Lecture Note 1/26 CERAMIC AND RELATED MATERIALS [Adopsi dari: Zbigniew D Jastrzebski, “The Nature And Properties of Engineering Materials”, John Wiley & Sons, ISBN 0-471-63693-2, 1987, CHAPTER 9.] Ceramics are inorganic, nonmetallic materials that are processed or used at high temperatures. They include a broad range of silicates, metallic oxides, and combinations of silicates and metal oxides. Furthermore, elements such as carbon, boron, and silicon, carbides, borides, and nitrides, various refractory hydrides, sulfides, and selenides are usually considered as ceramics. A common feature of ceramic materials is that they depend on either ionic or covalent bonding or a combination of both. The bonding electrons therefore tend to be localized and ceramics are generally relatively poor conductors of heat and electricity. Because the bonds are strong, ceramics usually have high heat and chemical resistance. Ceramics are often compounds formed between metallic and nonmetallic elements and their crystal structures tend to be more complex than those of pure metals. They can be fairly simple as in the case of magnesium oxide (MgO), beryllium oxide (BeO), silicon carbide (SiC), or silicon nitride (Si 3 N 4 ), but they are often quite complicated as in various silicates. Furthermore, they may contain both crystalline and glassy phases. Ceramics can be grouped into three broad divisions—clay products, refractories, and glasses—according to their common characteristic features. Closely related to ceramics in chemical composition are inorganic cements that are used as binding materials to produce concrete, mortars, and similar products. CLAY PRODUCTS Clay products include important engineering materials such as bricks, tiles, porcelain, stoneware, and various chemical ware. All these products are made of various clays by manufacturing procedures that are essentially similar. 9-1 PLASTICITY OF CLAYS The plasticity of clay can be defined as its ability to form a plastic mass (dough) with water. The mass can be molded to shape easily, but it retains sufficient rigidity to prevent deformation on standing. Dry clays are not plastic and a certain amount of added water is always necessary to produce the required plasticity. The function of water is to form a film around the flaky clay particles so that their parallel orientation and movement under pressure are facilitated (Fig. 9-1). The amount of water required to make a clay plastic depends on the size and shape of the clay particles, their surface characteristics, and he presence of electrolytes. There is a certain minimum water content below which a clay ceases to behave as a plastic material and becomes friable or crumbly. This is called the plastic limit of a clay. As the proportion of water increases, the clay becomes more plastic until the point is reached at which the clay begins to flow and becomes wet and sticky. This is called the liquid limit. The difference in the water content between the liquid Limit and the plastic limit is called the plasticity index, which represents the plasticity range of the clay. Depending on the methods employed in the shaping and forming of ceramic wares, different plasticities of raw mixes are required to produce a product of desired size and properties. Hand molding and machine extrusion require a mass of plastic consistency, classified as stiff mud. Machine pressing is best accomplished on stiff-plastic and semidry mixes, whereas slip casting requires a mix of semiliquid consistency so that it can be easily poured in a mold. The required plasticity is secured by different methods, such as weathering and grinding, blending with other clays or finely pulverized non- plastic ingredients, and addition of alkalies, acids, and salts.
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Lecture Note
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CERAMIC AND RELATED MATERIALS
[Adopsi dari: Zbigniew D Jastrzebski, “The Nature And Properties of Engineering Materials”, John Wiley & Sons, ISBN 0-471-63693-2, 1987, CHAPTER 9.]
Ceramics are inorganic, nonmetallic materials that are processed or used at high temperatures. They include a broad range of silicates, metallic oxides, and combinations of silicates and metal oxides. Furthermore, elements such as carbon, boron, and silicon, carbides, borides, and nitrides, various refractory hydrides, sulfides, and selenides are usually considered as ceramics. A common feature of ceramic materials is that they depend on either ionic or covalent bonding or a combination of both. The bonding electrons therefore tend to be localized and ceramics are generally relatively poor conductors of heat and electricity. Because the bonds are strong, ceramics usually have high heat and chemical resistance. Ceramics are often compounds formed between metallic and nonmetallic elements and their crystal structures tend to be more complex than those of pure metals. They can be fairly simple as in the case of magnesium oxide (MgO), beryllium oxide (BeO), silicon carbide (SiC), or silicon nitride (Si3N4), but they are often quite complicated as in various silicates. Furthermore, they may contain both crystalline and glassy phases.
Ceramics can be grouped into three broad divisions—clay products, refractories, and glasses—according to their common characteristic features. Closely related to ceramics in chemical composition are inorganic cements that are used as binding materials to produce concrete, mortars, and similar products.
CLAY PRODUCTS Clay products include important engineering materials such as bricks, tiles, porcelain, stoneware, and various chemical ware. All these products are made of various clays by manufacturing procedures that are essentially similar.
9-1 PLASTICITY OF CLAYS
The plasticity of clay can be defined as its ability to form a plastic mass (dough) with water. The mass can be molded to shape easily, but it retains sufficient rigidity to prevent deformation on standing. Dry clays are not plastic and a certain amount of added water is always necessary to produce the required plasticity. The function of water is to form a film around the flaky clay particles so that their parallel orientation and movement under pressure are facilitated (Fig. 9-1). The amount of water required to make a clay plastic depends on the size and shape of the clay particles, their surface characteristics, and he presence of electrolytes. There is a certain minimum water content below which a clay ceases to behave as a plastic material and becomes friable or crumbly. This is called the plastic limit of a clay. As the proportion of water increases, the clay becomes more plastic until the point is reached at which the clay begins to flow and becomes wet and sticky. This is called the liquid limit. The difference in the water content between the liquid Limit and the plastic limit is called the plasticity index, which represents the plasticity range of the clay.
Depending on the methods employed in the shaping and forming of ceramic wares, different plasticities of raw mixes are required to produce a product of desired size and properties. Hand molding and machine extrusion require a mass of plastic consistency, classified as stiff mud. Machine pressing is best accomplished on stiff-plastic and semidry mixes, whereas slip casting requires a mix of semiliquid consistency so that it can be easily poured in a mold. The required plasticity is secured by different methods, such as weathering and grinding, blending with other clays or finely pulverized non-plastic ingredients, and addition of alkalies, acids, and salts.
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The effect of weathering or grinding is to produce a uniform clay—water paste in which clay particles are well dispersed and surrounded by a water film. This treatment greatly improves the plasticity and homogeneity of the mass.
Blending is the term given to mixing two or more ingredients in such proportions that a mass of required composition and plasticity is obtained. This is a particularly important step in the manufacture of high-grade products such as white-wares, chemical wares, and porcelain. The usual ingredients are clay, non-plastic materials, such as finely ground quartz or flint, and fluxes.
FIGURE 9-1 Plasticity of clay. (a) Random arrangement of clay particles in dry clay. (b) Parallel orientation of clay particles surrounded by water film under shearing
stress.
Non-plastic materials reduce the proportion of water in the mass necessary for a desirable plasticity, thereby preventing an excessive drying shrinkage. Fluxes are substances added to the mix to lower its fusion point, thereby making possible a more complete vitrification at not too high temperatures.
9-2 DRYING
Drying removes the water from the formed plastic body before it is subjected to firing. The total water present in the mass consists of the shrinkage water and the pore water. The shrinkage water is the water that is held between the particles of the clay, and it accounts for the plasticity of clay. The pore water is held in the internal pores of the particles. The removal of the shrink age water is accompanied by a contraction of the body, since the water lost comes from the interstices between the particles, thereby causing them to come closer together. This increases the attraction forces between the particles, resulting in a much higher strength of the dry clay as compared to its wet strength. Excessive drying shrinkage may result in cracking and warping of the body if the drying rate is too high. Drying, therefore, must be performed very carefully to prevent damage to the product from a high shrinkage rate. Cheap wares are usually dried in the atmosphere under a roof with open sides for many days. More expensive products are dried in special ovens at 85 to 96°C (185 to 205°F) using air of high humidity. The use of air with a high moisture content prevents excessive drying on the surface, and the elevated temperature in the oven reduces the viscosity of the water within the body, thus increasing its diffusion rate toward the surface. The moisture content of the drying air and the drying temperature are adjusted so that the rate of evaporation of the water from the surface is nearly equal to the rate of diffusion of water from the inside to the surface of the body. This permits relatively high drying rates without the danger of cracking and warping of the material. The amount of drying shrinkage increases with the plasticity of the clay, since this latter requires higher water content to become plastic. Hence, highly plastic clays are always blended with non-plastic ingredients such as fine sand to reduce the water content and subsequently the drying shrinkage.
The removal of pore water does not involve shrinkage, and it can be carried out with dry air at 110°C (230°F) or higher. This can also be made a part of the firing process. The removal of pore water does not involve shrinkage, and it can be carried out with dry air at 110°C (230°F) or higher. This can also be made a part of the firing process.
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9-3 FIRING
Firing has the aim ol converting a molded and dried clay article (green) into a permanent product possessing required strength, durability, and very often a better appearance. The firing temperature depends on the character of the clay and the desired properties of the product, and it may vary from 900 to 1400°C. In the initial stage of firing, from 110 to 260°C (230 to 500°F), the last traces of hygroscopic moisture are removed (Fig. 9-2). There is little further change until temperatures of 425 to 650°C (765 to 1200°F) are reached.
Here the clay minerals break down to silica and alumina, liberating the chemically combined water according to the reaction
Al 2O3.2SiO2.2H20 → Al2O3 + 2SiO2 + 2H2O (9-1)
At this point the clay loses its ability to form a plastic dough with water, and it cannot be remolded again. There is little change, however, in the strength and porosity of the body. In the temperature range 800 to 900°C (1472 to 1652°F), oxidizing conditions in the furnace should be maintained to secure the burning of any contained organic matter and the oxidation of iron pyrites. At this stage, all other gas-forming reactions should also be completed before vitrification temperatures are reached. At temperatures of 900 to 1000°C (1652 to 1832°F) fusion or vitrification begins, the porosity decreasing as firing shrinkage commences. Vitrification is the result of the gradual formation of liquid that fills up the pore spaces. When cooled, the liquid solidifies to a vitreous or glassy matrix by cementing the inert particles together. This decreases the porosity of the body and greatly increases its strength. With a further rise in temperature more liquid is gradually formed until full vitrify is reached at about 1400°C (1550°F). A still further rise in temperature does not cause any further shrinkage or any decrease in porosity, but it
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results in fusion, which greatly increases the strength of the body. When the proportion of liquid is so high that the specimen softens and collapses, the fusion point of the clay has been reached.
The degree of vitrification controls properties of ceramics such as cold strength, durability, porosity, and density. Common building blocks are usually fired in the temperature range 800 to 900°C (1472 to 1652°F) and are vitrified to various degrees. For a poorly fired brick the cold compressive strength may be about 7 MPa (1 ksi), while for a well-fired brick it may be 140 MPa (20 ksi) and sometimes higher. Paving bricks, called clinkers, are nearly fully vitrified bodies showing a uniformly dense structure, low porosity, high hardness, high compressive strength, and good abrasion resistance. Highly vitrified ceramics are stoneware and porcelain. Porcelain is such a highly vitrified body that it becomes nonporous and nearly translucent in thin sections. Stoneware is somewhat less vitrified, showing a porosity from 2% to 4%.
Ceramic products undergo considerable shrinkage from molding to the fired stages. This shrinkage is not uniform and can be controlled within a range of plus or minus 2% to 3%. Closer tolerances within 0.05 mm are obtained by grinding with silicon carbide or diamond wheels, but this is an expensive operation. The size of stoneware is limited to pieces of 2 m in diameter and 2.5 to 3 m in height; porcelain wares are restricted to still smaller sizes. Both stoneware and porcelain are used widely in the chemical industry because of their high chemical resistance to all acids except hydrofluoric acid. They are, however, attacked by concentrated alkalies and show low thermal shock resistance and low resistance to tensile stresses.
9-4 POROSITY AND PERMEABILITY
A fired ceramic product shows porosity to a variable degree; porosity is a measure of the volumes of all pores present in a material. The pores may be open or closed. The open pores are generally interconnected with each other by channels or capillaries, thereby making the material permeable to liquids or gases. The closed pores may be enclosed within individual particles or may form isolated spaces within the matrix of the body so that the material is impermeable to liquid or gas, despite its high porosity. Accordingly, two kinds of porosity can be distinguished: apparent porosity and true porosity. Apparent porosity, called also effective porosity. is expressed as the percentage of the volume of the open pores with respect to the exterior volume of the material under consideration. For consolidated masses such as certain ceramics and rocks the percentage of the apparent porosity can be found from the relationship
%100% ××××−−−−−−−−====
SW
DWP (9-2)
where P = the apparent porosity
D = the weight of dry solid
S = the weight of the suspended solid in water, after having been soaked in water, so that all open pores in the body are completely filled with water
W = the weight of the soaked body determined by weighing the soaked specimen from which the excess surface water has been removed by dabbing with a damp cloth
The difference between the weight of the soaked body W and the weight of the dry body D is equal to the weight of the water that filled all the open pores within the body; hence it represents the volume of pores in the body. The difference between the weight of the soaked body W and the weight of the suspended solid in water S is equal to the weight of
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the water in the volume of the body; hence it represents the volume of the body, including the pores. Consequently, the ratio of these two values multiplied by 100 gives the percentage apparent porosity of the material.
The true porosity represents the volume of both open and closed pores in the volume of the body. The percentage true porosity can be found from the relationship
%100porosity true% ××××−−−−
====g
dg
S
BS (9-3)
where Sg = the specific gravity or the density of the solid
Bd= the bulk density of the solid
The true specific gravity or true density refers only to the solid matter within the body and does not include any pores. The bulk density is the weight per unit volume of the material, which includes any pore space that may be present. it can be determined from the relationship
V
D
SW
DBd ====
−−−−==== (9-4)
where V is equal to W — S, the volume of the body with the pores, and D, S, and W are the same as in Equation 9-2. The true specific gravity is an inherent property of the material, but the bulk density is affected by the way in which the material has been manufactured, and it varies considerably with many factors. The true specific gravity or true density of a porous ceramic body is determined by crushing the material to a fine powder to eliminate all the internal pores inside the particles. Then the powder is placed in a pycnometer containing a suitable liquid, so that the volume of the fine particles is measured by the volume of the displaced liquid. The weight of the powdered material divided by the displaced volume of the liquid gives the value of the true density.
The presence of pores in a body adversely affects the strength of ceramics. This is because the pores reduce the cross-sectional area exposed to an applied load and also act as stress concentration raisers, which are particularly effective in brittle ceramics. The decrease in the strength of a ceramic body with porosity can be given by’
Pne φσσ 0==== (9-5)
where σ and σ0 the fracture strengths of porous and nonporous body, respectively
φP = the volume fraction of pores and
n = a constant having a value from 4 to 7
About 10% porosity by volume reduces the rupture strength by half of that for a nonporous material. Similarly, the Young modulus, E, is affected by porosity.
(((( ))))20 9.09.11 PPEE φφ ++++−−−−==== (9-6)
where E and E0 are Young moduli of the porous and nonporous body, respectively. Furthermore, the shape of the pores and their distribution are important factors.
Equations 9-2 and 9-3 permit us to determine the total porosity and open porosity; however, they do not provide us with information regarding the size of the pores and their distribution. An important method with which to determine the size and distribution of pores employs the mercury porosimeter. In this method, the mercury is brought into
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contact with the porous specimen, first under vacuum; hereafter, the pressure is gradually increased up to 100 MPa (14.5 ksi) and higher. Under this pressure the mercury enters the interconnected pore, and the volume of mercury penetrating into the pores is measured as a function of pressure. Assuming an ideal shape of the pores as cylindrical channels of diameter d, we obtain the relation
Pd
θγ cos4==== (9-7)
where γ = the surface tension of mercury
θ = the wetting angle between the mercury and the solid surface of the pore which is also dependent on the shape of the pore entry
P = the applied pressure
Permeability. Permeability of any porous body is the property that permits fluids to flow through its pores under a pressure gradient. It is obvious that only the open pores, which are interconnected, have the capacity to pass a fluid through the porous medium. For the same apparent porosity (effective porosity) the permeability will also be affected by the size of the pores, their uniform distribution, the internal surface area, and capillary effects. Consequently, any general correlation between porosity and permeability cannot exist, and two porous media of the same porosity may have entirely different permeabilities.
The flow rates of incompressible fluids (liquids) through porous masses can be determined from the empirical relation known as Darcy’s law. Darcy’s law states that the pressure gradient of a liquid is proportional to the specific flow rate through a porous mass and that the proportionality constants are the fluid viscosity η and the permeability α of the porous mass.
ul
p××××××××==== η
α∆∆ 1
(9-8)
where l
p
∆∆
= a pressure gradient across the porous medium of thickness l
u = the specific flow rate that can be expressed as either volumetric flow rate (u = QV /A) or mass flow rate (u = Qm/A) per unit area
Equation 9.8 can be written in terms of the volumetric flow rate as
η∆
∆α 1××××××××××××====
l
pAQv (9-9)
The permeability coefficient a is a measure of the volume of fluid flowing through a cross section in a unit time under the action of a unit pressure gradient and having a unit viscosity. If the volumetric flow rate of fluid of viscosity 1 mPa.s is 1000 mm3/s across a 100-mm2 area of a 10-mm-thick porous medium under a pressure difference of 101.3 kPa (1 atm), the permeability unit as calculated from Equation 9-9 will be
26
32
33
mm 109869.0
Pa/10mm 103.101mm 100
Pa.s 10/mm 1000
/−−−−
−−−−
××××====××××××××××××====
××××××××
====s
lpA
QV
∆∆η
α
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The permeability coefficient must be determined experimentally for each particular porous mass because it varies widely, even for the same effective porosity, according to the size of the pores, their distribution, and internal specific area.
The Darcy equation is applicable to the laminar flow of incompressible fluids through a horizontal bed of finite thickness. It can be applied for compressible fluids provided the changes in fluid properties (density), occurring during flow across the porous medium are accounted for in Equation 9-8. The flow rates through a porous mass may also change owing to possible structural changes in the mass as a result of swelling of the particles or blocking of the pores by solid impurities or corrosion. Furthermore, the effective porosity of the mass may decrease, owing to blocking of the pores by gases, which are usually dissolved in liquid and may escape from liquid as the pressure decreases during flow through the porous mass. Under low gas pressure a slip at wall surfaces may occur, thereby requiring a correction for viscosity. Also, in a free molecular flow, when the pore size is smaller than the mean free path, the flow is independent of the walls and the viscosity and depends only on the partial pressure and on the ratio of the passage diameter to the length. Furthermore, the effect of surface tension of liquid γ necessitates another correction to the total pressure, which can be determined from Equation 9-7 as
dP
θγ∆ cos4====
REFRACTORIES Refractories are special materials of construction capable of withstanding high temperatures in various industrial processes and operations. The main bulk of the commercial refractoriies comprises complex solid bodies consisting of high-melting oxides or a combination of oxides of elements such as silicon, aluminum, magnesium, calcium, and zirconium, with small amounts of other elements present as impurities. In recent years intensive work has been con ducted to develop new materials of construction for service at very high temperatures, such as are encountered in gas turbines, ram-jet engines, missiles, nuclear reactors, and various other high-temperature processes and operations. These highly refractory materials are relatively simple crystalline bodies composed of pure metallic oxides, carbides, borides, nitrodes. sialons, and sulfides. Finally, combinations of these refractory compounds with metals yield cermets, which show better thermal shock resistance than ceramics but, at the same time, retain their high refractoriness. Refractoriness is the ability of a material to withstand the action of heat without appreciable deformation or softening under particular service conditions.
9-5 COMMON REFRACTORY MATERIALS
Common refractory materials represent the main bulk of commercial refractories used in high-temperature processes and operations because of their relatively low price and ready availability. They consist of crystalline or partly amorphous constituents held together by a more or less glassy matrix of variable composition. One of the most widely used refractoriies is based on alumina—silica compositions, varying from nearly pure silica, through a wide range of alumina—silicates, to nearly pure alumina.
Silica—Alumina Phase Equilibrium Diagram. The silica—alumina phase equilibrium diagram is of great importance in understanding and predicting the properties and behavior of various clay products and silica—alumina refractories (Fig. 9-3).
The curved line ABCD is the liquidus line separating the liquid phase from the heterogeneous solid—liquid phase. The areas bounded by the curved and horizontal lines
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are the solid—liquid regions. The existing phases in these regions are indicated on the diagram. The horizontal line at 1470°C (2678°F) corresponds to the transformation of tridymite into cristobalite.
Both silica and alumina can be regarded as refractory oxides with melting points of 1710°C (3110°F) and 2050°C (3722°F), respectively. Small additions of alumina to silica act as a powerful flux for silica, rapidly lowering its melting point. The mixture of 5.5% alumina and 94.5% silica represents the eutectic composition with a melting point of 1545°C (2814°F). If a mixture of composition somewhere between 0% and 5.5% alumina, say 2.5%, is heated, it remains solid until the eutectic temperature of 1545°C (2814°F) is reached. At and just above this temperature, part of the mixture will melt, forming a liquid phase of the eutectic composition (5.5% A1203, 94.5% Si02), whereas the excess of silica remains in the solid state. The amount of the eutectic formed can be calculated from the lever rule = X 100 = 45.5% liquid and the amount of the solid silica left will be correspondingly 100% — 45.5% = 54.5%.
FIGURE 9-3 Equilibrium diagram of the system Al2O3-SiO2. (Journal of the American Ceramics Society, 7, 2381. 1924.)
Further heating will raise the temperature along line xx, and more solid silica will dissolve in the eutectic liquid whose composition will vary from B to the intersection of line xx with line AB (see Fig. 9-3). At point 2 the composition of the liquid is given by point 2’ on the liquid curve AB. The amounts of liquid and solid can be easily found from the lever rule, as previously. Similar considerations can be applied to a mixture of compositions between 5.5% and 55% alumina. When heated just above the eutectic temperature. a liquid of eutectic composition is formed first in an amount corresponding to 5.5% alumina. Any excess alumina left will react with the remaining silica to form mullite, a solid of composition 3Al2O3.2SiO2, which corresponds to 71.8% alumina and 28.2% silica by weight. With increasing temperature the amount of liquid formed increases, becoming progressively richer in alumina because of the solution of mullite in the original liquid. Thus the composition of the melt changes, as indicated by the curved liquidus line BC. For compositions lying between 55% and 71.8% alumina not all the mullite will be melted. The excess mullite will dissociate at a temperature of 1810°C (3290°F), forming corundum (crystalline alpha alumina) and liquid containing 55% alumina. This temperature point is known as the incongruent melting point, at which a
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compound dissociates to another compound and to a liquid of different composition from the original compound.
At temperatures below 1810°C (3290°F) mixtures of compositions greater than 71.8% alumina consist of the solid phases of corundum and mullite. At 1810°C mullite will dissociate into corundum and liquid (point C) and, with a further temperature increase, the corundum will gradually dissolve into liquid.
The amount of liquid phase and the mineralogical nature of the solid body can be calculated at any temperature for any known chemical composition of a mixture from the phase equilibrium diagram by means of the lever principle. For example, a mixture of 45% alumina (line yy) at a temperature below 1470°C (2680°F) is a solid body composed of tridymite and mullite coexisting in equilibrium. The tridymite present can be calculated from
%3.37%1000%8.71
%45%8.71 ====××××−−−−
−−−− tridymite
and the mullite will be 100% — 37.3% = 62.7%. The amount of liquid formed at the eutectic temperature is found from
%4.40%100%5.5%8.71
%45%8.71 ====××××−−−−−−−−
and the amount of solid phase mullite is 100% — 40.4% = 59.6%.
Under equilibrium conditions, a well-fired product containing 45% alumina and 55% silica will consist of 62.7% mullite and 37.3% cristobalite. At the eutectic temperature, the mixture will melt to a very viscous liquid in the amount of 40.4%. With increasing temperature the amount of liquid will increase until a temperature is reached at which the material will be completely melted. This temperature corresponds to the intersection of line yy with the liquidus curve BC.
The silica—alumina phase diagram data are applicable only to equilibrium conditions between reacting constituents and in pure binary systems. In practice, equilibrium is seldom reached because reactions between solid and solid, and even between solid and liquid, are sluggish, requiring a very long time for completion. Most commercial alumina—silica refractories always contain a certain amount of impurities such as basic oxides of iron, calcium, magnesium, and smaller amounts of alkaline metal oxides. These impurities greatly affect equilibrium relations between the two major components, silica and alumina, and considerably lower the eutectic temperature and alter the eutectic composition. The fluxing action of the basic oxides follows the order MgO < CaO < FeO < Na2O < K2O, the latter two being much more effective than the former three. Ferric oxide behaves as part of the refractory portion up to about 1300°C (2372°F). Above this temperature it begins to break down to ferrous oxide (FeO), especially in the presence of a high proportion of molten silicates.
The following practical conclusions can be drawn from the silica—alumina phase diagram.
1. Refractories of composition between 3% and 8% alumina should be avoided because they are close to the region of low eutectic temperature.
2. Refractoriness increases with an increase in alumina content. This applies particularly to refractoriness under load, which is determined almost en tirely by the amount of liquid formed and its viscosity.
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3. Alumina—sillica refractories show a wide softening temperature range extending from the temperature at which the liquid begins to form to the temperature at which the entire body melts. This accounts for the shrink age and deformation of these refractories under load at temperatures well below their fusion points. The compositions from 20% to 40% alumina are known as fireclay refractories, which are classified as super-duty, high-duty, medium-duty, and low-duty bricks, depending on their alumina content and degree of firing.
4. Crystals of mullite and alumina (corundum) are the only stable compounds of silica and alumina at temperature above 1710°C (3110°F). Mullite does not show any abnormality on heating, and its coefficient of expansion is fairly low. It follows that for high refractoriness, “green” bricks should be of a composition that yields the maximum mullite content on firing. Furthermore, in bricks of high mullite content, the glassy bond of the regular brick is replaced by a crystalline bond. This has led to the development of mullite refractories. which are made by fusing alumina and silica materials in any required proportion or by calcining sillimanite, a naturally occurring mineral of the composition Al2O3.SiO2 (63% Al2O3 by weight).
5. In the range of composition of 63.5% to 71.8% alumina, corundum appears due to the dissociation of mullite at 1810°C (3290°F). Compositions above 71.8% alumina yield a solid phase consisting of mullite and corundum only. It can be seen from the diagram (Fig. 9-3) that the first Iiquid begins to form at 1810°C. With an increasing amount of alumina the refractory consists mainly of crystalline corundum bonded with a glassy matrix formed from the molten impurities.
9-6 REFRACTORINESS VERSUS THE CERAMIC BOND
The refractoriness of refractory materials depends on their chemical and mineralogical composition. on their dimensional stability on heating, and to some extent on their texture. To obtain high refractoriness, manufacturing methods different from those for porcelain or stoneware need to be used. The main ingredient of the body is a highly refractory, non-plastic material that should have a sufficient dimensional stability at high temperatures. For this reason ingredients that tend to shrink considerably during firing, such as fireclay, diaspore clay, sillimanite, and magnesium oxide, must be well prefired to reduce their subsequent shrinkage on firing of the brick. The material is crushed into fractions of three different sizes that are mixed in suitable proportions to produce a mix of maximum density. The loose refractory aggregate is mixed with a suitable bonding agent to provide a mass with adequate workability for shaping and forming operations and to develop a ceramic bond on firing. Refractories are generally fired at much higher temperatures than ordinary ceramic wares. Firing produces a ceramic bond and insures the necessary dimensional stability of the product when it is used for high-temperature applications. A ceramic bond2 can be defined as a glassy matrix formed on cooling the liquid produced from the more fusible constituents of the mixture at firing temperatures.
The presence of the ceramic bond greatly increases the cold strength of a refractory, but it lowers its refractoriness at high temperatures. For high refractoriness the amount of glassy matrix should be as low as is compatible with the strength requirements of the refractory at room temperature.
The effect of the ceramic bond on refractoriness can be illustrated by referring to a few typical examples. Fireclay brick is made of a non-plastic, refractory material, which is well-fired clay, or old fireclay brick crushed to suitable size fractions, called grog. The grog is mixed with plastic fireclay as a bonding agent, which makes up as much as 50% of the total mixture. Such a mix gives a considerable amount of the ceramic bond on firing, accounting for gradual softening and low refractoriness-under-load of fireclay refractories (Fig. 9-4). When temperature during firing is sufficiently high and the time is long enough, the glassy matrix may be gradually replaced by crystals. This is due to the
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dissolution of certain compounds and the crystallization of elongated crystals of mullite. These tend to interlock with each other, giving rise to a strongly bonded mass, considerably increasing the refractoriness under-load. Magnesite brick is made of crushed. well-prefired magnesite bonded by an active magnesium oxide. The firing of “green” brick is carried out at a temperature above 1400°C (2550°F) and causes the amorphous magnesium oxide to convert into a crystalline, dense form called periclase. Peri-clase crystals, however, being of spherical shape, do not interlock with one another and thus do not contribute appreciably to the strength of the bond. This latter is determined only by the glassy matrix formed from the low-melting impurities present in the mixture. As this glassy matrix has a low melting point and its melt a low viscosity, the magnesite brick shows a low refractoriness-under-load, only 1500°C (2732°F), although its fusion point is about 2150°C (3900°F). (See Fig. 9-4.)
FIGURE 9-4 Refractoriness-under-load, 345 kPa (50 psi) of fireclay, magnesite, and silica brick.
Silica brick is made of crushed quartz, which is a non-plastic, refractory ingredient. This is mixed with only 2% lime which, on firing reacts with fine particles of silica to form calcium silicate (CaO.SiO2), functioning as a ceramic bond. This results in a relatively small amount of liquid having high viscosity, thereby explaining the high refractoriness of silica brick, which is close to the melting point of pure silica. In the presence of even small amounts of alumina or alkalies the liquid immiscibility ceases to exist, causing a considerable lowering of refractoriness for the brick. For high refractoriness, therefore, silica brick should be made of very pure quartzite. free from any appreciable amounts of such impurities as alumina and alkalies.
Dimensional Stability. Dimensional stability can be defined as the resistance of a material to any volume changes that may occur on its exposure to high temperatures over a prolonged time. These dimensional changes can be considered as permanent (irreversible) and reversible.
Irreversible changes may result in either the contraction or the expansion of a refractory. The permanent contraction is due to the formation of increasing amounts of liquid from the low-fusible constituents of the brick when it is subjected to a long period of soaking at high temperatures. The liquid gradually fills the pores in the body, causing a higher degree of vitrification and shrinkage. A typical example of such behavior is fireclay brick. The shrinkage of a refractory can also be caused by the transformation of one crystalline form into another. For example, magnesite brick, an amorphous magnesium oxide that is relatively light (specific gravity 3.05), is converted gradually to a dense
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crystalline form, periclase, of a specific gravity of 3.54. Such an increase in density is naturally accompanied by considerable shrinkage of the material.
On the other hand, the transformation of residual quartz in silica brick to tridymite and cristobalite at high service temperatures is accompanied by a decrease in the specific gravity and consequently by a volume increase. The specific gravity of quartz is 2.65, whereas those of tridymite and cristobalite are 2.26 and 2.32, respectively. This transformation accounts for the characteristic permanent expansion of silica brick in service. Reversible volume changes are directly related to the coefficient of thermal expansion.
9-7 CASTABLE AND FUSED REFRACTORIES
Castable refractories are made by mixing refractory aggregate of suitable grading, such as alumina—silicates or high alumina, with a refractory high-alumina cement and water to desired consistency. The mix is then either cast, rammed, gunned, or sprayed into shape and permitted to set until it becomes hydraulically bonded (see Section 9-11). On subsequent heating to high temperature this cementitious bond is dehydrated and replaced by a refractory bond that is developed between the matrix and aggregate particles. Castable refractories have been used in applications where abrasion resistance at elevated temperatures is required and as a protective barrier against corrosive attack by hot gases and liquids that are highly detrimental to other structural materials.
Other types of castable refractories are phosphate-bonded refractory bricks, mortars, ramming mixes, and plastic and cold setting castables. They may contain, as aggregate refractory oxides, carbides such as SiC and other mixed with phosphoric acid or aluminum acid phosphate, or alkali polymetaphosphates and other acid phosphates, and give rise to a phosphate bond between the aggregate particles. The phosphate-bonded alumina materials are highly resistant to thermal shock but have poor resistance to erosion.
To secure castable refractories and, to some extent, brick linings safely in a place, a special anchoring system in the form of either a hexagonal grid and/or studs of various design and shapes is required. These are made of carbon steel or stainless steels or some other heat-resisting alloys depending on the service temperature.
Fusion-cast refractories are produced by mixing suitable refractory ingredients and melting them in an electric-arc furnace at temperatures of 1760—2480°C (3200—4500°F). The resultant liquid is then poured into a mold made of graphite plates buried in refractory powder where the material solidifies and cools slowly in the mold to room temperature. The refractory ingots are then withdrawn from the mold and sawed into desired shapes and sizes. The fusion-cast process produces a unique refractory having a high density with little porosity due to large isolated voids, high hot strength, improved abrasion resistance, and better resistance to corrosive attack by molten liquids and hot gases. Fusion castings of ceramics has been limited mainly to alumina, mullite, zirconia, silica, chromia, and AZS (47% A12O3, 36.5% ZrO2, 16.5% SiO2) refractories.
9-8 SUPERREFRACTORIES
The equilibrium phase diagrams indicate that, in most cases, higher refractoriness can be attained by using pure oxides of high melting points. The presence of even small amounts of impurities considerably lowers the melting point and reduces refractoriness-under-load to a much greater extent than could be expected from the corresponding phase diagrams. Furthermore, the presence of the ceramic bond in a refractory represents an inherent weakness because it reduces their load-bearing characteristics, decreases their chemical resistance to slags and fluxes, and may adversely affect their other properties. Consequently, the development of high-refractory materials has been carried
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out along two lines—first, the use of very pure ingredients of high refractoriness and second, the elimination of the ceramic bond by adoption of special methods and techniques in manufacturing procedures.
These techniques may involve compacting of fine powders followed by sintering at a suitable temperature at which gradual crystallization at the grain boundaries occurs, binding the particles into a coherent, strong body. The resulting bond is the crystalline bond, whereas the sintered article is called a self-bonded refractory. Since the bond is composed of crystals of the same material as the particles, self-bonded refractories exhibit high refractoriness, approaching that of the pure material itself.
Hot pressing and liquid phase sintering are frequently used in shaping refractory compounds from powders. These methods increase the rate of densification and lead to strong products. Such methods contributed to the developments of technical ceramics based on alumina, zirconia, silicon carbide, silicon nitride, and various borides that provided materials of high temperature capabilities, excellent wear resistance, and improved brittleness.
Carbides. Carbides are characterized by very high melting points, but they lack oxidation resistance at high temperatures. The most important refractory materials are carbides of silicon and boron and interstitial carbides of the transition elements, such as zirconium and titanium. (See Table 9-1.) The most widely refractory carbide used is silicon carbide (SiC). It is hard but it has excellent resistance to oxidation to 1650°C (3000°F) because of formation of a protective SiO2 coating. Low thermal expansion and high thermal conductivity are factors contributing to its excellent thermal shock resistance. The principal bonds used in silicon carbide ceramics are the following: (1) oxide or silicate bond, (2) silicon nitride and oxynitride bonds, (3) recrystallized or sintered silicon carbide.
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For high-temperature usage in excess of 2500°C (4532°F) in vacuo, the carbides and borides are about the only suitable materials available because of their low volatility. Carbides of zirconium, tungsten, molybdenum, tantalum, niobium, and cerium can be used above 2000°C (3635°F) in neutral or reducing atmosphere. Titanium carbides, vanadium, and niobium carbides can be used up to 2500°C (4532°F) in a nitrogen atmosphere. Some of the carbides have the highest known melting temperature of materials, for example, hafnium carbide (HfC) at 3930°C (7100°F). Boron carbide is the hardest and most abrasion-resistant material available in massive form; its melting point is 2430°C (4400°F). It is used as armor because of high strength, high elasticity, and low density. Borides have poor oxidation resistance at elevated temperatures. TiB2 and ZrB2 have the electrical resistivity of the order of copper. They have strong covalent bonds and exists in two modifications: a low-temperature cubic crystal structure β transforming at about 2100°C (3810°F) to a high-temperature form a having a hexagonal zinc blend structure.
Nitrides. Nitrides are characterized by high melting points, but they have a low resistance to oxidation and poor chemical resistance. The two most industrially important nitrides are silicon nitride and boron nitride.
Boron nitride has the graphite structure and it resembles graphite in its lubricating properties. A cubic crystalline form of boron nitride, known as Borazon. has been produced under high pressure, 145 MPa (106 psi), and temperatures above 1650°C (3000°F) and has a hardness equal to that of diamond. Borazon can withstand temperatures up to 1930°C (3500°F) without becoming appreciably oxidized.
Silicon nitride (Si3N4) has a covalently bonded structure resulting from the tetrahedral arrangement of valence orbitals with 4 nitrogen atoms similar to SiO4 tetrahedra. These tetrahedra form a three-dimensional network by sharing corners such that each N is common to three tetrahedra. Silicon nitride (Si3N4) exists in two polymorphic forms: hexagonal β-Si3N4 and hexagonal α-Si3N4; the β form is the stable one at high temperatures. Silicon nitride can be made as a powder by a number of methods; the most commercially available one is by nitriding silicon at 1400°C according to the reaction
3Si + 2N2 = Si3N4 (α+ β) (9-11)
Because of its crystal structure and strong covalent bonding Si3N4 shows excellent intrinsic properties such as low thermal expansion, moderate elastic modulus, high thermal shock resistance, high strength, wear resistance, oxidation resistance, and thermal stability. Si3N4 powder is relatively easy to produce, but it is not easily converted to high-density products because the bonding is of covalent nature and the structure has only a few intrinsic vacancies.
A possible solution to the fabrication problem is to treat the β-Si3N4 structure with metallic oxides such as Al203, Y2O3, MgO, BeO, and others. A simultaneous replacement of silicon and nitrogen by aluminum and oxygen takes place giving the system Si—AI—O—N. Other metal atoms can also be substituted giving rise to new materials called Sialons with the three-dimensional structure formed by (Si, M)(O, N)4 tetrahedra. Here M stands for Al, Mg, Be, Y, or others. For example, the reaction sintered mixture of 50 mol% Si3N4, 25 mol% Al2O3 and 25 mol% AlN gives the sialon Si4Al 2N6O2 with a 97.1% theoretical density of 3.09 g/cm3. This material is stronger than reaction bonded Si3N4 and at the same time retains excellent thermal shock resistance. Sialons are of much scientific and industrial interest because their interatomic bonding may cover a wide spectrum from highly covalent to partial ionic bonding.
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Carbon and Graphite. Graphite and carbon are excellent refractory materials, but they can be used only under neutral or reducing conditions because they oxidize readily in air at elevated temperatures. For very high temperature applications, graphite is a more stable form than amorphous carbon, which is converted to crystalline graphite on prolonged heating at about 2500°C (4525°F). Of all the highly refractory materials, graphite is the easiest to shape by machining technique. At ambient temperatures, graphite is an exceptionally inert material; however, at higher temperatures, it becomes very reactive particularly in oxidizing gases. Up to about 800° (1472°F) in air the rate of oxidation is controlled by the structure and the purity of the graphite, following the Arrhenius equation but, at higher temperatures, the rate rapidly increases. Graphite has no melting point, but it sublimes at a temperature of 4200°C (7592°F). Graphite has become a very valuable refractory in high-temperature applications such as rocket nozzles and nozzle inserts. Many attempts have been made to improve graphite oxidation resistance by applying coatings of silicon carbide or molybdenum suicide or impregnating its surface by controlled melting of metals such as zirconium. The molten zirconium forms zirconium carbide, accounting for a good chemical bond with the graphite surface, whereas an outer layer of zirconium is oxidized to zirconia, protecting the graphite base from excessive oxidation.
Another type of artificial graphite can be made by pyrolytic processes. The pyrolytic process involves the thermal decomposition of a natural gas containing mainly methane on a heated surface and deposition of a solid product on the substrate surface. Generally, the temperature of the substrate material, which is usually commercial graphite, is maintained in the optimum range of 1750 to 2250°C (3180 to 4080°F), which is at about one-half or less of the melting point of the solid deposit, so that solid bulk diffusion is practically eliminated. During the decomposition of methane or its homologues, gaseous carbon condenses on the prepared graphite substrate surface. Carbon atoms are arranged in an orderly fashion, layer on layer, producing an ordered structure that has a higher strength-to-weight ratio than commercial graphite.
The strength of the pyrolytic graphite at 2200°C (4000°F) may be as high as 40 to 140 MPa (5.8—20.3 ksi). It appears that the nucleation of the pyrolytic solid occurs on the substrate surface in such a way that the growth of the crystal occurs along a low-index crystallographic plane. Thus, in pyrolytic graphite, the c axis of the deposit is oriented normal to the graphite substrate, regardless of its orientation. This results in a strong preferred orientation of the crystals that form the columnar structure common to pyrolytic materials. A high degree of anisotropy in pyrolytic graphite results in marked differences in thermal conductivity, electrical conductivity, coefficient of thermal expansion, and strength between the directions parallel to the surface and that perpendicular to the surface along the c axis. The most recent development in carbon materials is a glasslike carbon that exhibits a surface reactivity much lower than that of highly ordered pyrolytic graphite. Glasslike carbon is produced by slow carbonization of a cross-linked polymer with or without applied pressure. The porosity resulting from the evolution of decomposition products is controlled by subsequent heat treatment so that only closed micro-pores are produced.
INORGANIC CEMENTS Inorganic cements are materials that exhibit characteristic properties of setting and hardening when mixed to a paste with water. This makes them capable of joining rigid solid masses into coherent structures. Inorganic cements can be divided into hydraulic and non-hydraulic types, according to the way in which they set and harden. Hydraulic cements like Portland cement are capable of setting and hardening under water, whereas non-hydraulic cements like lime harden in the air and cannot be used under water.
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9-9 PORTLAND CEMENT COMPOSITION
Portland cement is the most important hydraulic cement used extensively in various types of construction, as in mortars, plasters, grouting, and concrete. Portland cement is obtained by burning an intimate mixture, composed mainly of calcareous and argillaceous materials, or other silica-, alumina-, and iron oxide-bearing materials, at a clinkering temperature of about 1400°C (2552°F).
The partially sintered material, called clinker, is then ground to a very fine powder. A small amount of gypsum, from 2% to 4%, is usually added to the clinker before grinding. The chemical analysis of Portland cement reveals its composition of calcium oxide, silica, alumina, iron oxides, magnesium oxide, and sulfur trioxide, but this does not indicate its complex chemical character. Microscopic investigations have proved that these oxide constituents exist in Portland cement mainly as calcium silicates and aluminates. They are mainly tricalcium silicate (3CaO.SiO2), dicalcium silicate (2CaO.SiO2), tricalcium aluminate (3CaO.Al2O3), and tetracalcium aluminoferrite (4CaO.Al2O3.Fe2O3). In the nomenclature of the cement industry these compounds are usually written as C3S, C2S, C3A, and C4AF, respectively, where C stands for CaO, S for SiO2, A for Al2O3, and F for Fe2O3. Small quantities of pentacalcium trialuminate (5CaO.3A12O3), free magnesium oxide and calcium oxide, calcium sulfate, and even smaller quantities of titanium dioxide and potassium and sodium oxide may also be present.
FIGURE 9-5 Comparison of compressive strengths of cement compounds. (From R. H. Bogue and W. Lerch Industrial and Eigineering Chemistry, 26 837, 1934.)
The properties of the four main cement compounds are illustrated by Fig.9-5, indicating that the most desirable constituent is the tricalcium silicate (C3S) because it hardens
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rapidly and accounts for the early high strength of the cement. The dicalcium silicate (C2S) hardens more slowly and behaves in a more complex way because it exists in three crystalline forms: α, β and γ. The α and β forms are stable at high temperatures and are slow-setting, but they gradually develop a strength nearly equal to that of the tricalcium silicate. The low-temperature γ form is relatively stable toward water and does not possess cementing properties. If the amount of dicalcium silicate (C2S) formed during the burning of the mixture is not too high, a rapid cooling of the clinkerwill inhibit the transformation of the a and forms to the y form.Both tricalcium aluminate (C3A) and tetracalcium aluminoferrjte (C4AF)give, on hardening, a product of low strength, which would tend to make them undesirable constituents. The presence, however, of some alumina andiron oxides in the raw mixture is necessary because they function as fluxes to lower the fusion temperature, thereby facilitating the recrystallization of the desirable tricalcium silicate from the liquid phase.
There are five major types of Portland cement covered by ASTM and federal specifications. The compositions of these cements in terms of their compounds are shown in Table 9-2.
Type I is the most universally used cement in concrete construction when the special properties specified for the other types are not required.
Types II, IV, and V are characterized by lower contents of tricalcium silicate and tricalcium aluminate. This accounts for their moderate or low heat evolution and the fact that smaller volume changes occur during hydration than in the Type I cement. Type IV is used for massive concrete work in which a low evolution of heat is required, whereas Type Vis used when high resistance to sulfate attack is essential. Type II also shows improved resistance to moderate sulfate action.
Type III contains a high proportion of tricalcium silicate and is known as high-early-strength cement, which hardens rapidly and shows high heat evolution. It is made by increasing the lime content of the cement and by finer grinding.
9-10 SETTING AND HARDENING OF PORTLAND CEMENT
Setting and hardening of hydraulic cements are the result of hydration reactions occurring between the cement compounds and water. When the cement is mixed with water to a paste, hydration reaction begins, resulting in the formation of gel and crystalline products. These are capable of binding the inert particles of the aggregate into a coherent mass. Setting is defined as the stiffening of the originally plastic mass of cement and water to such a consistency that no significant indentation of the mass is obtained when it is subjected to certain standardized pressures. Hardening follows setting and is the result of further hydration processes advancing gradually into the interior of the particle core. The strength developed by cement depends on the amount of gel formed and the degree of crystallization.
Hydration Reactions The course of hydration reactions is illustrated by the following chemical equations:
First, hydrates are formed from the corresponding anhydrous products that passed into the solution. The hydrates have lower solubility than their corresponding anhydrous
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products and begin to crystallize from solution when it becomes saturated with respect to anhydrous products. Hydration of the C3A occurs rapidly with the formation of hydrate crystals (C3A.6H2O), resulting in “flash set.” The hydrate crystals form a film over the silicate particles, inhibiting their further hydration, so that subsequent development of strength is slow and incomplete. The addition of gypsum retards the dissolution of the tricalcium aluminate because of the formation of the insoluble calcium sulfoaluminate of variable composition 3CaO.Al2O3.xCaSO4.yH2O, where x = 1 to 3 and y = 10.6 to 32.6. This reaction presents a high concentration of alumina in the solution, thus retarding the initial set of the cement. In the presence of iron oxide the amount of C3A in the cement is reduce because the corresponding amount of alumina combines with the iron oxide to form tetracalcium aluminoferrite. As this latter hydrates more slowly than tricalcium aluminate, less gypsum is required. Tetracalcium alumino ferrite combines with water to form crystals of tricalcium aluminate and a gel that is probably hydrated monocalcium ferrite.
Both dicalcium and tricalcium silicate hydrate at a slower rate than tricalcium aluminate and yield an amorphous mass (gel) of dicalcium silicate. Tricalcium silicate also releases excess lime as calcium hydroxide, which precipitates out of the saturated solution as crystals. This is believed to account for the high rate of hardening and early high strength of cement. Any water in excess of that which entered into the chemical reactions will fill the capillaries because the vapor pressure in the capillaries is less than that of the water in bulk. The capillary-held water tends to diffuse slowly into the inner cores of the cement particles, causing hydration. This results in a slow but continuous expansion of the hardened cement when totally immersed in water. This expansion is only of the order of a 0.1% increase in length per annum, but it must be allowed for in laying large masses of concrete. Heat of hydration may be immaterial in many cases, but it cannot be easily dissipated in certain engineering structures involving large masses of concrete. This may cause the temperature to rise by as much as 50°C (122°F), resulting in the possible cracking of the structure on cooling and the lowering of the strength and quality of the concrete. On the other hand, heat of hydration can be beneficial in cold-weather concreting.
Structure of Cement Paste. The hardened Portland cement paste consists of the calcium—silicate hydrate (C—S—H) products which, like other gels, contain a network of capillary pores and gel pores (Fig. 9-6). The total porosity of the paste is about 30% to 40% by volume, having a very wide pore-size distribution ranging from 10 to 0.002 m in diameter. This imparts to the hardened paste an extremely large surface area of 200 to 400 m2/g. The gel porosity, consisting of very small pores, is about 26%; the remaining porosity is due to a capillary network. The latter can be regarded as the remnants of the water-filled space of the initial fluid paste that is gradually filled with hydration products. The total porosity of the paste is an important factor in determining the strength and durability of the cement paste.
Fineness. Fineness of cement greatly affects the setting time and the strength of the hardened cement because the chemical activity of a solid is directly proportional to its surface area, which greatly increases with the increased fineness of particles. As hydration proceeds from the outside to the inner core of the particle, the smaller the particle, the greater the probability that nearly the whole core will be converted to gel and crystals. For coarser particles a considerable portion of the inner part will not be available for hydration. Consequently, the finer cement will develop more gel per unit weight than the coarser cement of the same composition. This accounts for a more rapid hardening and a greater strength of the finer cement as compared with that of the coarser one. On the other hand, too fine a cement tends to give considerable shrinkage on setting, and a compromise must be sought to obtain the optimum properties.
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Extremely high strength Portland cement pastes can be produced using specially ground cement with the assistance of surfactant grinding aids to make surface areas ranging from 0.6 to 0.9 m2Ig. When mixed with water and plasticizing agents, the hardened pastes show very low porosity and high compressive strength of 196 MPa (28.4 ksi). This is about twice the strength of the cement paste produced by conventional methods.
Very high strengths are also obtained by hot pressing conventional cement pastes under pressures of 196 to 392 MPa (28.4 to 56.8 ksi) at 150°C (302°F). A nearly zero porosity is obtained for the hardened cement paste that shows typical strength: 490 MPa (71 ksi) (compression), 44 MPa (6.4 ksi) (tensile) and 83 MPa (12 ksi) (shear). These values are about four times greater than the strength values for cement pastes produced by conventional methods. These techniques are still at the experimental stage, but they clearly indicate the potential possibilities of making the Portland cement concrete much stronger than that produced today.
When the hardened cement is exposed to dry air, it shrinks because of the loss of capillary water but then expands on rewetting in moist air. This causes reversible shrinking and expansion of the hardened cement on drying and wetting, respectively.
FIGURE 9-6 Structure of cement paste showing elongated crystals, X 3000.
9-11 ALUMINOUS CEMENTS
Aluminous or high-alumina cement is made by fusing a mixture of bauxite and limestone and grinding the resulting mass to the same fineness as that of Portland cement. The burning is accomplished at a temperature between 1490 and 1600°C (2714 and 2912°F) in a rotary kiln, blast furnace, or arc-type electric furnace. The typical composition of cement shows 35% to 40% CaO, 35% to 55% A1203, 5% to 15% FeO and Fe203, and 5% to 10% SiO2. The most important cement compounds formed on fusion are monocalcium aluminate (CA) and tricalcium pentaluminate (C3A5); furthermore, some pentacalcium pentaluminate and small amounts of dicalcium aluminosilicate. di-calcium silicate, and tetracalcium aluminoferrite are also present. Both monocalcium aluminate and tricalcium pentaluminate hydrate initially to a gel CaAl2(OH)8.6H2O, which gradually changes to a very stable, crystalline complex Ca3AI2(OH)10.3H2O and a gel of aluminum hydroxide Al(OH)3. Since the crystalline complex is stable on heating and even on dehydrating, the aluminous cement retains its strength at high temperatures, thereby accounting for its high refractoriness. hi contrast. Portland cement loses its strength rapidly and disintegrates at a temperature of 500°C (932°F) owing to the
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dehydration of the gel. The setting of high-alumina cement is similar to that of Portland cement, but its rate of hardening is very rapid, and full strength is attained in 24 h.
The advantage of rapid hardening of aluminous cement is offset to some extent by a rapid evolution of heat of hydration, which cannot be easily dissipated in a short time. This causes a considerable rise in the temperature of the Concrete mass in layers more than 50 or 80 mm (2—3 in.) thick, but it is an asset when concreting under freezing conditions. Overheating during the hardening process of aluminous cement affects the course of hydration reactions adversely and results in a product of low quality and strength. High-alumina cement, as compared with Portland cement, has superior chemical resistance to seawater, sulfate-bearing groundwater, and acid solutions that frequently occur in industrial wastewaters. However, high-alumina cement is less resistant than Portland cement to alkalies.
GLASSES Glass can be defined as an inorganic product of fusion that has been cooled to a rigid condition without crystallization. Although silica is a perfect glass-forming material, it has a very high melting point and cannot be melted alone at reasonable cost. Basic metal oxides are added to lower the fusion point and viscosity of the melt and thus make easier the fabrication of glass-wares.
The addition of about 25% by weight of sodium oxide results in the formation of sodium disilicate (Na2O.2SiO2). giving a eutectic mixture with silica with a melting point of 793°C (1460°F). Such glass shows little tendency to devitrification but, unfortunately, it is water soluble, making such a mixture of little use as a material of construction. The addition of suitable amounts of calcium oxide to the mixture gives a soda lime glass, which is insoluble in water.
9-12 COMMERCIAL GLASSES
Commercial glasses can be classified as soda lime or lime glasses, lead glasses, borosilicate glasses, and high-silica glasses. Their typical chemical compositions are given in Table 9-3.
Soda lime glasses have compositions approximating the formula Na2O.CaO.6SiO2. Additional small amounts of alumina and magnesium oxide are introduced to improve the chemical resistance and durability of glass. To mask the colors developed by contained iron compounds, minute amounts of coloring agents can be added. Soda lime glasses are produced in largest quantity because they are low in cost, resistant to devitrification, and relatively resistant to water. They are easily hot-worked and are widely used as window glass, electric bulbs, bottles, and cheaper tableware, where high-temperature resistance and chemical stability are not required.
Lead glasses, also called “flint” glasses, usually contain from 15% to 30% lead oxide. They are used for high-quality tableware, optical purposes, neon sign tubing, and in art objects because of their high luster. The glasses of high lead content, up to 80%, are used for extra dense optical glasses and for windows and shields to protect personnel from X-ray radiation. Lead glasses have a relatively low melting point, but they exhibit good hot work ability, high electrical resistivity, and high refractive indices. Borosilicate glasses contain virtually only silica and boron with a small amount of alumina and still less alkaline oxide. The substitution for alkali and basic alkali oxides of the lime glasses by boron and aluminum results in a glass of low thermal coefficient of expansion and high chemical resistance. The glass is known under the trade name Pyrex. Because of their high chemical stability, high thermal shock resistance, and excellent electrical resistivity, borosilicate glasses are extensively used in industry as piping, gauge glasses, laboratory ware, electrical insulation, and for some domestic purposes. Aluminosilicate glasses
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contain about 20% alumina, which imparts to them a high heat shock resistance and a heat resistance greater than that of borosilicate glasses.
Ninety-six percent silica glasses are made by chemically removing the alkalies from a borosilicate glass. After it has been melted and shaped to the desired oversized dimensions, the borosilicate glass, is heat treated. This causes a separation into two layers: one high in alkalies and boron oxide, the other high in silica. The alkali layer is dissolved in immersing the article in hot acid, leaving a porous. high-silica layer. By reheating the article at about 1200°C (2192°F), the glass becomes perfectly clear and vacuum tight. Ninety-six percent silica glasses are much more expensive than other types of glasses. They are used mainly where extreme thermal shock resistance and high temperature resistance up to 900°C (1652°F) are required.
Glasses possess high chemical resistance to most corrosive agents. Commercial silicate glasses are corroded only by hydrofluoric acid, hot concentrated phosphoric acid, and concentrated alkaline solutions. Borosilicate and high-silica glasses have much higher chemical resistance, and fused silica has even higher resistance. These glasses are actually used in the construction of chemical plants.
Fused, also called vitreous, silica is almost pure silica (99.6% to 99.9% SiO2) made by fusing pure quartz crystals or glass sand in an electric arc or a high-frequency furnace or in the oxyhydrogen flame. Since there are no fluxing constituents present. the fusion temperature is about 1750°C (3182°F), even though the molten glass is so viscous that it is very difficult to obtain complete homogeneity and freedom from bubbles.
Fused silica is available in a translucent and transparent variety. Transparent silica, also called fused quartz, is highly transparent to ultraviolet, visible, and infrared radiation and is much stronger mechanically, more resistant to devitrification, and less permeable to gases than the translucent form. Transparent silica is mostly used for optical instruments and other instruments for which high transparency to a wide range of radiation is required. It is very expensive material. The translucent form, or vitreous silica, owing to its lower price, is used mainly for wares for chemical plants, for chemical laboratory wares, and for electrical insulating materials in electrical heaters, furnaces. and the like.
Fused silica has a very low and regular coefficient of thermal expansion, which makes it highly resistant to thermal shock. Its high fusion point gives it stability over a wide range of temperature. The useful temperature range is limited, however, to about 1100°C (2012°F) because of flow and a tendency to devitrification. Various specialty glasses, such as optical glass, photosensitive glass, opal glass, radiation-absorbing glass, and metal-coated glasses, are also available.
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9-13 GLASS MANUFACTURE
Glass is manufactured by melting suitable materials in required proportions and fabricating the molten glass into desired articles. The melting is carried out in a glass-tank furnace. This process is always used for the mass production of glasses that can tolerate direct contact of the reacting mixture with the flame. The raw materials, together with cullet (broken glass), are fed at one end of the furnace, while the molten glass is continuously withdrawn at the other end so that the level of glass in the furnace remains constant. The flow is controlled, so that sufficient time is allowed for the complete melting and refining of the mass. The furnace temperature required to secure melting at a desirable rate is about 1500°C (2732°F). This corresponds to the viscosity of molten glass of about 10 Pas (100 P). From the refining section the molten glass travels slowly to the working pit, from which it is drawn for fabrication.
The temperature at this section is only about 1000°C (1852°F), giving the glass a viscosity of about i0 Pa’s (10e P). For small amounts or for special glasses, the melting is done in pots that are placed in the furnace. A pot is a one-piece refractory container for molten glass. The pots may be open or closed: closed pots are used for glasses that cannot be exposed directly to the flame.
Forming and Shaping. Formation and shaping of glass articles are usually accomplished by various casting techniques. Flat glass is produced by rolling a continuous stream of glass from a tank furnace passing between water-cooled rolls. Rods and tubes are made by a drawing process, while various containers and specific articles can be made by pressing, blowing, and similar operations. During the shaping of glass, internal stresses are produced due to temperature gradients developed within the glass during cooling. The most recent process involves casting on molten tin, which results in a nearly perfect surface of a plate. Most glass articles are now formed by highly complicated machines although. in certain cases, the old method of hand blowing has survived. The molten glass must possess an adequate range of working plasticity so as to be easily formed into articles of various shapes. The working plasticity is determined by the viscosity of glass, which varies with the temperature, as shown in Fig. 9-7. In the working range the viscosity of glass is from 10 to 1066 Pa’s (1076 P), which is suitable for shaping and forming operations. During working operations, the temperature decreases and the glass viscosity increases, making it stiff enough to support its own weight without deformation. At room temperature, the viscosity of glass is about 1O’ Pa’s (1020 P).
Annealing. The cooling of glass from its working range to room temperature is relatively rapid in practice and results in thermal stresses in the glass, which adversely affects its strength and physical properties. This adverse effect of rapid cooling can be eliminated by a proper heat treatment that consists of heating glass for a sufficiently long period in the annealing temperature range and cooling it slowly to room temperature Experience has shown that to prevent stresses in glass, cooling should be very slow during a short interval in the neighborhood of the glass transition temperature; after that. it may proceed at a more rapid rate. A proper annealing treatment produces a glass free from internal stress or strain and results in its higher density and higher refractive index. At the annealing temperature, the viscosity of glass is sufficiently low to permit a slight viscous flow in the mass. which results in relaxation of stress according to the Maxwell relation, as given by Equation 7-55. It is estimated that the relaxation time in the annealing range is about 100 s. although it may vary for different types of glass. In practice annealing schedules are based on experience, and an optimum cooling rate depends on the required properties of glass and the size of the specimen. Optical glasses are annealed for a longer period and are cooled very slowly in the neighborhood of the glass transition temperature (1/2 to 1°C/h). since any internal stress in the glass wall cause double refraction, which cannot be tolerated in optical glasses. Ordinary glassware, however, can be cooled at a
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much higher rate. The larger the size, the slower the cooling rate, for example, the mirror (5000 mm in diameter) built for the Mount Palomar observatory was cooled at a rate of less than 1°C/day (2°F/day) in the range 500 to 300°C (930 to 570°F).
Strengthening. Since the strength of the glass is determined by its surface conditions, it can be greatly increased by eliminating the larger surface flaws or introducing residual compressive stresses in the surface to counteract any present internal or applied tensile stresses. This process, called prestressing. can be accom plished by tempering or by chemical strengthening. Tempering or thermal strengthening involves heating the glass uniformly to the annealing temperature range to induce a slight viscous flow and then chilling the two outside glass surfaces very rapidly by blasts of air below the glass transition temper ature. This causes the glass skin to become rigid, while its interior is still in a viscous state. On further cooling the interior contracts, causing the compressive stress in the outside surfaces, while the glass interior is in tension (Fig. 9-8). The introduced compressive stress will counteract any tensile stress that may develop on loading the specimen, thereby considerably increasing the strength of glass. Tempered glass exhibits a strength up to 140 MPa (20 ksi) and an impact resistance from three to five times greater than that of annealed glass, but it retains the same appearance, clarity, hardness, and coefficient of expansion as the original glass. Chill-tempered glass cannot be cut, machined, or ground. since this would disturb the system of prestresses, resulting in disintegration of the glass into small, but fairly harmless, fragments. For this reason all machining operations must be done before the glass is tempered.
FIGURE 9-7 Viscosity—temperature curves for glasses. (Properties of Glasses and Glass-Ceramics, Cornrng, 1973)
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Chemical strengthening involves changes in the composition of the surface layer of the glass. which results in a material with a very low or sometimes zero coefficient of thermal expansion. The glass interior, however, maintains its high coefficient of thermal expansion. Thus, on cooling, the interior contracts much more than outside surfaces, causing compressive stresses in the glass surface (Fig. 9-8).
FIGURE 9-8 Distribution of residual stresses across the sections of glasses, tempered and chemically strengthened. (Engineering Glass, Modern Materials.
Vol.6, edited by B. W. Gonser, Copyright Academic Press Inc., New York. 1968.)
Chemical strengthening can be accomplished by surface crystallization, ion exchange, or surface glazing processes. Surface crystallization involves nucleation of crystals of lithium—aluminum—silicate glass, using titanium oxide as a nucleating agent. The resultant 13-eucryptite crystals have a negative expansion coefficient. On cooling, these crystals expand, introducing compressive stresses in the surface. The ion exchange process consists of heating a soda—alumina—titania—silica glass in a bath of molten lithium sulfate at 600°C (1110°F). Small lithium ions diffuse into the glass, replacing the larger sodium ions and forming -eucryptite, as above. When the same glass is immersed in a molten potassium salt, sodium ions are replaced by larger potassium ions, causing the glass surface to be in compression.
Finally, in surface glazing, the glass surface is coated with a finely powdered glass or crystalline material of the composition mentioned above and baked in an oven to produce a hard enamellike glazing. Chemically strengthened glass may attain a strength as high as 690 MPa (100 ksi).
9-14 GLASS-CERAMICS
Glass-ceramics cover the crystallized or devitrified glass produced by a controlled crystallization of a solid glass body. The term glass-ceramics should not be confused with ceramics made by bonding glass or other powder, even though crystallization may occur during the bonding process. Crystallization in glass can be induced in certain circumstances, although it is avoided in making transparent glass. Glasses that will crystallize reasonably easily usually have a relatively high proportion of modifying oxides. This weakens the three-dimensional glass network by introducing non-bridging oxygen ions, making possible the atomic rearrangements necessary for crystallization. It appears that the smaller cations of greater polarizing power enhance crystallization more than the larger cations. The following compositions are typical of glasses in which nucleation and crystallization have been commercially produced:
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Li 2O−A12O3 −SiO2 (9-16)
MgO−A12O3−SiO2 (9-17)
Li 2O−MgO−SiO2 (9-18)
Li 2O−ZnO−SiO2 (9.19)
A process of controlled crystallization involves the addition of nucleating agents such as TiO to the molten glass. These nucleating agents can be TiO2, ZrO2, CaF2, or metallic colloidal particles such as Pt, Au, Ag, and Cu. Then the melt is shaped by the usual glass-forming techniques to clear glass articles, which are subjected to a special heat treatment to convert the glass to a microcrystalline ceramic. The heat treatment consists of two steps (Fig. 9-9). First, the object is heated to a nucleation temperature T, which corresponds to glass viscosities in the range 1010 to 1011 Pa.s, and is soaked at this temperature. Second, after the nucleation period the temperature of the glass is raised at a rate of about 5°C/mm (9°F/mm) to a temperature of optimum crystal growth, Tcr. This is usually about 100°C (212°F) below the liquidus temperature. The resultant microstructure consists of very fine crystals ranging from 0.01 to 1 µm which ideally should be uniformly dispersed in a concentration from 1018 to 1021 nuclei/m3. On prolonged heating the number of crystals decreases and their size increases. This crystallization process is accompanied by optical changes from a transparent glass to an opaque polycrystalline material. The opacity is due to light scattering at interfaces between the crystal and the residual glass matrix, which have different refractive indexes. When crystals are very small, the glass-ceramic may be translucent and even transparent. Polycrystalline glasses are used in industrial and domestic applications under the trade name Pyroceram.
FIGURE 9-9 Heat treatment of glass ceramics. Tn is the nucleation temperature
Tcrist is the crystallization temperature.
Photosensitive Glasses. Photosensitive glasses have been developed using a lithium—alumina—silicate composition and inducing crystallization by metals such as Cu, Ag, and Au, which are photosensitive constituents. When such glasses are irradiated with ultraviolet light through a mask or a negative, a latent image forms in the glass because of the production of atoms of the photosensitive metals. On subsequent heating to a temperature just below the annealing point, the submicroscopic crystals of copper, silver, or gold are first formed by the aggregation of the irradiated metal atoms. These crystals serve as nucleation centers for lithium metasilicate crystals to form and grow. Since this crystallized region of glass is more soluble in hydrofluoric acid than the original glass matrix, intricate patterns can be etched. After proper machining, the glass may be exposed again to high temperatures for enhanced crystallization to produce a strong
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glass-ceramic article. To protect the glass surface from accidental mechanical damage or atmospheric attack and to impart specific properties to the glass surface, various coatings can be applied. These coatings may be transparent or reflective, they may impart vivid colors and decorative effects, they may enhance conductivity or resistivity, they may be opaque, they may filter light, and they may reflect or generate heat. Inorganic, organic, and metallic coatings are used, depending on the specific characteristics required of the product.
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