October 2013 – For Students Attending EL3004 Not for commercial

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.

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.SiO 2 ), dicalcium silicate (2CaO.SiO 2 ), tricalcium aluminate (3CaO.Al 2 O 3 ), and tetracalcium aluminoferrite (4CaO.Al 2 O 3 .Fe 2 O 3 ). 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 SiO 2 , A for Al 2 O 3 , and F for Fe 2 O 3 . Small quantities of pentacalcium trialuminate (5CaO.3A1 2 O 3 ), 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 (C 3 S) because it hardens The properties of the four main cement compounds are illustrated by Fig.9-5, indicating that the most desirable constituent is the tricalcium silicate (C 3 S) because it hardens

possess cementing properties. If the amount of dicalcium silicate (C 2 S) 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 (C 3 A) and tetracalcium aluminoferrjte (C 4 AF)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 First, hydrates are formed from the corresponding anhydrous products that passed into the solution. The hydrates have lower solubility than their corresponding anhydrous

anhydrous products. Hydration of the C 3 A occurs rapidly with the formation of hydrate crystals (C 3 A.6H 2 O), 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.Al 2 O 3 .xCaSO 4 .yH 2 O, 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 m 2 /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.

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 (C 3 A 5 ); 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 CaAl 2 (OH) 8 .6H 2 O, which gradually changes to a very stable, crystalline complex Ca 3 AI 2 (OH) 10 .3H 2 O 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 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 (Na 2 O.2SiO 2 ). 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 Na 2 O.CaO.6SiO 2 . 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 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% SiO 2 ) 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.

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 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

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)

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:

Li 2 O − A1 2 O 3 − SiO 2

(9-16)

(9-17) Li 2 O − MgO − SiO 2

MgO − A1 2 O 3 − SiO 2

(9-18) Li 2 O − ZnO − SiO 2

A process of controlled crystallization involves the addition of nucleating agents such as TiO to the molten glass. These nucleating agents can be TiO 2 , ZrO 2 , CaF 2 , 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

10 corresponds to glass viscosities in the range 10 11 to 10 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, T cr . 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 µ m which ideally

18 21 should be uniformly dispersed in a concentration from 10 3 to 10 nuclei/m . 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. T n is the nucleation temperature

T crist 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 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