mse 352 engineering ceramics ii lecture notes · pdf filethe course is intended to teach...

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Prepared by: A. Andrews

MSE 352 ENGINEERING CERAMICS II

LECTURE NOTES

OBJECTIVES

The course is intended to teach students studying Materials Science and Engineering the

fundamental science underlying glass technology, namely: the kinetics and conditions necessary for

formation of glass (and glass ceramics), the structure of glass, and the relationship between

structure, composition and properties of glass. The course is also intended to teach students on

cement and concrete technology.

PREREQUISITE

The following courses should be revised.

Principles of Materials Science I & II (MSE 154, MSE 255)

Phase Transformation (MSE 260)

Engineering Ceramics I (MSE 351)

COURSE OUTLINE

Introduction to glasses; The glass state; Theories of glass formation; Kinetics of glass formation;

Kinetic treatment of glass; Glass forming ability and glass stability; Oxide glasses; Properties of

glasses including viscosity, chemical stability, conductivity, and strength; Glass ceramic processing

including raw materials, batch calculations, homogenizing of melts, glass forming, heat treatment of

glasses, glass ceramics and devitrification; Bioactive glasses; Cement, Concrete Technology.

RECOMMENDED BOOKS

1. Materials science and engineering: An introduction, 2010 – W.D. Callister Jr. and D.G.

Rethwisch

2. The science and engineering of materials, 2010 - D.R. Askerland, P.P. Fulay, and W.J.

Wright

3. Fundamentals of Inorganic Glasses. Academic Press, Inc., New York, 1994 - A.K.

Varshneya.

4. Physical Ceramics. – Y.-T Chiang, D.P. Birnie, W.D. Kingery, Wiley, New York, 1997

5. Advances in Ceramics, Volume 18: Commercial Glasses. American Ceramic Society Inc.,

Columbus, Ohio, 1986 - D.C. Boyd and J.F. MacDowell (Editors)

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1.0 Introduction

Most ceramic materials fall into an application–classification scheme that includes the following

groups: glasses, structural clay products, whitewares, refractories, abrasives, cements, and the

newly developed advanced ceramics. Fig. 1.1 presents taxonomy of these several types. For this

course the focus would be on Glasses and Cements.

Figure 1.1: Classification of ceramic materials on the basis of application (Callister & Rethwisch, 2010)

1.1 Glasses

When a liquid phase is cooled to below its freezing temperature, it usually transforms into a

crystalline solid, i.e. it crystallizes. Some liquids, because of complex molecular configuration or

slow molecular transport, do not “crystallize” (assume an ordered configuration) upon being cooled

to low temperatures, but instead form a rigid disordered network, known as glass, which is very

similar in structure to that of the liquid. Most metals and alkali halides crystallize easily upon

cooling through the freezing temperature because the structural rearrangement from the liquid to the

crystalline state is simple and bonding is nondirectional. At temperatures just above the freezing

temperature, most metals and molten salts have fluidities approximately equivalent to that of water

at room temperature. Thus, the required structural rearrangements can take place relatively easily.

In contrast, many inorganic silicates form glasses upon cooling because the fluidity at and even

above the freezing temperature is very low. This glass formation is related to the high silicon-

oxygen single-bond energies and the directional bonding requirements imposed by sp3 hybridization

of silicon. The disordered liquid cannot flow easily and thus cannot undergo the rearrangements

required for crystallization. Moreover, the fluidity decreases very rapidly as the temperature is

lowered. For large organic and polymeric liquids, crystallization is difficult because of their chain

lengths. In polymer systems the intermolecular bonding (Van der Waals) is weak and expected to

permit individual molecules to readily slide past one another. Thus, it is the “difficult” structural

rearrangement required for crystallization that induces glass formation in these systems.

Noncrystalline silicates contain other oxides such as CaO, Na2O, K2O, and Al2O3, which influence

the glass properties.

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2.0 The Glass State

The properties and chemical composition of the glass used for optical purposes differ greatly from

those of the glass used for baking dishes. Likewise, the glass used for window panes differs from

the glass used for artwork or optical purposes. However, most inorganic glasses have one thing in

common: they are ceramic materials (compounds that contain metallic and non-metallic elements,

most often oxygen) with the amorphous structure of a frozen liquid.

Quartz glass is not crystalline at room temperature because the rearrangement of interatomic bonds

between Si and O required for the establishment of order is impeded: the viscosity of glass is so

high at room temperature that millions of years would be required for it to crystallize. Hence, glass

can be viewed as a perfectly rigid “liquid”, being exceptional in that its extraordinarily high

viscosity slows down its flow properties even on an expanded time scale.

Upon cooling of liquid “glass” we observe that ΔV/ΔT (the contraction with lowering of T)

changes. This may be seen by following the three basic steps in the production of glass:

1. the melting of quartz sand (minute crystals of silica),

2. the shaping of the glass while in a viscous state and

3. the controlled cooling of the shaped article.

The SiO4 tetrahedra in the sand (crystals) are arranged in an ordered pattern (Fig. 2.1), but in the

molten state the crystal matrix breaks up into strings and rings of tetrahedra in irregular patterns

(Fig. 2.2). Because the temperature is very high, the groupings continually break up and reform,

making the mass fluid.

Figure 2.1: Basic Si - O - Si - units as established with sp3 hybridization of Si.

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Figure 2.2: Two-dimensional view of tetrahedra as in silicon dioxide structures: (a) in crystalline, ordered

structures and (b) in disordered glassy (amorphous) structures.

Upon cooling of molten SiO2, the tetrahedra form larger groupings and the glass becomes more

viscous, permitting it to be shaped by blowing, rolling, pressing, etc. With continuing cooling,

larger groupings of tetrahedra form as their kinetic energy decreases. These two phenomena are

responsible for the increase in viscosity and the shrinkage that takes place during the cooling period.

Beyond the glass transformation, temperature changes in configuration of the tetrahedra virtually

cease because the glass, to all intents and purposes, has lost its fluidity.

2.1 Theories of glass formation

Goldschmidt proposed that glasses of general formula RnOm form most easily when the ionic radius

ratio of the cation, R, to the oxygen ion lies in the range 0.2 to 0.4. Since the radius ratios in this

range tend to produce cation surrounded by four oxygen ions in a form of a tetrahedral,

Goldschmidt believed that only melts containing tetrahedrally-coordinated cations form glasses

during cooling.

Later Zachariasen also formulated a set of rules that are analogous to Pauling’s rules for crystal

structure stability. The thoughts can be summarized by his statement that formation of oxide glass

may occur if:

1) the material contains high proportion of cations, which are surrounded by either oxygen

triangles or oxygen tetrahedral

2) these polyhedral are connected only by their corners and

3) some oxygen atoms are linked to only such two cations and do not form additional bonds

with other cations.

Item (1) states that sufficient network cations must be present to allow continuous structure to form;

Item (2) states that the network is an open structure and item (3) states that sufficient bonds linking

the network polyhedral exist for the formation of continuous network structure. These rules are

quite useful for explaining the network forming capability of oxides such as B2O3, GeO2, P2O5, and

of course SiO2.

(a) (b)

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The replacement of a covalent bond between Si and O by an ionized oxygen and the cation (Na, K

etc.) is the crucial structural change that shortens the effective average chain length. Oddly, in the

borate glasses, the charge redistribution that occurs with alkali additions allows BO4 units to

participate on network formation, thereby increasing average network lengths. This effect explains

the apparent boron anomaly, and the minimum in thermal expansion coefficient that occurs as

alkalis are added to a borate glass.

2.2 Kinetics of glass formation

Crystallization refers to a combination of two processes: nucleation and crystal growth.

Crystallization requires the presence of a nucleus on which the crystals will subsequently grow to a

detectable size. The nucleus may either be homogenous (i.e. forming spontaneously within the melt)

or heterogeneous (i.e. forming at the preexisting surface) such as that due to impurity, crucible wall

etc. if no nuclei are present, crystal growth cannot not occur and the material will form a glass.

Even if some nuclei are present, but no growth has occurred, the extremely small size and low

volume fraction of the nuclei prevents their detection, so that the solid is, for all practical purposes,

still a glass.

The growth step in a phase transformation begins once an embryo has exceeded the critical size, r*,

and becomes a stable nucleus. Note that nucleation will continue to occur simultaneously with

growth of the new phase particles; of course, nucleation cannot occur in regions that have already

transformed to the new phase. Furthermore, the growth process will cease in any region where

particles of the new phase meet, because here the transformation will have reached completion.

2.3 Kinetic treatment of glass formation

Formation of a glass involves cooling from the melt in such a way as to prevent significant crystal

formation. Nucleation and crystal growth occurs simultaneously during cooling of a melt, with rates

that change continuously as the temperature decreases. A pragmatic approach to glass formation

must therefore deal with the interaction between these processes.

Most inorganic glasses can be made to transform from a noncrystalline state to one that is

crystalline by the proper high-temperature heat treatment. This process is called crystallization, and

the product is a fine-grained polycrystalline material that is often called a glass-ceramic. The

formation of these small glass-ceramic grains is, in a sense, a phase transformation, which involves

nucleation and growth stages. As a consequence, the kinetics (i.e., the rate) of crystallization may be

described using the same principles that were applied to phase transformations for metal systems.

For example, dependence of degree of transformation on temperature and time may be expressed

using isothermal transformation (TTT) and continuous cooling transformation (CCT) diagrams. The

continuous cooling transformation diagram for the crystallization of a lunar glass is presented in

Fig. 2.3; the start and end transformation curves on this plot have the same general shape as those

for an iron–carbon alloy of eutectoid composition. Also included are two continuous cooling

curves, which are labelled “1” and “2”; the cooling rate represented by curve 2 is much greater than

that for curve 1. As also noted on this plot, for the continuous cooling path represented by curve 1,

crystallization begins at its intersection with the upper curve, and progresses as time increases and

temperature continues to decrease; upon crossing the lower curve, all of the original glass has

crystallized. The other cooling curve (curve 2) just misses the nose of the crystallization start curve.

It represents a critical cooling rate (for this glass, 100 oC/min)—that is, the minimum cooling rate

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for which the final room temperature product is 100% glass; for cooling rates less than this, some

glass ceramic material will form. A nucleating agent (frequently titanium dioxide) is often added to

the glass to promote crystallization. The presence of a nucleating agent shifts the start and end

transformation curves to shorter times.

Figure 2.3: Continuous cooling transformation diagram for the crystallization of a lunar glass (35.5 wt%

SiO2, 14.3 wt% TiO2, 3.7 wt% Al2O3, 23.5 wt% FeO, 11.6 wt% MgO, 11.1 wt% CaO, and 0.2 wt% Na2O).

Also superimposed on this plot are two cooling curves, labelled “1” and “2.”

Thus TTT diagrams must be considered as a model for aiding our understanding of the glass

formation process. This is however, not an experimental tool. Melt viscosity is an important factor

in glass formation. The kinetic barrier to crystallization will be very high if the melt viscosity is

large at the temperature corresponding to the critical cooling rate. Melts which exhibit a large

barrier to nucleation also exhibit good glass forming behaviour. Inhibition of crystal growth due to

factors other than simply high viscosity also improves glass forming behaviour.

2.4 Glass forming ability and glass stability

Glass forming ability is defined in terms of resistance to crystallization of melt during cooling.

Glass forming ability is most important during processes requiring production of an initial glass.

Glass forming ability can be determined by estimating the critical cooling rate required to prevent

crystallization of a specified volume fraction of the sample. For two melts of identical size, the melt

with the smaller cooling rate has a better glass forming ability. A series of compositions can

therefore be ranked in order of improving glass forming ability, by decreasing values of their

critical cooling rates.

Glass stability is defined in terms of resistance to crystallization of glass during heating. Glass

stability is most important during processes involving re-forming of existing glass. Glass stability is

characterized by the difference in temperature between the onset of glass transformation region (Tg)

and the occurrence of crystallization, for a sample heated at a specified linear rate.

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2.5 Oxide glasses

The bonding forces responsible for the formation of network structure in crystalline SiO2 produce

similar networks in glasses of other oxides such as B2O3 and P2O5 which are thus also considered as

primary network formers. If an oxide, such as Na2O, is added to silica glass, a bond in the network

is broken and the relatively mobile sodium ion becomes a part of the structure (Fig. 2.4). Examples

of such network modifiers are N2O, K2O, Li2O, CaO, MgO and PbO.

Figure 2.4: Schematic glass networks; (a) SiO

2 network (b) SiO2

network modified through addition of Na2O

With increase in the amount of modifier, the average number of oxygen-silicon bonds forming

bridges between silicon atoms decreases as follows:

O/Si Ratio in Glass Bridging – O-Si Bonds/Si atom

2.0 4 (Fig. 2.4)

2.5 3

3.0 3

The bridging bonds per silicon atom will correspond, for example, to a linear chain structure (Fig.

2.5) and any further increase in modifier will reduce the length of the chain. The principal effect of

a modifier is to lower the melting and working temperature by decreasing the viscosity. An excess

of modifier can make the structural units in the melt sufficiently simple and mobile that

crystallization occurs in preference to the formation of a glass.

Although radius ratios are an excellent guide to the stability of ionic structures and other crystal

structures, the conditions under which glasses are stable require additional criteria as discussed

earlier under theories of glass formation.

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Figure 2.5: Schematic of SiO2 network; O to Si ratio is 2:1 as indicated by the chemical formula.

The compositions and properties of typical oxide glasses are listed in Table 2.1. Soda-lime glass

not only has the advantage of being made from inexpensive raw materials, but also the practical

convenience of low working temperatures (Fig. 2.6). Most glass-forming operations (pressing,

drawing, etc.) begin at the temperature corresponding to the working point, but can continue while

the glass cools.

Table 2.1: Composition and properties of typical oxide glasses

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Figure 2.6: Important temperatures in fabrication of silicate glasses (the temperatures are related to viscosity

which in turn controls workability).

3.0 Properties of Glasses

The solidification of glassy materials is different from that of crystalline materials. Upon cooling, a

glass becomes more and more viscous in a continuous manner with decreasing temperature; there is

no definite temperature at which the liquid transforms to a solid as with crystalline materials. One

of the distinctions between crystalline and noncrystalline materials lies in the dependence of

specific volume (or volume per unit mass, the reciprocal of density) on temperature, as shown in

Fig. 3.1.

For crystalline materials, there is a discontinuous decrease in volume at the melting temperature Tm.

However, for glassy materials, volume decreases continuously with temperature reduction; a slight

decrease in slope of the curve occurs at what is called the glass transition temperature, or fictive

temperature, Tg. Below this temperature, the material is considered to be a glass; above, it is first a

supercooled liquid, and finally a liquid.

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Figure 3.1: Contrast of specific volume versus temperature behavior of crystalline and noncrystalline

materials. Crystalline materials solidify at the melting temperature Tm. Characteristic of the noncrystalline

state is the glass transition temperature Tg (Callister & Rethwisch, 2010).

3.1 Viscosity of glasses

Also important in glass-forming operations are the viscosity–temperature characteristics of the

glass. The fluidity of liquids (the reciprocal of the viscosity) is a measure of their ability to flow. In

turn, the viscosity (η) (Fig. 3.2) is a measure of their resistance to flow. Viscosity is formulated as

the ratio of the applied stress to the resultant shear strain rate – its dimension is in Newton seconds

per meter squared (Nsm-2).

Figure 3.2: Dimensional analysis of viscosity.

Liquid flow, involving the motion of molecules past one another, requires the breaking and making

of new bonds. Thus the fluidity, like chemical reactions and diffusion in solids is expected to

exhibit exponential temperature dependence and may be modelled as:

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Fig. 3.3 plots the logarithm of viscosity versus the temperature for fused silica, high silica,

borosilicate, and soda–lime glasses. On the viscosity scale, several specific points that are important

in the fabrication and processing of glasses are labelled:

1. The melting point corresponds to the temperature at which the viscosity is 10 Pa.s (100 P);

the glass is fluid enough to be considered a liquid.

2. The working point represents the temperature at which the viscosity is 103 Pa.s (104 P); the

glass is easily deformed at this viscosity.

3. The softening point, the temperature at which the viscosity is 4 x 106 Pa.s (4 x 107 P), is the

maximum temperature at which a glass piece may be handled without causing significant

dimensional alterations.

4. The annealing point is the temperature at which the viscosity is 1012 Pa.s (1013 P); at this

temperature, atomic diffusion is sufficiently rapid that any residual stresses may be removed

within about 15 min.

5. The strain point corresponds to the temperature at which the viscosity becomes 3 x 1013

Pa.s (3 x 1014 P); for temperatures below the strain point, fracture will occur before the onset

of plastic deformation. The glass transition temperature will be above the strain point.

Most glass-forming operations are carried out within the working range – between the working and

softening temperatures. The temperature at which each of these points occurs depends on glass

composition. For example, the softening points for soda–lime and 96% silica glasses from Fig. 3.3

are about 700 and 1550 oC, respectively. That is, forming operations may be carried out at

significantly lower temperatures for the soda–lime glass. Thus formability of a glass is tailored to a

large degree by its composition.

Figure 3.3: Logarithm of viscosity versus temperature for fused silica and three silica glasses (Callister &

Rethwisch, 2010).

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3.2 Chemical stability

All glasses are immune to oxidation (decay) because their atoms are fully oxidized. Glass composed

of 100 percent silica tetrahedra is extraordinarily inert. It resists the action of most acids (the most

notable exception is hydrofluoric acid, utilized to etch glass), but it is attacked by strong alkalis (the

bottles containing liquid alkalis on a chemist’s shelf have a clouded surface).

3.3 Conductivity

The ability of a material to conduct electricity is dependent upon the presence of electrons in the

conduction band, separated in glasses by as much as 10 eV from the valence band. Because the

electrons in glass are tightly bonded, glass is a very poor conductor of electricity - in fact, the large

energy gap makes glass an excellent insulator. Glass is also a poor conductor of heat. Thus, hot

common glass may crack when cold water is poured onto it because the surface exposed to the cold

water will shrink while the dimensions of the interior remain unaffected.

3.4 Strength

Glass is harder than many types of steel and is also very elastic, as evidenced by the speed and

accuracy with which glass marbles rebound when they meet. Glass has “no” crystal structure, and

hence the phenomenon of slip cannot take place. This, together with the strong bonding between

atoms, gives glass a very high compressive strength and a theoretical tensile strength of about 107

kN/m2 (significantly higher than that of steel). Glass fibers with an actual tensile strength of 4 x 107

kN/m2 have been produced. Since glass has a “liquid structure” it may be considered “saturated with

dislocations”.

If a piece of metal has a very high dislocation density (e.g., due to cold working) the dislocations

interfere with each other’s movements. Therefore, with increasing dislocation density a piece of

metal becomes harder and stronger, but also more brittle.

Since the molecular structures in glass are unable to move, the presence of minute cracks or

imperfections in glass permits stress concentrations to localize and exceed the bond strength

between atoms - common glass will crack. Thus, in actual practice, the strength of glass is, by a

factor of 100 to 1000, less than the theoretical strength, and glass is brittle. For example, a freshly

made electric light bulb may not crack when initially dropped, but its surface becomes damaged

and, after it rebounds and strikes the floor a few more times, it breaks.

The inherent high strength of fibrous glass is utilized in the fiberglass sections employed for boats

and automobile bodies. Before being pressed into shape, fiberglass is mixed with a synthetic resin

which serves to protect the fibers from scratching. Since the fibers thus retain their high strength,

fiberglass sections are very strong for their weight.

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4.0 Glass Ceramic Processing

4.1 Raw materials

Glass is produced by heating the raw materials to an elevated temperature above which melting

occurs. Most commercial glasses are of the silica–soda–lime variety; the silica is usually supplied as

common quartz sand, whereas Na2O and CaO are added as soda ash (Na2CO3) and limestone

(CaCO3). For most applications, especially when optical transparency is important, it is essential

that the glass product be homogeneous and pore free. Homogeneity is achieved by complete

melting and mixing of the raw ingredients. Porosity results from small gas bubbles that are

produced; these must be absorbed into the melt or otherwise eliminated, which requires proper

adjustment of the viscosity of the molten material.

It is important that during the melting of the components of glass (high quality silica sands, soda

ash, modifying oxides), adequate mixing in what are often relatively viscous melts (compared with

metals), are ensured and remove product on a continuous or semi-continuous basis. Recycled glass

is useful in order to provide a low melting point material (the cullet) that then dissolves the solid

oxides in the feedstock (silica, alumina, soda ash). Gas fired reverberatory furnaces are typical,

although electrical heating is also very useful, especially if it is important to avoid contamination.

Mixing is a significant challenge and additives such as sulphate powders are used ("fining") to

provide large volumes of gas bubbles which provide a strong stirring action. Refractories are very

important to glass melting furnaces: high temperature oxides such as zirconia-alumina-silica

mixtures are used. For very high purity systems, platinum crucibles are used, e.g. glass fibers. The

names of many of the raw materials and their compositions are listed in Table 4.1. Gravimetric

factors which allow calculation of the yield of the desired glass component for each weight unit of

raw material are also listed in the table.

Table 4.1: Raw materials for glass making

Common Name Nominal Composition Gravimetric Factor

Albite Feldspar Na2O-Al2O3-6SiO2 Na2O=8.46; Al2O3=5.14; SiO2=1.45

Alumina Al2O3 Al2O3=1.00

Anorthite feldspar CaO-Al2O3-2SiO2 CaO=4.96; Al2O3=2.73; SiO2=2.32

Whiting CaCO3 CaO=1.79

Dolomite CaCO3-MgCO3 CaO=3.29; MgO=4.58

Bone ash 3CaO-P2O5 CaO=1.84; P2O5=2.19

Borax Na2O-B2O3.10H2O Na2O=6.14; B2O3=2.74

Regardless of the source of the components used to produce a specific glass, the batch materials can

be divided into five categories on the basis of their role in the process: glassformer, flux, property

modifier, colorant and fining agent.

The same compound may be classed into different categories when used for different purposes. The

most essential component of any glass batch is the glassformer. They are also called network

formers or glass forming oxides in many glass oxides. The identity of these components usually

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serves as the basis for the generic name used for the glass. For example, silicate glass means the

glassformer is silica. Borosilicate means major components are boric acid and silica.

Fluxes reduce the processing temperature within acceptable limits. The most common fluxes

include alkaline oxides (Na2O) and PbO. The use of PbO is being discouraged due to environmental

concerns. While addition of fluxes decreases cost of glass formation, the addition of large amount

of alkali oxides results in serious degradation in many properties. Degradation in properties is

usually countered by addition of property modifier which includes alkaline earth and transition

metal oxides and most importantly aluminium oxide.

Colorants are used to control the colour of the final glass product. In most cases, colorants are

oxides of either the 3rd transition metals or the 4f rare earths. Gold and silver are used to produce

colours by formation of colloids in glasses. When colorants are used to counteract the effect of

other colorants to produce slightly grey glass, they are referred to as decolourant.

Fining agents are added to glass forming batches to promote the removal of bubbles from the melt.

Fining agents include the arsenic and antimony oxides, potassium and sodium nitrates, NaCl,

fluorides such as CaF2, NaF and Na3AlF6. These materials are present in small quantities (<1wt%).

4.2 Batch calculations

Glass batch calculations can range from very simple to very complex, as a function of the

complexity of composition and the raw material used to prepare the mixture.

Steps for batch calculation:

1. Determine the molecular weight of each component required to produce the desired molar

composition

2. Multiply the molecular weight by the mass fraction of each component

3. Total these contributions to determine the molecular weight of the glass

4. Divide each individual contribution by molecular weight of the glass to determine the

weight fraction of each component

5. Multiply the weight fraction of each component by the amount of glass to be produced

6. The batch weight of any component which decomposes during melting is adjusted by

multiplying the weight fraction of that component by appropriate gravimetric factor for the

raw material actually used in the batch.

Work examples

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4.3 Homogenizing of melt

The fluid produced during the initial batch decomposition process is very heterogeneous. This

heterogeneity is gradually reduced by stirring action of rising bubbles during the firing process.

Production of acceptable homogenous glass requires additional time for diffusion process to

improve the homogeneity of the melt. Gross defects such as bubbles, seed and stones (particles of

undissolved materials) are often clearly visible and results in rejection of commercial glasses.

Density and refractive index measurements on crushed glasses are used as relative measurement of

intensity of inhomogeneity of glass. Poor homogeneity frequently results from poor mixing of the

original batch materials.

4.4 Glass forming

Five different forming methods are used to fabricate glass products: pressing, blowing, drawing,

sheet and fiber forming. Pressing is used in the fabrication of relatively thick-walled pieces such as

plates and dishes. The glass piece is formed by pressure application in a graphite-coated cast iron

mould having the desired shape; the mould is ordinarily heated to ensure an even surface.

Although some glass blowing is done by hand, especially for art objects, the process has been

completely automated for the production of glass jars, bottles, and light bulbs. The several steps

involved in one such technique are illustrated in Fig. 4.1. From a raw gob of glass, a parison, or

temporary shape, is formed by mechanical pressing in a mould. This piece is inserted into a

finishing or blow mould and forced to conform to the mould contours by the pressure created from

a blast of air.

Figure 4.1: The pressand-blow technique for producing a glass bottle (Callister & Rethwisch, 2010).

Drawing is used to form long glass pieces such as sheet, rod, tubing, and fibers, which have a

constant cross section. The technique normally used is the float process shown in Fig. 4.2. With this

technique, the molten glass passes (on rollers) from one furnace onto a bath of liquid tin located in a

second furnace. Thus, as this continuous glass ribbon “floats” on the surface of the molten tin,

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gravitational and surface tension forces cause the faces to become perfectly flat and parallel and the

resulting sheet to be of uniform thickness.

Furthermore, sheet faces acquire a bright “fire-polished” finish in one region of the furnace. The

sheet next passes into an annealing furnace (lehr), and is finally cut into sections. The success of

this operation requires rigid control of both temperature and chemistry of the gaseous atmosphere.

Figure 4.2: Schematic diagram showing the float process for making sheet glass (Callister & Rethwisch,

2010)

4.5 Heat treatment of glasses

Annealing of glass

When a ceramic material is cooled from an elevated temperature, internal stresses, called thermal

stresses, may be introduced as a result of the difference in cooling rate and thermal contraction

between the surface and interior regions. These thermal stresses are important in brittle ceramics,

especially glasses, because they may weaken the material or, in extreme cases, lead to fracture,

which is termed thermal shock. In order to relieve the internal stresses that may develop during

glass formation, all commercial glassware is annealed (slowly cooled) immediately after shaping.

This slow cooling operation is carried out in industry by placing the shaped objects on a conveyor

belt which transports them first through a reheating zone (annealing point) and then through

sections having successively lower temperatures in a furnace called the lehr to room temperature.

Strengthened glass

The glass scientist has produced objects made of glass which can be struck with a hammer or

dropped from a tall building without breaking. Two techniques are employed in strengthening glass

- one physical (glass tempering) and one chemical (ion exchange). Both techniques are based on

the fact that, when there is the slightest imperfection (minute scratch) on the surface, glass remains

extraordinarily strong in compression but becomes weak in tension. Thus the strengthening

treatment consists of prestressing a glass object by inducing compressive strains in its exterior and

thereby enabling it to counteract any tensile stresses which develop under tension.

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In physical prestressing, the shaped object is heated to just below its softening point (surface flaws

are annealed out) and its surface is then chilled by means of a blast of air or an oil bath. Under these

conditions the exterior of the glass cools and contracts immediately, but since glass is a poor

conductor of heat the interior will not contract. The glass will not crack because the interior remains

plastic. When the interior starts to cool, it cannot contract because the exterior has already set, but,

in attempting to contract, the interior continues to draw the exterior together. A built-in compressive

stress then develops in the outer layers of the glass. When a tensile stress is applied to this glass, it

is counteracted by these compressive stresses. Thus the prestressed glass will not shatter until the

surface compression is exceeded. As long as its surface remains intact, prestressed glass continues

to be strong and shock-resistant. However, if a deep scratch develops on the surface, the tensile

stresses set up in the interior by the surface compression are released and the glass will shatter into

thousands of tiny particles. Therefore prestressed articles must be cut to exact size and all holes

drilled prior to prestressing. Among the many uses for this type of glass are plate-glass doors, side

windows of automobiles and eyeglass lenses.

Ion exchange is a chemical technique for prestressing glass objects. The first step is to place the

shaped object into a molten salt bath containing potassium ions, which replace the sodium ions on

the surface of the glass. The potassium ions are larger than the sodium ions and, in accommodating

them, the surface of the glass becomes more crowded - thus inducing compressive strains on the

exterior. Glass of this type, characterized by very high strength, is useful for a great variety of

applications. Its flexural strength can be as high as 106 kN/m2

compared with 105 kN/m2

for

untreated glass.

4.6 Recrystallized glass (glass ceramics)

Recrystallized glass, also known as polycrystalline glass, is commonly produced by adding

nucleating agents to the glass batch. Subsequently the glass can be formed into a desired shape by

any of the conventional glass forming processes (e.g. blowing and drawing) and then be heat-treated

to promote recrystallization. One such set of heat treatments for a Li2O–Al2O3–SiO2 glass-ceramic

is detailed in the time-versus-temperature plot of Fig. 4.3. After melting and forming operations,

nucleation and growth of the crystalline phase particles are carried out isothermally at two different

temperatures. Recrystallized glass possesses increased impact strength, hardness and thermal shock

resistance compared with conventional non-crystalline glasses. One common place application of a

recrystallized glass is in the manufacture of the so-called refrigerator-to-oven cooking dishes.

4.7 Devitrification of glass

Under certain conditions glass will become “contaminated” with crystalline particles. A glass in this

condition is termed devitrified, which is simply another way of stating that it has partially or

completely crystallized. Devitrified glass, unless of the very special variety that is deliberately

produced, is undesirable since the crystalline areas form large crystals and are extremely weak and

brittle, as well as being translucent only. Crystalline segregations in glass are known as stones.

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Figure 4.3: Typical time-versus-temperature processing cycle for a Li2O–Al2O3–SiO2 glass-ceramic

(Callister & Rethwisch, 2010).

5.0 Bioactive Glasses

Bioactive material: A material that elicits a specific biological response at the interface of the

material, resulting in the formation of a bond between the tissues and the material.

Materials that can be classified as bioceramics include alumina, zirconia, calcium phosphates, silica

based glasses or glass ceramics, and pyrolytic carbons

The use of bioceramics has become very important in recent times because of the many advantages

they possess. Bioceramics are inert in the body (or bioactive in the body), chemically inert in many

environments, high wear resistance (orthopaedic and dental applications), high modulus and

compressive strengths, and has esthetic value for dental applications. They are however, brittle, has

low tensile strength (except fibers) and poor fatigue resistance.

Bioceramics are available as micro‐spheres, thin layers or coatings on a metallic implant, porous

networks, composites with a polymer component, and large well polished surfaces.

Bone is considered as one of the natural ceramic composites. The mineral component of bone is

hydroxyapatite (Ca5(PO4)3OH). The apatite (reinforcement) is brittle whilst the soft part (matrix) is

collagen. Hence synthetic calcium phosphates are used as biomaterials. Basic application areas

include its use as orthopaedic – bone plates and screws, total and partial hip components, coatings

of metal prostheses for controlled implant/tissue interfacial response, space filling of diseased bone,

vertebra prostheses and vertebra spacers. Table 5.1 summarizes some of the application areas of

bioceramics.

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Table 5.1: Biomedical applications of bioceramics (Trends Biomater. Artif. Organs, Vol 18 (1), 2004

6.0 Cement

Several familiar ceramic materials are classified as inorganic cements: cement, plaster of paris, and

lime, which, as a group, are produced in extremely large quantities. The characteristic feature of

these materials is that when mixed with water, they form a paste that subsequently sets and hardens.

This trait is especially useful in that solid and rigid structures having just about any shape may be

expeditiously formed. Also, some of these materials act as a bonding phase that chemically binds

particulate aggregates into a single cohesive structure. Under these circumstances, the role of the

cement is similar to that of the glassy bonding phase that forms when clay products and some

refractory bricks are fired. One important difference, however, is that the cementitious bond

develops at room temperature.

Of this group of materials, Portland cement is consumed in the largest tonnages. The properties of

Portland cement, including setting time and final strength, to a large degree depend on its

composition. Portland cement is termed a hydraulic cement because its hardness develops by

chemical reactions with water. It is used primarily in mortar and concrete to bind aggregates of inert

particles (sand and/or gravel) into a cohesive mass; these are considered to be composite materials.

Other cement materials, such as lime, are nonhydraulic; that is, compounds other than water (e.g.

CO2) are involved in the hardening reaction.

6.1 Types of Portland cement

The great majority of Portland cements made throughout the world are designed for general

constructional use. The standard specifications with which such cements must comply are similar,

but not identical, in all countries and various names are used to define the material, such as Class

42.5 Portland cement in current European and British standards (42.5 is the minimum 28-day

compressive strength in MPa), Types I and II Portland cement in the ASTM (American Society for

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Testing and Materials) specifications used in the USA, or Ordinary Portland Cement (OPC) in

former British standards.

Standard specifications are, in general, based partly on chemical composition or physical properties

such as specific surface area, and partly on performance tests, such as setting time or compressive

strength developed under standard conditions. The content of MgO is usually limited to 4-5%,

because quantities of this component in excess of about 2% can occur as periclase (magnesium

oxide), which through slow reaction with water can cause destructive expansion of hardened

concrete.

Free lime (calcium oxide) can behave similarly. Excessive contents of SO3 can also cause

expansion, and upper limits, typically 3.5% for ordinary Portland cements, are usually imposed.

Alkalis (K2O and Na2O) can undergo expansive reactions with certain aggregates, and some

specifications limit the content, e.g. to 0.6% equivalent Na2O (Na2O + 0.66 K2O).

Other upper limits of composition widely used in specifications relate to matter insoluble in dilute

acid, and loss on ignition. Many other minor components are limited in content by their effects on

the manufacturing process, or the properties, or both, and in some cases the limits are defined in

specifications.

Rapid-hardening Portland cements have been produced in various ways, such as varying the

composition to increase the alite content, finer grinding of the clinker, and improvements in the

manufacturing process, e.g. finer grinding or better mixing of the raw materials. The alite contents

of Portland cements have increased steadily over the one and a half centuries during which the latter

have been produced, and many cements that would be considered ordinary today would have been

described as rapid hardening only a few decades ago. In the ASTM specifications, rapid-hardening

Portland cements are called high early strength or Type III cements. For both ordinary and rapid-

hardening cements, both lower and upper limits may be imposed on strengths at 28 days, upper

limits being a safeguard against poor durability resulting from the use of inadequate cement

contents in concrete.

Destructive expansion from reaction with sulphates can occur not only if the latter are present in

excessive proportion in the cement, but also from attack on concrete by sulphate solutions. The

reaction involves the A12O3-containing phases in the hardened cement, and in sulphate-resisting

Portland cements its effects are reduced by decreasing the proportion of the aluminates phase,

sometimes to zero. This is achieved by decreasing the ratio of A12O3 to Fe2O3 in the raw materials.

In the USA, sulphate resisting Portland cements are called Type V cements.

White Portland cements are made by increasing the ratio of A12O3 to Fe2O3, and thus represent the

opposite extreme in composition to sulfate resisting Portland cements. The normal, dark colour of

Portland cement is due to the ferrite, formation of which in a white cement must thus be avoided. It

is impracticable to employ raw materials that are completely free from Fe2O3 and other

components, such as Mn2O3, that contribute to the colour. The effects of these components are

therefore usually minimized by producing the clinker under slightly reducing conditions and by

rapid quenching. In addition to alite, belite and aluminate, some glass may be formed.

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The reaction of Portland cement with water is exothermic, and while this can be an advantage under

some conditions because it accelerates hardening, it is a disadvantage under others, such as in the

construction of large dams or in the lining of oil wells, when a cement slurry has to be pumped over

a large distance under pressure and sometimes at a high temperature. Slower heat evolution can be

achieved by coarser grinding, and decreased total heat evolution by lowering the contents of alite

and aluminate. The ASTM specifications include definitions of a Type II or 'moderate heat of

hardening' cement, and a more extreme Type IV or 'low heat' cement. The Type II cement is also

suitable for conditions exposed to moderate sulfate attack, and is widely used in general

construction work. Heat evolution can also be decreased by partially replacing the cement by flyash

(pulverized fuel ash; PFA).

6.2 Chemical composition of Portland cement

Modern straight Portland cement is a very finely ground mix of Portland cement clinker and a small

amount (typically 3% to 7%) of gypsum (calcium sulfate dihydrate) and/or anhydrite (calcium

sulfate). Cement chemistry is generally denoted in simple stoichiometric shorthand terms for the

major constituent oxides. Table 6.1 provides the shorthand notation for the major oxides in the

cement literature. Table 6.1 also shows the typical chemical composition of modern Portland

cement and its clinker. For clinker, the oxide compositions would generally not vary from the rough

averages shown by more than 2% to 4%. The oxide composition of Portland cement would vary

slightly depending on its actual gypsum fraction or whether any other additives are present.

Table 6.1: Chemical Shorthand and Composition of Clinker and Portland Cement

6.3 Mineralogy of Portland cement and its clinker

The major oxides in clinker are combined essentially into just four cement or clinker minerals,

denoted in shorthand: tricalcium silicate or ‘alite’ (C3S); dicalcium silicate or ‘belite’ (C2S);

tricalcium aluminate (C3A); and tetracalcium aluminoferrite (C4AF). These formulas represent

averages, ignoring impurities commonly found in actual clinker. It is the ratios of these four

minerals (and gypsum) that determine the varying properties of different types of Portland cements.

Table 6.2 provides the chemical formulas and nomenclature for the major cement oxides as well as

the function of each in cement mixtures.

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As indicated in Table 6.2, some of the minerals in clinker serve different functions in the

manufacturing process while others impart varying final properties to the cement. The proportion of

C3S, for example, determines the degree of early strength development of the cement. The “ferrite”

mineral’s (C4AF) primary purpose, on the other hand, is to lower the temperature required in the

kiln to form the C3S mineral, and really does not impart a specific property to the cement.

Table 6.3 presents the common mineralogical compositions of Types I through IV cements and the

unique properties of each type.

Table 6.2: Typical Mineralogical Composition of Modern Portland Cement

Table 6.3 Typical Range in Mineral Composition in Portland Cements

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6.4 Physical properties of Portland cement

Portland cement consists of individual angular particles with a range of sizes, the result of

pulverizing clinker in the grinding mill. Approximately 95% of cement particles are smaller than 45

micrometers, with the average particle around 15 micrometers. The fineness of cement affects the

amount of heat released during hydration. Greater cement fineness (smaller particle size) increases

the rate at which cement hydrates and thus accelerates strength development. Except for AASHTO

M 85, most cement standards do not have a maximum limit on fineness, only a minimum. The

fineness of Types I through V Portland cement are shown in Table 6.4 (Kosmatka, 2002). Values

are expressed according to the Blaine air-permeability test (ASTM C204 or AASHTO T 153),

which indirectly measures the surface area of particles per unit mass.

Table 6.4 Fineness of Portland Cement

The specific gravity of portland cement typically ranges from 3.10 to 3.25, with an average of

3.15. Bulk densities can vary significantly depending on how the cement is handled and stored.

Reported bulk densities range from 830 to 1,650 kg/m3 (Kosmatka, 2002).

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6.5 Clinker manufacturing process

Portland cement manufacturing is a two-step process beginning with the manufacture of clinker

followed by the fine grinding of the clinker with gypsum and other additives to make the finished

cement product. Grinding can occur on site or at offsite grinding plants. The first step in clinker

manufacture is the quarrying, crushing, and proportioning of raw materials. Due to the low unit

value of these raw materials, they typically are mined within a few miles of the cement plant. The

cost of transport renders long-distance transport of these low-cost raw materials uneconomical.

Once the raw mix, or raw meal, is prepared, it is fed into a cement kiln and converted into the

clinker minerals through a thermochemical conversion, referred to as pyroprocessing because it

involves direct flame interaction. Figure 6.1 provides a generalized flow diagram of the cement

manufacturing process (van Oss, 2005).

The raw materials for clinker manufacture consist primarily of materials that supply four primary

oxides: Calcium oxide (CaO), silicon dioxide (SiO2), aluminum oxide (Al2O3), and ferric oxide

(Fe2O3). The composition of the raw mix typically includes about 80% calcium carbonate, about

10% to 15% silica, and small amounts of alumina and iron. Depending on the quality and quantity

of these oxides available to the facility, other raw materials, referred to as accessory or sweetener

materials, are added to correct for any deficiencies in the primary raw materials.

Certain types of fuel burned in the cement kiln can also contribute oxides (e.g., ash from coal

combustion contributes silica oxides, steel belts in waste tires contribute iron oxide).

Figure 6.1: Cement Manufacturing Flow Diagram

Calcium oxide (CaO or simply C in shorthand) is the primary ingredient in clinker, comprising

about 65% of clinker by mass. A cement plant typically examines its source of C (typically

limestone, marl, or chalk) and determines what other oxides need to be added to achieve the desired

clinker composition. Clay, shale, slate, or sand provide the silica and alumina component, while

iron, mill scale, or other ferrous materials provide the iron content. Preparing the raw mix for

clinker production requires constant sampling, chemical testing, and adjusting of the inputs to

maintain the desired clinker composition.

On average, it takes about 1.7 tons of nonfuel raw materials to produce 1 ton of clinker. Of the 1.7

tons of raw materials, approximately 1.5 tons is limestone or calcium oxide rich rock (van Oss,

2005). The lost mass takes the form of carbon dioxide (CO2) driven off by the calcination of

25


limestone and the generation of CKD (Cement Kiln Dust). Nearly one ton of CO2 is produced for

every ton of clinker manufactured (van Oss, 2005). The CKD that is produced during clinker

manufacture is carried “up the stack” and captured by emission control devices. A large portion of

the CKD, though not all of it, is returned to the kiln as part of the feed stream.

After clinker has been cooled to about 100°C, it is ready to be ground into finished cement in a

grinding mill, more commonly referred to as a finish mill. Generally, separate grinding and/or

blending finish mill lines will be maintained at a plant for each of its major product classes (finished

Portland cements, blended cements, masonry cements, ground slag). Additives that commonly

require grinding at the mill include gypsum, limestone, granulated blast furnace slag, and natural

pozzolans. Additives that generally do not require significant grinding include coal fly ash, GGBFS,

and silica fume, but the finish mill does provide intimate mixing of these with the Portland cement

base.

7.0 Concrete

Concrete is a common large-particle composite in which both matrix and dispersed phases are

ceramic materials. Concrete and cement are sometimes incorrectly interchanged. In a broad sense,

concrete implies a composite material consisting of an aggregate of particles that are bound together

in a solid body by some type of binding medium, that is, a cement. The two most familiar concretes

are those made with Portland and asphaltic cements, in which the aggregate is gravel and sand.

Asphaltic concrete is widely used primarily as a paving material, whereas Portland cement concrete

is employed extensively as a structural building material.

7.1 Portland cement concrete

The ingredients for this concrete are Portland cement, a fine aggregate (sand), a coarse aggregate

(gravel), and water. The aggregate particles act as a filler material to reduce the overall cost of the

concrete product because they are cheap, whereas cement is relatively expensive. To achieve the

optimum strength and workability of a concrete mixture, the ingredients must be added in the

correct proportions. Dense packing of the aggregate and good interfacial contact is achieved by

having particles of two different sizes; the fine particles of sand should fill the void spaces between

the gravel particles. Ordinarily these aggregates constitute between 60% and 80% of the total

volume. The amount of cement–water paste should be sufficient to coat all the sand and gravel

particles; otherwise, the cementitious bond will be incomplete. Furthermore, all the constituents

should be thoroughly mixed. Complete bonding between cement and the aggregate particles is

contingent on the addition of the correct quantity of water. Too little water leads to incomplete

bonding and too much result in excessive porosity; in either case, the final strength is less than the

optimum.

The character of the aggregate particles is an important consideration. In particular, the size

distribution of the aggregates influences the amount of cement–water paste required. Also, the

surfaces should be clean and free from clay and silt, which prevent the formation of a sound bond at

the particle surface.

Portland cement concrete is a major material of construction, primarily because it can be poured in

place and hardens at room temperature, and even when submerged in water. However, as a

structural material, it has some limitations and disadvantages. Like most ceramics, Portland cement

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concrete is relatively weak and extremely brittle; its tensile strength is approximately one-fifteenth

to one-tenth its compressive strength. Also, large concrete structures can experience considerable

thermal expansion and contraction with temperature fluctuations. In addition, water penetrates into

external pores, which can cause severe cracking in cold weather as a consequence of freeze–thaw

cycles. Most of these inadequacies may be eliminated or at least improved by reinforcement and/or

the incorporation of additives.

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