Engineering materials
1
Cement Lime and Gypsum
Introduction
Inorganic cementing materials are most widely used bonding agents for construction purposes. They include cement lime plaster of paris etc. These materials, when made into a paste with water are capable of binding rigid solid masses into coherent structure, because of their characteristic setting and hardening properties.
Cement
Cement is a substances which acts as a binding agent for materials like stones, bricks etc.. It possesses adhesive and cohesive properties.
Classification
Cements are classified into the following two types: 1. Natural cement
2. Artificial cement. Natural cement
Natural cement stones contain 20 to 40% argillaceous matter (clay) and the rest calcarious matter. It is calcined at high temperature and crushed to powder. Example: Roman cement from clay nodules, pozzuolana cement from volcanic powder and medina cement from septaria. They possess hydraulic property. They are quick setting and hence have low strength.
Artificial cement
A mixture of argillaceous and calcareous raw materials and calcined at 1500-1700°C and powdered to get artificial cement called Portland cement.
Composition of Portland cement
Lime (CaO) - 60%
Silica (SiO2) - 22%
Alumina (Al203) - 5%
Magnesium (MgO) - 4%
Gypsum (CaSO4) - 4%
Iron oxide (Fe203) - 3%
Sulphur trioxide (SO3) - 1%
Engineering materials
2 Manufacture of Portland Cement:
Raw materials required for the manufacture of cement are 1. Calcareous materials (CaO) – lime stone, chalk 2. Argillaceous material (Al2O3.SiO2) – clay, shale, slate
3. Gypsum (CaSO4.2H2O)
Manufacture of cement involves the following steps: 1. Mixing, 2. Burning and 3. Grinding. Mixing
Mixing is done either by dry process or wet process. Dry process
Dry process is suitable for hard raw materials like lime stone. The raw materials are crushed separately and then mixed in proper proportion.
Wet process
Engineering materials
3 Burning
The dry mixture from dry process or the fine slurry from wet process is burnt in a rotary kiln.
The kiln is a steel cylinder of length 100 m and diameter 35 m with a slight inclined at discharging end. It is lined inside with refractory bricks, the cylinder can be rotated on its axis at 1 rmp. Pulverized cool or oil or gas is used as fuel fro heating.
The feed is introduced at the upper end of the kiln. It descends down slowly towards the lower end of the kiln due to rotation of the cylinder on its axis. The feed gets heated up by the burning fuel. Various changes take place at different temperatures prevailing inside the kiln in different zones.
(i)Drying Zone:
The temperature prevailing in this zone is about 400°C. Heat the slurry dries up due to evaporation of water.
(ii) Calcining Zone(nodule zone)
The temperature at this zone is around 1000°C. At this temperature limestone decomposes to form small lumps called nodule zone.
CaCO3 CaO + CO2
(iii) Clinker Zone
In this zone at 1500°C the lime and clay react to form aluminates and silicates. These compounds fuse together to form clinkers.
2CaO + SiO2 Ca2SiO4(C2S)
3CaO + SiO2 Ca3SiO5(C3S)
3CaO + Al2O3 Ca3Al2O6(C3A)
4CaO + Al2O3+ Fe2O3 Ca4Al2Fe2O10(C4AlF)
Grinding:
The clincker thus produced emerges out of the rotary kiln. It is rapidly cooled by steam of air. The cooled clinker is powdered with 4% gypsum in ball mills. Gypsum is added to retard the early setting of cement.
3CaO.Al2O3+ xCaSO4.7H2O 3CaO.Al2O3xCaSO4.7H2O
Engineering materials
4 The setting and hardening of cement is due to hydration and hydrolysis reactions
of various constituents of the cement. These reactions lead to the formation of gel and crystalline products which bind the inert particles like sand and stone into coherent mass.
Stiffening of the plastic mass of cement is called setting. It is followed by the hardening. Hardening is the development of strength due to interlocking of crystallized products from the hydration reactions.
When cement is made into a paste with water, hydration of tricalcium aluminate (C3A) takes place rapidly. Hydrated crystals of C3A.6H2O are formed resulting in intial
set orflash set.
C3A + 6H2O C3A.6H2O
These soluble crystals prevent the hydration of other constituents by forming a barrier over them. To retard this flash set, gypsum is added to cement during the manufacturing process. Gypsum reacts with C3A forming an insoluble complex of
calcium sulphno aluminate. Hence the dissolution of C3A is reduced.
C3A + xCaSO4.2H2O C3A.xCaSO4.yH2O
Tetra calcium alumino ferrite hydrates with water forming crystalline C3A and a
gel of hydrated mono calcium ferrite.
C4AF +7H2O C3A.6H2O(crystals ) + CF.H2O (gel)
These products do not contribute appreciably to the development of strength. Due to the hydration of C4AF, small capillaries are formed enabling water to reach C3S and
C2S. Now tricalcium silicate undergoes hydration.
C3S +H2O C2S.xH2O (gel) + Ca(OH)2
This reaction begins in 24 hours and gets completed in 7 days. C3S develops high
early strength and maximum ultimate strength. The excess Ca(OH)2 formed in the
hydration of C3S precipitates out as crystals. This accounts for the high rate of hardening
and early high strength of cement. Dicalcium silicate begins to hydrate and the reaction gets completed in 28 days.
C2S + xH2O C2S.xH2O
C2S develops less early strength but the ultimate strength is high.
Special cements
Engineering materials
5 1. White cement
Portland cement free from iron oxide is called white cement. To avoid the presence of iron oxide, refined china clay and pure white chalk are used as raw materials. It is used for ornamental works and decorative purposes. It is mixed with pigments to get coloured cements.
2. Quick setting cement
It is also called rapid hardening cement. It is similar to Portland cement but its lime content is high and gypsum content is less. It contains more tricalcium silicate (C3S)
which enables rapid strengthening. Initial setting takes place within 5 minutes. It is used for emergency constructions.
3. High alumina cement
Bauxite (40%0, lime (40%), iron oxide (15%), silica and magnesia (5%) etc. Are fused together and powdered to get high alumina cement. During fusion mono-calcium aluminate, tricalcium pentaluminate, dicalcium alumino silicate and tetra calcium alumino ferrate are formed.
Monocalcium aluminate and tricalcium pentaluminate initially undergo hydration to form a gel which gradually changes to a stable crystalline complex. This is stable on heating and also on dehyudrating and hence retains its strength at high temperatures.
Setting time if alumina cement is similar to Portland cement but its rate of hardening is rapid. Full strength is attained in 24 hours. It is resistant to dilute acids where as Portland cement is easily attacked. But it is easily attacked by alkali. It is resistant to sea water and fire.
The kiln is a steel cylinder of length 1000m and diameter with a slight inclination at the discharging end. It is lined inside refractory bricks. The cylinder can be rotated on its axis at 1 pulverised coal or oil or gas is used as fuel for heating.
Engineering materials
6
Refractories
Refractories are inorganic materials that can with stand high temperature without softening or undergoing any deformation in shape. They are mainly used in engineering as a material to confine heat (Inner lining of a furnace) to maintain high temperatures and to resist the abrasive and corrosive action of molten metals, slag and gasses at high operating temperatures without distortion in shape. Refractories are used for the construction of the lining of the furnace, kilns, etc., which are employed as for manufacturing of cement, glass, metals etc.,
Refractories are available in different shapes such as bricks, crucibles, tubes and granules.
Characteristics of a good
refractory:-The following are the essential characteristics of a good refractory. It should; 1. Not to fuse at the operating temperature.
2. Its physical, chemical and mechanical properties should not undergo substantial changes at high temperatures.
3. Be chemically inert towards the corrosive action of molten metals, slag’s and gases produced in the furnace.
4. Be capable of retaining its original form without cracking and splitting when subjected to sudden temperature changes.
5. Be strong enough to bear the charge load at the working temperature. 6. Resist the abrading action by molten metal or slag.
7. Not cause contraction and expansion in a sudden, but uniform manner. Properties of
Refractories:-1.
Engineering materials
7 The softening temperature of a refractory is determined by the standard
pyrometric cone equivalent(PCE) test or Seger cone test. In this test pyrometric cones or seger cones of standarded dimensions (38mm height with triangular base of 19mm sides) are prepared from heated on a base under standard condition (10C/minute), at one stage, the apex of the cone bends and touches the base. The temperature at which this occurs is the measure of the softening temperature of the cone. Depending upon the softening temperature the cone are given some numbers know as PCE:
PCE Softening temperature(C)
1 1110
2 1120
3
-4
--
-37 1825
38 1850
To obtain the refractoriness of a material the finely ground Refractories is moulded into small cones of standard dimensions. This is heated with other standard cones. The PCE value of the test or given refractory is taken as the number of standard cone in which softens along with the test cone.
Engineering materials
8 2. Spalling Resistance or Thermal
Spalling:-Spalling is the breaking or cracking or peeling of a refractory material. A good refractory should be resistant to thermal spalling. Uneven thermal expansion or contraction due to temperature differences, arising out of rapid heating or cooling is the main cause for thermal spalling. The spalling tendency is directly proportional to the coefficient of expansion. Hence to avoid spalling, a refractory should have a low coefficient of expansion.
3. Refractoriness under Load
(RUL):-Refractories used in metallurgical operations and industries have to withstand varying loads. Refractories should have high mechanical strength under operating temperatures. The load bearing capacity of a refractory can be measured by means of RUL test.
Refractories under load or strength of a refractory is its ability to withstand high temperatures under the influences of maximum load without breaking. RUL is performed on a test specimen of standard size (cylinder diameter 50 mm and height 50 mm). The specimen is heated in a furnace at a rate of 10°C/ minute under a load of 1.75 Kg/cm2.
RUL is expressed in terms of the temperature at which 10% deformation occurs on the specimen. A good refractory should have a high RUL value.
4.
Porosity:-Due to porosity, slags, gases etc., are likely to enter more easily to greater depth and also react with a refractory. Hence porosity reduces the strength and resistance to corrosion. Porosity of a material is given by the ratio of its pores volume to that of its bulk volume. Thus porosity is given as
P = W-D/ W-A x 100
Where W= weight of saturated specimen in air D = weight of dry specimen in air
A = Weight of saturated specimen submerged in water.
Engineering materials
9 as insulator and hence it can be used for lining in furnace, oven etc., A good refractory in
general should have lower porosity. 5. Dimensional Stability:
Dimensional stability is the resistance of a refractory to any changes when exposed to high temperature over a prolong time. These dimensional changes may be reversible ore irreversible (permanent).
Irreversible changes may lead to contraction or expansion of refractory. When a refractory is exposed to a high temperature for a long period, the low fusible constituents of the refractory will from increasing amount of liquid. The liquid gradually fills the pores of the refractory will form increasing amount of liquid. The liquid gradually fills the pores of the refractory body causing high degree of shrinkage.
In the case of reversible dimensional changes, the refractory material will result in uniform expansion and contraction. A good refractory material should have reversible dimensional changes.
Classification of Refractories
Refractories are mainly classified into three types: (a) Acid Refractories, (b) Basic Refractories, (c) Neutral Refractories.
Acid Refractories
These are made up of acidic materials like silica. Acid Refractories are not attacked by acidic substances but are easily attacked by basic substances, e.g. silica and fire-clay Refractories.
Engineering materials
10 Properties: Fire-clay Refractories are slightly acidic in nature. They possess lower
refractoriness (1500-1600 ºC), lower porosity and higher resistance for thermal spalling. Allotrope: Quartzite Tridymite Cristobalite
Uses: The steel industries are the largest users of these bricks. They are mainly used for lining of blast furnaces, open hearths and lime kilns.
Basic Refractories
Basic Refractories are made up of basic materials like calcium oxide and magnesium oxide. They are not attacked by basic substances, but are easily attacked by acidic substances, e.g. magnesite and dolomite Refractories.
Magnesite Refractories
Magnesite is calcined at 1700ºC to form a dense, hard crystalline variety of magnesia. This magnesia is called dead burnt magnesite. It is ground, mixed with iron oxide (binder) and water, moulded at high pressure, dried and fired at about 1500ºC to give fire bonded bricks. They contain 80-95 % magnesium oxide, 2-7 % iron oxide and < 4% aluminium oxide.
Properties:The following are the properties of magnesite refractoriess:
They are high refractiveness (2000ºC) but poor spalling resistance due to high coefficient of thermal expansion. They have high thermal conductivity.
Uses: They are largely used in steel making furnace and rotary clean. Magnesite Refractories are also used in copper smelting furnace and furnace processing antimony ores.
Neutral Refractories:
These are made of neutral materials like graphite, Zirconia and Silicon carbide. Neutral Refractories are inert towards both acidic and basic substances e.g. carbon and graphite Refractories.
Engineering materials
11 Properties: They have high refractiveness (2500ºC). They possess high resistance to
spalling. They have high thermal conductivity, low thermal expansion and high mechanical strength.
Engineering materials
12 Glass
Glass is an amorphous, hard, brittle, transparent, super cooled liquid of infinite viscosity obtained by fusing a mixture of a number of metallic silicates, most commonly Na, K, Ca and Pb. It posses no sharp melting point, definite formula or crystalline structure. Commonly represented as
xR2O.yMO. 6SiO2
Where R = Monovalent alkali metal (Na, K, etc.,) M = Bivalent metal like Ca, Pb, Zn etc., x,y = whole number
Manufacture of glassRaw materials
:-a. Sodium is soda (Na2CO3)- soft glass
b. Potassium is potash (K2CO3)- Hard glsass
c. Calcium as limestone, chalk & lime d. Lead are litharge 7 red lead – flint glass e. Silica –quartz, white sand & ignited flint f. Zinc is inc oxide (heat & shock proof glass)
g. Borate are borax & boric acid (heat & shock proof glass) h. Cullets or pices of broken glass to increase the fusiblity. i. Colours : Yellow- ferric salts
Green – Ferrous & chromium salts Blue – cobalt salts
Red – Nickel
Purple – Maganese etc.,
Manufacturing
Engineering materials
13 CaCO3 + Sio2 CaSiO3+ CO2
Na2CO3 + SiO2 Na2SiO3 + CO2
When all Co2 has escaped out of molten mass, decolorizers (MnO2 or nitrite
) or colouring salts are added at this stage. Heating is continued till the molten mass is free from bubbles and glass balls, then cool to about 800°C.
ii) Forming and shaping: Molten glass is then worked into articles of desired shapes by blowing or moulding or pressing between rollers.
iii) Annealing : Glass articles are then allowed to cool gradually to RT by passing different chambers with descending temperature. The longer the annealing period the better the quality of the glass.
iv) Finishing : All glass articles, after annealing, are subjected to finishing processes such as cleaning, grinding , polishing, cutting, sand blasting etc..
Soft glass and Soda lime:
Raw materials : Silica, calcium carbonate and soda ash Composition : Na2O.CaO.6SiO2.
They are low in cost, reristant to devitrification and resistant to water. They melt easily hence can be moulded into desired shape under hot condition. Soft glasses are attacked by common reagents like acids.
Uses : Window glasses, electric bulbs, plate-glasses, bottles, jars, building blocks and cheaper table ware, where high temperature-resistant and chemical-stability are not required.
Hard glass or potash-lime:
Raw materials : silica, calcium carbonate and potassium carbonates. Composition : K2o.Cao.6Sio2
They possess high melting point,here with difficulty and less acted upon by acids, alcohols, alkalies and other solvents than ordinary glass.
Engineering materials
14 Pyrex glass or Jena glass or Boro silicates glass: Most common of hard glasses of
commerce.
Raw materials : Silica, boron, alumina and some alkali oxides. Composition : SiO2 - 80.5%
B2O3 - 13%
Al2O3- 3%
K2O - 3%
Na2O - 0.5%
Presence of boron and alumina results in low-thermal co-efficient of expansion and high chemical resistance. Boro silicates glasses have very high softening points and excellent resistivity.
Engineering materials
15
CERAMICS
The word ceramic is derived from the Greek keramos means Burnt Stuff. In modern usage ceramics are polycrystalline inorganic metallic or non-metallic materials that are processed and used at high temperature. Ceramic materials include a wide range of silicates, metallic oxides and combination of silicates and metal oxides. Ceramic materials having wide range of properties are produced for different applications. In general ceramic materials have the following characteristics.
1. They are usually hard and brittle in nature and generally being in the form of amorphous or glassy solids.
2. The atomic bonding in these materials is of mixed ionic or covalent character. 3. They have good electrical resistance and act as insulators.
4. They have high temperature resistance.
5. They have good resistance to chemical attack and weathering. 6. They have high compressive strength and tensile strength. Ceramics generally consists of the following three major components.
1. Plastic Portion: This is usually provided by clay, which imparts the necessary plasticity and workability.
2. A Flux or a glassy materials:This is generally provided by feldspar (K2O.
Al2O36H2O), which helps in bonding and cementing the ingredients together.
3. A Non-Plastic Refractory Crystalline Portion: This is generally provided by Silica, which contributes mechanical strength.
Classification of Ceramic
Materials:-Generally ceramic materials are three types based of their characteristic properties. They are clay products, Refractories and glasses. Closely related to ceramics in composition are natural rocks, and their disintegration products such as clay, sand and gravel. Ceramic materials are classified in the following way.
Engineering materials
16 Functional Classification of Ceramics
Group Example
1. Abrasives Alumina, carborundum.
2. Pure oxide ceramics MgO, Al2O3, SiO2 3. Fire clay products Porcelain, bricks, tiles etc. 4. Inorganic glasses Hard glass, window glass. 5. Cementing materials Lime, Portland cement etc.
6. Rocks Granite, Sand stone etc.
7. Minerals Quartz, calcite etc.
8. Refractories Silica bricks, magnesite bricks etc.
Structural Classification of Ceramics
Group Examples
1. Crystalline Ceramics Single phase like MgO or muliphase from MgO to Al2O3
2. Non-crystalline Ceramics Natural and synthetic inorganic glasses
3. Glass bonded ceramics Fire – clay products, crystalline phases are held in glassy matrix
4. Non-crystalline phases Cement
Ceramic Materials:
Clay:The term clay denotes certain earths which are highly plastic when wet, but on heating to redness lose its plasticity and are converted into hard mass, which is unaffected by water. Clay is composed of hydrated aluminum silicates (Al2O32SiO22H2O) together
with other substance such as mica and quartz. Calcined fire clays are called grog. Primary clay burns white and it is called Kaolin or china clay.
Engineering materials
17 Manufacture of Structural Clay Product:The raw materials are ground and then screened.
The powdered raw materials are mixed with water to increase plasticity. The sticky and plastic clay is moulded in the desired shapes using stiff mud process or soft mud process or dry pressing process. The moulded articles are then dried in outdoors and finally fired in kiln at 875 to 1100ºC for about 7 days.
White Wares or White Pottery (Triaxial Ware):The term white ware or white pottery usually refers to glazed or unglazed ceramic materials which have white or cream color after firing and have fine structure. White wares are usually prepared from suitable proportions of china clay, feldspar and quartz and so it is called Triaxial Ware. The three raw materials are powdered and made into paste by mixing suitable amount of water and subjected to desired shape by moulding process. This is followed by careful drying and then firing at 1350ºC to 1500ºC when a partial vetrification takes place. White ware product consisting of a refractory body with a glossy coating called the glaze. These are usually made by any of the following two processes.
1. In the Porcelain process the body and the glaze are developed in one firing only. 2. In the China process the glaze is developed in the second stage.
Earthenwares and Stonewares: Clay products, which are hard and strong like stone come under this category. Relatively softer type of clay products which are obtained by burning at lower temperatures are called Earthen Wares, while the clay products which are denser and harder but are obtained by firing at high temperatures are called Stone wares. Earthen wares and Stonewares are usually glazed to render them strong, compact and impervious to water. Earthen wares are two types- Coarse and Fine. Earthen wares are obtained from mixed earths of clays or clay mixed with sands.
Stone wares : A typical formulation of producing stoneware is as follows-Clay 50%,
Feldspar 20%, Flint 15%, Kaolin 5% and grog 10%. The shaping is done by casting or moulding. The shapes are dried and fired at 1000C followed by suitable glazing.
Uses of Stone Ware : Stonewares are used for the construction of sanitary fixtures, pipes, piping vessels, drainage pipes.
Engineering materials
18 materials of proper composition such as lead silicate, borosilicate etc. A glaze is applied
on a ceramic body to achieve the following objects. 1. To produce decorative effect
2. To make surface impervious to liquids, water etc. 3. To increase durability
4. To provide a smooth and glossy surface and
5. To protect the surfaces from the environment / atmospheric action. Application of Ceramics:
The older ceramics refer to white wares and are widely used in tiles, sanitary ware, insulators and high frequency applications. White wares also used in chemical industries as crucibles, jars and compounds of chemical reactors. Pyrometer tubes burner, burner tips and radiant heater supports are somewhat heat resistant items, which are also used as white wares.
The newer ceramics include borides, nitrides, and single oxides, mixed oxides, silicates, metalloid and intermetallic compounds. These have high hardness and high resistance to heat and oxidation. Therefore they are widely used in diverse applications such as Refractories for industrial furnaces and fuel for nuclear reactors. Ceramics are now being used in electrical and electronic industries for insulators, semi-conductors, super conductors, dielectric crystals, piezoelectric crystals, glass, porcelain, alumina, quartz, mica etc. Ceramics such as UC2, UC and UO2 are used in nuclear applications as fuel
