silicate board will be most ideal. Where buildup and alkali attack are the major problem, aluminosilicate refractories containing SiC (silicon carbide) or zircon help to reduce the buildup and alkali attack. SiC reacts at high temperature with atmospheric oxygen to form a glassy layer of SiO2 on the refractory surface which prevents buildup; similarly the zircon at high temperature decomposes to form SiO2, which forms a glassy coating over Refractory surface. Zircon is better,

because it has lower thermal conductivity than SiC which helps to save anchors from overheating [5].

The ideal lining thickness for any area of the preheater is the minimum thickness required to attain the specified shell temperature. In the case of monolithic lining, if it is too thin, the anchor may get overheated which can accelerate corrosion, scale formation, fatigue, etc. and may lead to collapse of the lining. If the lining is too thick, then sharp temperature gradient inside the lining may cause crack and spall-ing of the lining.

In the preheater the fine dust behaves like fluid, and very often the red-hot dust infiltration behind the refractory lining causes failure to the lining. The failures are caused by, sometimes, shearing of anchors or the shrinkage of insulation due to sintering in contact with red-hot dusts or by bulging of the brick lining due to push-ing of the lining from behind.

The Refractory used can be both in shaped or unshaped form, but which one is more preferable is a controversial matter. Performance wise both are good and both have some merits and demerits. The advantages and disadvantages of brick and monolithic lining are shown in Table 5.3.

The installation of monolithic lining is faster and easier than brick lining. But the installation of monolithic Refractory is more critical than brick lining. Brick is a finished product, but castable is a semifinished product and needs much attention and care during installation.

In the brick lining, the shape of the brick is important, especially when the diam-eter of the cyclone is more. The brick shape and size must be designed to have an interlocking arrangement to avoid bulging out of the lining. In the case of mono-lithic lining, the anchor design is very important to attain the stability of the lining.

The common mistake made during selection of Refractory is to put the speed of installation ahead of the lining stability. If the preheater lining life is 2–5 years with-out repair, then installation time should never get the priority.


The thermal efficiency of the cyclone system improves a lot when the recycled hot air from the cooler is delivered to the calciner through the tertiary air duct. Fuel cost is also reduced by using the cheaper alternate fuel in the calciner. At higher operat-ing temperature, chemical attacks are more severe in calciner compared to cyclone. The suitable refractory for the calciner must have high refractoriness, high thermal shock resistance, and high resistance to chemical attack by alkali, chlorides, and sulfur oxides. The refractory in calciner also should have high abrasion resistance because large volume of dust-laden gas is delivered in calciner at high velocity through the TAD. The selection of right kind of insulation is also very important to prevent energy loss. The use of alternate fuel like plastics, municipal wastes, etc. may pose serious problem like early wear in central pipe.

Besides reacting with the refractory material itself, alkali vapor penetrates inside Refractory lining and attacks the metallic parts like anchors, and the stability of the lining may be greatly reduced.

Three main areas of the calciner are cone, wall, and roof. Both the shaped and unshaped refractories can be used to line all these areas of the calciner, but the Refractories must have the desired properties as mentioned before to ensure satis-factory performance. It is better to line the roof with castable which has lower per-meability compared to brick lining.

The typical properties of the Refractories, suitable for calciner, are shown in Table 5.4.

Riser Duct

It recycles hot air from the lower stage cyclones to kiln improving the thermal effi-ciency of the process and reduces the fuel cost. Riser duct lining often faces the problem of buildup especially in the curved sections.

The closer the duct is to the hot end of the cyclone system, the greater is the alkali penetration and spalling of the Refractory. The alkali vapor penetration can cause severe alkali bursting, where the Refractory volume can increase by ten times from normal volume [6]. The typical properties of the Refractories, suitable for riser duct lining, are shown in Table 5.5.

Smoke Chamber

It experiences the worst possible exposure to high temperature (1300  °C), alkali attack, buildup, anchor corrosion, etc. The Refractory selected for lining inside this equipment must have low porosity and low permeability, and the best choices are dense bricks or low cement castables. The typical properties of the Refractories, suitable for smoke chamber lining, are shown in Table 5.6.

Meal Pipes

It connects the cyclones and has smaller diameters. The meal pipes are made in small sections. They are lined in ground level and then are transported to the area of installation. The main property required here is to withstand the continuous abra-sion of the meal material. It can be lined with both bricks and castables. The typical properties of the Refractories, suitable for meal pipe lining, are shown in Table 5.7.


The total heat loss by radiation in the preheater is less than 2% of the total heat input. Most preheater vessels and ducts are insulated to protect the steel shell. The use of refractory fiberboard as insulation, in the hotter sections of the preheater, may induce premature lining failure, if the fiberboards fail behind the dense lining. Fiberboard contains a certain amount of organic matter that, when exposed to mod-erately high temperatures, burns out with emission of fumes and loses both its mechanical strength and insulating property. The loss of insulating property exposes both the steel shell and the anchor stem to higher temperatures resulting in relative movement and damage of the anchoring system and inducing cracks on the dense lining.

To avoid such problems, an insulating material with the minimum amount of loss on ignition, e.g., calcium silicate, diatomaceous earth, or vermiculite boards, or insulating castables that can be cast, gunned, sprayed, or pumped in place should be selected.

Kiln Hood

The kiln hood for lining purpose can be divided into two halves. The upper half, consisting of the ceiling and sidewall, experiences high-temperature radiation, abra-sion, exposure to alkali vapors and sulfur oxides from secondary air, and thermal shock, when there is kiln shutdown. It is best to line the area by 50–75% alumina bricks or dense low cement castables. There is report of good result of silicasol-bonded castable in this area [7–9]. If the temperature of the secondary air is above 900 °C, then magnesia-spinel refractory can be a good choice also.


Another interesting alternative, finding more and more acceptance in cement plants, is the modular lining which offers the advantages of both bricks and casta-bles. In this alternative the castable is precast and pre-fired into blocks that have the insulation and the anchoring system pre-attached.

The lower half of the hood towards the cooler side can be lined with silicon car-bide or zircon-containing high alumina Refractories. The buildup of coating occurs in this area, and the said Refractories will repel the coating. The typical properties of the Refractories, suitable for kiln hood lining, are shown in Tables 5.8 and 5.9.


The kiln hood lining must use the insulation at the back with the steel shell. When the working lining thins down by wear, the insulation will save the steel shell from overheating and warping. The integrity of the insulation is critical to the working lining integrity. If hot clinker dust penetrates the insulating layer behind the dense castable or brick, it quickly damages the anchoring system and pushes the lining away from the shell until it collapses.

Most insulating materials shrink at hood temperatures and make the situation even worse. The insulating material for application in hood is chosen such that it contains no organic binders and its limit of service temperature is above 1000 °C to retain its full integrity at that temperature. Either calcium silicate of 1100 °C grade or insulating castable is the best choice for that area of application.

Tertiary Air Duct and Cooler

TAD recycles the heat from the hot clinker to the kiln. Air from the grate cooler, laden with highly abrasive clinker dust, travels through the TAD at a very high velocity (25–30 m/sec) at high temperature (about 1000 °C) and poses a very high abrasion on the refractory lining especially at bends and dampers. Therefore the selected refractory to line the TAD must have very high abrasion resistance and alkali resistance.

Grate Cooler

Clinker enters the cooler at a temperature of around 1200 °C and heats up the grate and surrounding refractories rapidly. High refractoriness, high abrasion resistance, and resistance to thermal shock are necessary, especially at the clinker downfall area and bullnose. The cooler walls directly above the grate are also subjected to extreme wear. The partition wall, designed to limit the inflow of cold air to the kiln system, must withstand the abrasive effect of clinker dust entering the tertiary air duct as well as the temperature fluctuations, resulting from its exposed position.

The cooler is effectively divided into two zones – the “hot” zone and the “cold” zone. The first zone is from the clinker downfall to the bypass duct and partition wall. The purpose is to exchange the heat from the hot clinker and route it back to preheater via the tertiary air duct. The function of the second zone is to ensure a uniform reduction of temperature of the clinker body from 800 °C down to ambient. The Refractories to be used here must have high abrasion resistance and high ther-mal shock resistance.

Both bricks and monolithic can be used to line the kiln hood. To get good refrac-tory life and the fastest possible installation at the clinker downfall, bullnose, front sidewalls, and roof, precast refractory blocks, which can be quickly bolted into place and just as quickly removed, can be used.


Kiln is the heart of the cement plant where the clinkerization takes place at high temperature. The kiln is a cylindrical horizontal tube with a very little inclination of 1–2 degree. The kilns have diameter 3–7 m and length 50–100 m. The kiln rotates on its horizontal axis at a speed of 1.5–4 RPM.

The Refractory lining of a modern cement rotary kiln is performing different specific functions at different zones of the kiln, which include:

  1. Protection of the kiln shell and tires from attaining an excessive temperature which can deform the shell
  2. Protection of the kiln shell from the corrosive gases and liquids
    1. Protection of the kiln shell from mechanical abrasion by the feed material and

    the clinker

    1. Reduction of the thermal energy loss to keep it within tolerable limit
    2. Increase of heat exchange between the hot gases and the feed material


The kiln can be divided into several zones depending on its temperature at different zones and also function being performed. Table 5.10 shows the length norms of different zones in different types of kilns.

Refractory selection for different zones is done based on the operational environ-ment. The temperature profile and the refractory practices followed for different zones in the kiln are shown in Fig. 5.3 [10, 11].

Inlet Cone

The most important considerations for the bricks here are that those must be able to withstand high abrasion, thermal shock, and alkali attack. Therefore 40–50% alu-mina fireclay refractories are found to work best in this area. Both shaped and unshaped refractories can be used in this area. The typical properties of the refrac-tories suitable for lining are given in Table 5.11.

Preheating Zone

The Refractories in this zone must be able to withstand high mechanical abrasion by the moving calcined feed material and alkali attack. Fifty to sixty percent alumina refractory bricks are the best lining material for this area. The typical properties of the aluminosilicate refractories suitable for lining different zones of the Kiln are given in Table 5.9.


Transition Zone

Here the Refractory lining is exposed to:

  1. High temperature and also cyclic temperature fluctuations, because there will be no coating or unstable coating
  2. Radiant heat of the flame
  3. Attack by alkali salts and alkali vapor
  4. Attack by the liquid phase in the cement which will start and attack by other cor- rosive gases if alternate fuels are used
  5. Redox reaction cycles when there is variation in oxygen potential due to change in fuel

Magnesia alumina-spinel refractories are the best material to line this area, espe- cially when the furnace is with large diameter. The magnesia-alumina refractories have the property to withstand high thermal shock. It has low MOE to withstand high thermo-mechanical stresses, and it can withstand the chemical attack of alkali salts, corrosive gases at high temperature, and cyclic oxidizing and reducing conditions.

With smaller diameter kiln, normal high alumina refractories and phosphate- bonded high alumina refractories with 70% alumina also work well. High alumina refractories because of their pyroplastic property can withstand the high deforma-tion without getting crack. It has also a good thermal shock resistance. High alu-mina Refractories also take up good coating which protects it from all abuse.

Burning Zone

It is the most important area in a cement plant where the clinkerization takes place.This zone experiences:

1-Highest temperature.

2-Reaction with the liquid phase formed in cement.

3-Continuous temperature fluctuation between when the lining is in contact withthe clinkers at the bottom and when it is at the top farthest away from the clin-kers. The temperature fluctuation gets further aggravated, if the coating becomes unstable.

4-Reaction with the sulfur oxides and other gases, e.g., chlorine, generated from alternate fuels.

5-High abrasion from the clinkers.

6-Different kinds of thermo-mechanical stresses.

Fortunately a thick coating of the cement clinker is formed on the refractory lining in the burning zone, which minimizes the damaging effects of the said factors on the lining and enhances its campaign life. The process people in the cement plant always pay attention to maintain a stable and thick coating on the burning zone refractory lining. The stability of the coating depends on many factors which will be discussed later.

Thermal load is one of the most crucial parameters to decide the lining quality. The thermal load (TL) is defined as the heat consumption per hour per unit cross section of the kiln, and usually it is expressed in terms of GJ/m2h:

where Fi = fuel burning rate per hour (kg/hr), NHV = net calorific value of the fuel (KJ/kg), and Di = effective internal diameter of the kiln after Refractory lining.

With increasing production per net kiln volume (ton/dm2), the thermal load increases, and lining material must be designed accordingly to be able to take up that thermal load.

It is found through experience that the normal thermal load of cement kiln remains in the range 14–22 GJ/m2h, whereas the recommended maximum thermal load in the burning zone is 24.3 GJ/m2h. It is found by experience that up to a ther-mal load of 13 GJ/m2h, high alumina bricks are working satisfactorily, when other operational parameters are within tolerable limit. Beyond this value of thermal load-ing, basic refractories, e.g., magnesia spinel, work much better, and if the thermal loading exceeds 21 GJ/m2h, special care must be taken in selecting the refractory. In such case very pure grade of magnesia alumina or magnesia-zirconia lining should be used.

For large diameter kilns where the thermal loading is high and alternate fuels are used, the basic refractories are preferred over high alumina products, but the Bauxite based high alumina Refractories are also used very successfully in smaller diameter kilns. The raw material chemistry, fuel used, and operational discipline play a major role in deciding the refractory lining life in burning zone. The selection of the refractory for the burning zone depends upon the kind of stresses the refractory lining is exposed to. The operating conditions that prevail in different cement plants are not same.

The typical properties of some basic Refractories for the transition and the burn-ing zone are shown in Table 5.12.

Before selection of the Refractory, one should know the actual operating condi-tions in the cement plant, under consideration. Table 5.13 shows the suitable quality of basic bricks under different kinds of stresses. To enhance the life of the basic lining in the burning zone Magnesia-Spinel castable has been developed which has shown very promising result in actual field trial where the material was applied by gunning on the worn out lining in the burning zone to build a thickness of 60 mm [12].

Outlet Area (Retainer Ring and Nose Ring)

This is a very critical area in the kiln and highly stressed. The thrust of the whole kiln lining is borne by the retainer ring, and the refractory in contact with the retainer ring is to withstand it. Besides the thrust in this area, the bricks are to withstand high abrasion of the clinkers, and the bricks also experience high thermal shock due to temperature fluctuation. Various designs are tried to minimize the thrust, e.g., to put two retainer rings or to make a conical design to transfer part of the axial thrust in radial direction, but none is found to be a foolproof solution.

The stress generated on the lining in this zone, because of thermal expansion, was computer simulated and modeled. It shows that the deformation of the lining caused by thermal expansion depends upon the temperature profile across the brick height and the brick quality [13]. During initial lining the cardboards are used as expansion joints. It was found that in the initial heat up, the highest stress produced was confined to one third of the brick height from hot face [13] and the stress pro-duced was lower than the crushing strength of the bricks. But a completely different situation was encountered after kiln shutdown and reheating. The expansion joint provided during initial lining is lost due to downhill movement of the lining. Moreover all the open joints are filled up with clinker dust and the residual expan-sion of the lining made it further worse. The brick next to the retainer ring is sup-ported. Only 70 mm of the brick height, next to the retainer ring, is supported by the retainer ring, and a high shear force is generated which is much higher than the brick strength resulting in crushing of the brick (Fig. 5.4).

To combat this problem, RHI has developed a design to increase the contact sur-face area by providing slanting surface as shown in Fig. 5.5. Because of higher surface area, the force per unit will be lower to save the bricks from getting crushed [14].

In the nose ring area, the main factors, which make this area critical, are high abra-sion of the falling clinkers and high thermal shock as it is exposed to atmosphere.

The material to be used in the nose ring must have high volume stability, abra-sion resistance, and thermal shock resistance. The properties of some ideal material used for lining this area are shown in Table 5.14. Silica sol-bonded or no cement castable is also tried in this area with great success [7].

Along with the castable quality, the design of anchors is also very important for the successful performance. It has been found that specially designed precast refractory blocks with very high content of stainless steel fiber work very well in this area [15].

Brick Shapes for the Kiln Lining

The brick shape used for lining of the rotary kiln is side arch bricks. Normally the kilns are lined with brick shapes with two different tapers. Two different systems of refractory sizes are very popular, one is VDZ and the other is ISO, shown in Fig. 5.6. Two shape bricks system fit better in the rotary kiln lining because often the kiln shells are not perfectly circular. By changing the ratio of two different bricks, the distorted area in the kiln can be lined better. Some practical thumb rules used for the lining are:

  1. Basic bricks should be lined in the VDZ shapes because VDZ bricks are thinner compared to ISO sizes and the lining with VDZ shapes has more joints which can accommodate the higher thermal expansion of the basic refractory.
  2. Kiln lining with larger diameter needs bricks with more taper, and preferably ISO shapes should be used.
  3. At the kiln tires because of the higher shell ovality, VDZ brick shapes should be used because of their flexibility with more number of radial
  4. Handling of ISO bricks is more stressful for lining crews because of their greater weight compared to VDZ shapes.

There is no proven corelation between the brick lining thickness and its cam-paign life. Sometimes lining with 160 mm brick thickness gives better refractory life than 220 mm thick lining. Actually the radial stress, which crushes the brick, increases with lining thickness. The ideal thickness of the lining should be the mini-mum thickness of the lining which will not allow the shell temperature to exceed the acceptable limit. The normal recommended lining thickness is given in Table 5.15.

It is to be borne in mind also that the key effect of the bricks in the lining becomes more prominent with higher length of the bricks and in bigger diameter kiln the brick lining with longer bricks is more stable.

Calculation of Combination of Two Different Brick Sizes

The number of bricks of each size used in the lining can be calculated by using the mathematical formula given below.

If Na and Nb are the number of pieces required of shapes A and B to form a ring in a kiln with inside diameter D, Xa and Xb are the a and b sides of shape A, and Ya and Yb are the a and b sides of brick B, then

Refractory selection in burner pipe must take into consideration the following


  1. High temperature
  2. Thermal cycling
  3. Alkali attack
  4. Sulfur attack
  5. High abrasion of clinker
  6. Low thermal conductivity to protect the steel pipe inside

It is not possible to get the best of all these properties in one material, and there- fore a balance is struck to optimize the different properties to get a best suitable material. Castable is the type of Refractory used to line the burner pipe.

Since the main requirement of a burner pipe lining is to protect the metal pipe inside, so an insulation cloth or paper must be wrapped on the pipe and then casting is done. The lining thickness is normally 100 mm. The front 1.5–2 m of the burner pipe is the most vulnerable area where damage takes place in service. In order to reduce the cost, the zone lining can be done in the pipe. In zone lining 2 m from the tip is cast with the best castable, and beyond that lining can be made with normal castable with lower price.

The main reason for damage of the castable lining of the burner pipe is the dif- ferential expansion and contraction of the castable and the anchors [16]. To mini- mize this effect, the anchor design is very important. The anchor must be designed in such a way that it can accommodate the expansion of the castable lining material [17]. The anchors are to be covered with bitumen paint or wax, which when burns off, creates a gap for the play of the castable. Instead of rigid anchors, the floating anchors, which can accommodate the movement of castable, enhance the perfor- mance. The stainless steel fiber reinforcement in the castable resists the abrasion and thermal shock better and improves the performance. Properties of some cast- able suitable for lining the burner pipe are given in Table 5.16.

Silica sol-based castable has been tried successfully in the burner pipe lining [9, 18].

The report is also there that specially developed magnesia-spinel castable shows very promising result in the lining of the burner pipe [19].


Refractory Design, Installation, and Maintenance


Refractories can be a substantial investment for a cement industry, either in new construction or in repair work. Installation of refractories may comprise a small part of the entire budget, but it protects the equipment, protects the people, and protects the production, and their value is greater than its cost. Refractory performance depends on the quality of installation to a great extent. The best quality of Refractories does not ensure the best performance, unless the Installation quality is sound. Installation techniques are different for different types of Refractories. Installation methods of shaped and unshaped products are quite different. Complete knowledge on the character and properties of the refractory products under installa-tion is required for good installation. Refractory lining inside each equipment in a cement plant like calciner, cyclone, TAD, cooler, Kiln, etc. needs proper planning and utmost care and precaution. Any premature Refractory failure needs a shutdown to set it right and causes huge loss in profit because of production loss.

General methods and principles followed in Refractory installation are the same as followed in civil engineering construction, and it is the same irrespective of the equipment. But the major difference is that the whole refractory lined structure will remain at an elevated temperature during operation. Therefore some special tech-niques are followed to take care of stresses generated because of higher operational temperature. Installation of Refractories in some of the equipments, for example, Rotary kiln, because of its geometry and of its dynamicity, needs special care.

Refractory Lining in Rotary Kiln

Rotary kiln is the heart of a cement plant and single most important equipment in a cement industry. Most of the unplanned shutdowns for refractory maintenance, in cement plants, are due to the refractory problem in rotary kiln. It is to be borne in mind that Rotary kiln is huge in size and mass and is not stationary but dynamic in nature, which has made it very special from other static furnaces. A kiln of 6 m diameter and 60 m length, for example, including Refractory lining and feed weigh about 1100 MT [1]. When such a huge structure, with huge mass, rotates on two or three supports, different types of thermo-mechanical stresses are generated in the Refractory lining, and therefore lining perfection is very much needed to reduce the effect of those stresses. The following points must be taken into consideration before starting refractory installation, in any rotary kiln or in any other equipment as well.


The refractories for lining must be kept under covered space so that the bricks do not get wet, especially in case of basic bricks and monolithics. In case of lining basic brick along with other nonbasic refractories, all the products must be used in dry condition to avoid any kind of hydration of the basic bricks. Wet basic bricks should not be exposed to a temperature above 50 °C.

Tools and Tackles

All tools must be inspected before starting the lining job. The common tools used for installation are:

(a) Rubber or plastic hammers of different sizes to align the laid bricks and to close the joints

b) Brick cutting machine to cut the bricks as per requirement, specially to close a ring or a lining wall
(c) Trowels to apply and spread the mortars on brick surface

(d) Measuring tapes
(e) Spirit level
(f) Plumb
(g) Chisels, etc.

A pneumatic or hydraulic jack is preferred to assist in the jacking and closing out of each brick ring. The jack should have a pressure dial capable of reading up to 4000  psi. Automatic lining machine (Fig.  6.1) is also used largely for lining which makes it faster because the kiln need not to be turned after lining the lower 180 degree [2].

Preview of Drawings

A thorough review of the installation drawings, with the customer, installer, and refractory supplier, should be made, before installation begins.

Kiln Shell Inspection

Both internal and external appearance of the kiln shell must be inspected thoroughly before the lining starts. For any abnormalities in kiln shell, special care must be taken. The kiln shell inside must be clean and free of any debris. The brick must be seated firmly on the shell with tight brick-to-brick contact.

Reference Line: Longitudinal

As a first step to start the kiln lining, longitudinal reference line should be estab-lished on the lowest part of the shell (6 o’clock position) along the length of the kiln, to be brick lined. A bar is kept horizontal inside the kiln with the help of a spirit level (Fig. 6.2) [3]. A string, with a weight at one of its end, is hung from the center of the bar. The point, where it touches the shell, is noted. The similar readings are taken at several points along the length of the kiln. The points obtained are joined to get accurately the longitudinal line. This longitudinal reference line must be parallel to the axis of the kiln shell and will be used to start laying brick for each ring. Construction of these lines can also be done with the help of a long spirit level kept in a similar way as the wooden bar is kept. The level is adjusted and the center is marked on the kiln shell. Longer is the spirit level better is the accuracy.

Additional parallel lines at 1.5  m intervals should also be drawn on the shell (Fig. 6.3). Nowadays, more sophisticated method is available using laser to con-struct the longitudinal reference line (Fig. 6.4).

Reference Line: Circumferential

In the next step of the construction, circumferential reference lines are drawn. These lines are constructed perpendicular to the longitudinal reference lines made earlier. Obviously the circumferential reference lines are perpendicular to kiln axis also. The reference lines (Fig. 6.3) are essential to ensure the accuracy of the installation and to help keep the rings straight and to maintain the joint tight [3]. The bricks are laid on this mark, and the bricks are now put on both the sides of this line one by one to form the ring which will be perpendicular to the axis of the kiln. Nowadays more advanced method has been developed to fix the axial line by laser beam, shown in Fig. 6.4. Existing weld joints may be used as circumferential reference line. If the rings are not laid perpendicular to the axis of the kiln, there will always be a chance that lining gets spiraled or skewed during running of the kiln. This causes the loos-ening of the ring and falling down of the bricks from the ring and finally lead to the shutdown of the kiln.

Brick Shapes and Size

Side arch bricks of two different shapes following ISO or VDZ standard (Fig. 5.3) are used for lining rotary kilns. If the kiln diameter is known, then the ratio of two different shapes, to form a ring, can be determined from the available table (Appendix).

Some kilns still use refractory blocks (cupola shapes) having a curved surface to match the contour of kiln shell. Bricks are always installed starting at the downhill end of the section to be lined. During lining the bricks must be tapped with a leather hammer to ensure the proper contact with the shell.

Lining Thickness

The thickness of the refractory lining depends upon the kiln diameter. The standard norm is to use 200 mm thickness for up to diameter 4.2 m, 220 mm thickness for 4.2–5.2 m diameter, and 250 mm thickness for diameter above 5.2 m. The purpose of increasing lining thickness is to insulate the kiln shell from overheating. But it has been found, in actual practice, that with good and stable coating on the brick lining, lower refractory thickness also can be used without any problem.

Many other factors, besides the kiln diameter, should be taken into consideration while deciding a lining thickness. The rigidity of the kiln shell is very important factor. It is defined by its thickness. The thicker shell flexes less with the same grade of steel and same operating temperature. The rigidity of the kiln shell becomes very low above 350 °C because of the decrease of metal strength with increasing tem-perature. A flexible shell with high ovality poses compressive stresses on thick lin-ing and higher risk of brick detachment when the lining gets thinner (Sect. 7.5.1).

The relationship between lining thickness and radial stress on the brick [4] can be expressed as