Unit-I: POLYMERS& BUILDING MATERIALS

POLYMERSPolymer: “ A polymer may be defined as a high molecular weight compound formed by the combination of a large number of one or more types of smaller molecules of low molecular weight.” The repeating units in a polymer chain are termed as “monomer”.

Eg:- Polythene is a polymer of ethylene.

Polymerisation: The reaction by which monomeric units combine to form polymer chain is known as Polymerisation.

Degree of Polymerisation: (Dp):It is defined as the number of monomeric units or repeating units (n) in a polymer. Most of the polymers are in the molecular mass rage of 5,000-2,00,000.

For the homo polymer, the Degree of PolymerisationDp = Totalmolecular weight of the polymerMolecular weight of monomeric unit = MnMo

For the heteropolymer (co polymer), the above formula cannot be designed where the monomeric units are not identical.

FUNCTIONALITY: “The number of bonding sites (or) reacting sites in a monomer is referred to as Functionality”. A monomer molecule must have atleast two reactive (or) bonding sites. IN an olefin, the double bond can be considered as a site for two free valencies. When the double bond is broken, two single bonds become available for combination. Thus, ethylene is considered to be a bifunctional monomer.

Different types of structures can be obtained, depending upon the functionality of the monomeric units.

a) In case of a bifunctional monomer, two reactive groups attach side by side to each other, forming linear or straight chain molecule. Different chains are held together by weak Vanderwaal’s forces of attraction. This gives possibility of chain movement in one direction.

b) When the polymerization of a bifunctional monomer is carried out in presence of small amount of a trifunctional monomer, a branched chain polymer results. The movement of branched chain polymer is more restricted than that of simple straight chain molecule.

c) In case a polyfunctional groups, monomer molecules are joined to each other by covalent bonds, resulting in the formation of a three-dimensional network. In such polymers, the movement of individual molecules is totally restricted due to strong cross-links.

CLASSIFICATION (TYPES) OF POLYMERS:

1) Based on their methods of synthesis: There are two major methods used for preparing polymers

i) Addition polymerization (or) Addition polymers.ii) Condensation polymerization (or) Condensed polymers.

Addition Polymerisation: In this polymerization process, the monomers add together by repeating addition without formation of biproducts to form polymer. This process is called as addition polymerization and the resultant polymer is known as addition polymer.

Eg:- Polystyrene, Polyethene, Poly Vinyl Chloride (PVC).

Condensation Polymerisation: When the molecules of same monomers (or) different monomers are linked together by the elimination of small molecules like H2O, HCl, NH3, CH3OH etc.,is called as condensation polymerization and resultant polymer is known as condensed polymer. Eg:- Polyester, Urea-Formaldehyde, Bakelite, Nylon-6,6 etc.,

In some cases, condensation polymerization takes place without elimination of smaller molecules like H2O, HCl etc.,

Differences between Addition and Condensation Polymerisation:

S.No.

Addition Polymerisation Condensation Polymerisation

1 The functionality of the monomer is double bond which is bifunctional.

The functionality of monomer is bifunctional or tri or polyfunctinal which is due to functional groups ike –OH, -COOH, -NH2, -COOR etc. The monomers must be dibasic acids, diols, diamines or triols etc.,

2 The polymerization takes place by self-addition of monomers.

The polymerization is due to slow step-wise condensation of the functional groups.

3 No biproducts like H2O, CH3OH etc., are produced.

Biproducts like H2O, HCl, CH3OH etc., are produced.

4 The molecular weight of polymer is the sum of molecular weights of monomers.

The molecular weight of polymer is not the sum of molecular weights of monomers.

5 The mechanism of polymerization is carried out in three steps – initiation, propagation and termination.

The mechanism of polymerization is carried out by slow step-wise condensation.

6 The mechanism is rapid. The mechanism is slow.7 The mechanism is highly

exothermic because π bond is converted to σ bond liberating 20 kcal/mole of energy during mechanism.

Not exothermic.

8 An initiator is required to start the polymerization.

A catalyst is required for the reaction.

9 The polymer has same composition as that of monomerEg: Polyethylene, PVC, Polystyrene etc.,

The polymer has different composition from the monomer. Eg: Polyesters, Nylon, silicones etc.,

2) Based on thermal capacity: Based on thermal capacity, polymers can be classified into two types.i) Thermoplastic polymers.

Eg: Polythene, P.V.C., Poly Styrene etc.,ii) Thermosetting polymers (or) thermosets

Eg:- Phenol Formaldehyde (Bakelite), Epoxy plastics, Silicone plastics etc.,

Thermo plastics:Thermoplastics are those, which can be softened on heating and hardened on cooling. Their hardness is a temporary property, subject to change with temperature. Repeating of heating or cooling donot alter the chemical nature of T.P. because the changes involved are purely of physical nature. T.Ps are usually soft, weak and less brittle and can be reclaimed from the wastes. These are formed by addition polymerization and consists of long chain linear polymers with little or no cross linking. These resins are soluble in some organic solvents.

Eg: Polythene, P.V.C., Poly Styrene etc.,

Thermosetting Plastics:Thermosetting plastics are those, which change irreversibly into hard and rigid materials on heating. These are permanent setting resins and during moulding, acquire 3-dimensional cross linked structure with strong covalent bonds. The bonds donot break, even on heating. Thermosetting resins are usually harder, stronger and more brittle than termoplastics and they cannot be reclaimed from wasters. The resins are formed by condensation polymerization and consists of 3D network structure joined by strong covalent bonds which makes them insoluble in almost all organic solvents.

Eg:- Phenol Formaldehyde (Bakelite), Epoxy plastics, Silicone plastics etc.,

Differences between Thermoplastics and Thermosetting Plastics:S.No

Thermoplastics Thermosetting Plastics.

1 These resins become soft on heating and rigid on cooling reversibly.

During fabrication process these resins are moulded. Once they are solidified, they cannot be softened.

2 Thermoplastic resins are formed by chain Polymerisastion.

Thermoset resins are formed by step polymerization.

3 They consist of long chain linear polymers with weak vandar Waal’s

They have three dimensional network structures.

forces of attraction in between.4 They can be reshaped. They cannot be reshaped.5 These plastics can be reclaimed

from waste.They cannot be reclaimed from waste.

6 These are soft, weak and less brittle. These are hard, strong and more brittle.

7 These resins are usually soluble in organic solvents.

Due to strong bonds and cross links, they are insoluble in almost all organic solvents.

8 They are softened on heating readily because the vandar Wall’s force of attraction between the individual chain can break easily by heat, pressure or both.

The bonds retain their strength on heating hence do not soften on heating.

PLASTICS: An organic substance which changes its shape by the application of heat and pressure is called Plastics. This is done in presence of catalyst.

Properties: These are usually light in weight. These will have low thermal and electrical conductance. Corrosion resistant, insect resistant. Have low fabrication cost. Easily moldable. Easily workable. Chemically inert. Transparent Possess good shock absorption capacity. Impermeable to water. Have good strength and toughness. Possess dimensional stability. Have low maintenance cost.

Disadvantages: Expensive Combustible Poor ductility High softness Deformation under load Low heat resistance.

COMPOUNDING OF PLASTICS (OR) MOULDING CONSTITUENTS OF PLASTICS:Compounding of plastic is the process of addition of some external material to give specific properties to the product. These external materials not only give the required property to the plastic but also makes the plastic economical. The components of compounding of plastics are as follows:

1. Resins: The product of polymerization is called resin and this forms the major portion of the body of plastics. This acts as a binder which holds the different constituents together. Eg:- Thermoplastic and Thermosetting Plastic Resins.

2. Plasticisers: Plasticizers increase plasticity and flexibility of the polymer. Plasticisers neutralize the intermolecular forces of attraction between polymer chains. Thus imparts greater freedom of movement between the polymeric molecules. Eg:- Vegetable oils, Camphor, Tri Cresyl Phosphate, esters of steric acid etc.,

3. Fillers: Fillers give better hardness, tensile strength, opacity, finish and workability to the plastics. It reduce the cost of polymers. They decrease the shrinkage of the polymer on setting. Fillers also reduce the brittleness of the polymer.

Some special types of fillers are added to impact special properties to polymer like Barium Saltsmake polymers impermeable to x-rays and Asbestos provides heat and corrosion resistance to polymers, Carborandum, Quarts and Mica gives hardness to the polymers.

4. Lubricants : Lubricants make moulding of plastic easier. They impart flawless, glossy finish to the products. Lubricants prevent moulded article from sticking to the fabrication equipment. Eg:- Waxes, oils, stearates, oleates and Soaps.

5. Catalysts (or) Accelerators: They accelerate the polymerization of fusible resin during moulding operation into cross linked infusible form for thermosetting resins. Eg:- Benzoyl peroxide, hydrogen peroxide, acetyl sulphuric acid etc.,

6. Stabilisers: Stabilisers improve thermal stability during polymerization. Eg:- White lead, lead chromate, red lead, stearates of lead, Cadmium and Barium.

7. Colouring Materials: These are added to impart desired colour to the final product. The decorative colours are provided by both organic dye stuffs and inorganic salts. Eg:- Black Colour - Carbon Black.

White Colour - ZnO (or) CaCO3.Red Colour – Ferrix Oxide.Green Colour – Chromium Oxide.Crimson Colour – Antimony Sulphide.

MOULDING OF PLASTICS (or) FABRICATION OF PLASTICS:

The method by which plastics can be fabricated, depends on the thermal behavior of the plastics i.e., whether it is thermoplastic or thermosetting plastic and also on the shape of the finished product.

The following are the methods employed for the fabrication of Plastics:1) Compression Moulding:

This method can be adopted for both Thermosetting and Thermoplastic Resins.

The quantity of the resin, filler and other ingredients placed in the mould should be such that the volume of the compact mass is slightly more than the volume of the cavity of the mould. The mould is usually preheated. It is closed after placing the ingredients and heat and pressure are applied, wherein the ingredients soften and fill the cavity of the mould.

For T.P. resins, the mould is cooled before releasing the pressure, otherwise the object will distort. The thermosetting resin has to be kept in the mould for a period of time for the completion of the formation of permanent shape (Curing Time i.e., the time required for the polymer to set in the mould is called curing”).

2) Injection Moulding:

This method is widely used for the fabrication of Thermoplastic materials. In this method, the Thermoplastic material is softened by heating, and the hot plastic is injected into a mould where it sets and takes the shape of the mould. The mould is kept at cold temperature. It is a modification of compression moulding. The advantage of this process is that it is rapid.

Advantages:1) This method is characterized by high speed production and low mould

cost. 2) Loss of material will be less.

The disadvantage of this is a large number of cavities cannot be filled simultaneously, so there is a limitation to the designed articles.

3) Transfer Moulding:

Transfer moulding is a method which was the principle of injection moulding for T.S. Plastics. The powdered compounded plastic material is in a chamber applied with minimum temperature and high pressure till it begins to become soft and semisolid. Then it is injected into a mould by a plunger working at high pressure, due to high friction developed at the nozzle, the soft plastics material is ejected from the

orifice into the mould which is heated upto the curing temperature required for setting the fabricated article is then ejected out mechanically.

Advantages:1) Since the plasticized mix flows very slowly into the mould, very delicate

articles with intricate shapes can be produced.2) The articles are free from flow marks.3) Mould cost is less.

4) Extrusion Moulding:Extrusion molding is used for moulding of Thermo Plastic materials into articles of uniform cross section like tubes, rods, sheets, wires, cables etc., The Thermo Plastic ingredients are heated to plastic state and then pushed by means of a screw conveyor into a die, having the required outershape of the article to be fabricated. The extruded article gets cooled due to atmospheric exposure or artificially by air jets, or by water sprayer in a long conveyor which carries away the cooled product.

Individual Polymers preparation, properties and uses:

I) POLY ETHYLENE (PE):This polymer is obtained from ethylene. There are two kinds in polyethylene.

1) Low Density Polyethylene (LDPE) 2) High Density Polyethylene (HDPE).

Preparation of LDPE: It is obtained by polymerization of ethylene using oxygen as the initiator with the pressure of 1500 atmosphere at the temperature range of 80 – 2500C.

n CH2¿CH2 ⟶ -[CH2 – CH2]n-Properties of LDPE:

i) The melting point is 1100C – 1250C and 40% crystalline.ii) The density is 0.91 – 0.92 g/ml.iii) It is insoluble in any solvent at room temperature.

Uses: It is used for films, table clothes, packing materials, squeeze bottles, pipes etc.,

Preparation of HDPE: It is obtained by co-ordination polymerization of ethylene using Zigler-Natta Catalyst, relative under low pressure.

n CH2¿CH2 Ziegler−NattaCatalyst→

-[CH2 – CH2]n-

Properties of HDPE:i) The melting point is at 144-1500C and 90% crystalline.

ii) The density is 0.965 g/ml.iii) It is chemically more resistant than LDPE.

Uses: It is used for films, detergent bottles, pipes, industrial clothes and filters. * General Properties of Polyethylene:1) Polythene is a rigid waxy, white, non polar material and resistance to

acids, alkalies and salt solutions at room temperature.2) It is a good insulator of electricity. However, it is permeable to most oils

and organic solvents particularly kerosene.

* General Uses of Polyethylene:It is used for making domestic appliances, toys, sheets for packing material,tubes, pipes, coated wires and cables, bags for packing, bottle caps, flexible caps etc.,

II) Preparation of Poly Vinyl Chloride (PVC):

In the presence of peroxides (like Benzoyl Peroxide, Hydrogen peroxide etc.,), water emulsion of vinyl chloride when heated under high pressure give poly vinyl chloride (PVC).

Properties:1) It is colourless, odourless, non inflammable and chemically inert powder.2) It is resistant to light, atmospheric oxygen, inorganic acids and alkalies.3) It is soluble in chlorinated hydrocarbons and ketones.4) Pure resin possesses a high softening point and greater stiffness and

rigidity when compared to polythene.Uses:

a) Unplasticised PVC possess superior chemical resistance and high rigidity. This is used for tank linings, light fittings, safety helmets, refrigerator components, trays, cycle and motor cycle mud guards. It is also called rigid PVC.

b) Plasticised PVC which is obtained by the addition of plasticisers like dibutyl phthalate, tricresyl phosphate etc., is used for making rain coats, table clothes, curtains, electrical insulations like coverings of electrical cables, toys, tool handles, chemical containers, conveyor belts used in coal mines etc.,

III) Poly Tetra FluoroEtylene (PTFE) (or) Teflon (or) Fluon:It is obtained by the chain polymerisation of tetrafluoro ethylene in presence of benzoyl peroxide as an initiator.

Properties:The regular configuration of polytetrafluoro ethylene molecule results in strong attractive forces between different chains.

1) These strong attractive forces give the material toughness, high softening point, high chemical resistance, high density and extremely good electrical and mechanical properties.

2) It is highly crystalline and melts at 3500C into a very viscous and opaque mass, which can be moulded into different forms. It has a low dielectric constant and it is not soluble in any organic solvents.

Uses:a) It is used in making pump valves, tank linings, chemical carrying pipes,

packing tanks etc.,b) It is used in making non-lubricating bearings and non sticking stop cocks.

IV) Bakelite (or) Phenol-Formaldehyde Resin (or) Phenolic Resin (or) Phenoplasts:

Bakelite is formed by the condensation of phenol with formaldehyde. The reaction can be catalysed by either acid or alkali. The reaction results in the formation of o- (or) p-hydroxy methyl phenol, which further react with phenol to form linear polymer NOVOLAC. Novolac is heated with hexamethylenetetramine (hexamine) which converts the soluble, fusible NOVOLAC into hard, infusible and insoluble solid of cross linked structure.

The reaction proceeds as follows:

Properties: It is rigid, hard, water resistant and has resistance to acid salts and many organic solvents. It also possesses good electrical and thermal insulating property. Uses:

1) Bakelite is used for making electric insulator parts like switchboards, switches, plugs etc.,

2) For making moulded articles like telephone parts, cabinets for radio and TV.

3) As an adhesive for grinding wheels.4) IN paints and varnishes.5) As a cation exchanger resin in water softening.6) For making bearings used in paper industry and rolling units.

V) Nylon-6,6:It is obtained by the condensation polymerization of adipic acid with hexamethylenediamine.

Nylon-6:

It is produced by self polymerization of caprolactum.

Properties of Nylons:a) They have plastic and fibre property.b) They have high strength, elasticity, toughness and abrasion resistance.c) They possess good mechanical properties.d) They are insoluble in common organic solvents like methylated spirit,

benzene, acetone and soluble in phenol and formic acid.Uses of Nylons:

a) Nylon-6,6: It is primarily used for fibres which are used in making socks, undergarments, dresses, carpets etc.,

b) Nylon-6: is mainly used for moulding purposes for gears, bearings, electrical mouldings etc.,

VI) POLY ESTER (OR) DACRON (OR) TERYLNE:It is prepared by the condensation between a terephthalic acid and an ethylene glycol.

Properties:1) Its melting point is around 2650C.2) It is resistant to heat and moisture.3) It possesses good mechanicals trength.4) It is highly resistant to chemicals.

Uses:a) It is extensively used in making textile, fibres.b) It is also used in making films and manufacturing of magnetic recording

tapes.c) It is used in the manufacture of modern blended with cotton.d) Unsaturated polyesters are used as the resin matrix in fibre reinforced

plastics structures.

BUILDING MATERIALSCement: Cement is a dirty greenish heavy powder which finds its importance as a building material. It is described as a material which possesses adhesive and cohesive properties to bind rigid masses like stones, bricks, building blocks etc., Cements are hydraulic in nature i.e., it possesses the property of setting and hardening in the presence of water. Further the essential constituents of cement used for constructional purpose are compounds of calcium (calcarious) and argillaceous i.e., (Al + Si) materials.

Classification of Cement: Based on different chemical compositions, cement is classified into four types. They are1) Natural cement 2) Puzzolana cement3) Slag cement and 4) Portland cement

Natural Cement: This is obtained by calcining and pulverizing natural rocks consisting of clay and limestone. Calcium silicates and aluminates are formed because of the combination of silica and alumina with calcium oxide. Natural cement is usually used for construction of big structures such as dams.

Properties: a) It is hydraulic in nature with low strength andb) Its setting time is very less.

Puzzolana Cement: It is one of the ancient cements in the world and was identified by the Romans. It was used by them in making concrete for the construction of walls and domes. This cement was prepared from volcanic ash of Mount Vesuvius around the place called Puzzouli in Italy. The volcanic ash consisting of silicates of calcium, iron and aluminium mixed with lime and on heating result in puzzolanic cement.

Properties: a) It is hydraulic in nature and is mixed with Portland cement and is then used for different applications.

Slag Cement: This cement is prepared by mixing hydrated lime and blast furnace slag, which is a mixture of calcium and aluminium silicates in a stream of cold water. It is dried and then pulverized to fine powder. Sometimes, accelerators like clay, or caustic soda are added to hasten the hardening process.

Properties: a) It possesses low strength.b) The time required for setting and hardening is more i.e., one

week.

Because of these properties it has very few applications and is usually used in making concrete in bulk construction. It is also used as concrete in water logged areas, where the tensile strength is less important.

Portland Cement: It is produced by heating a mixture of limestone and clay and crushing the resulting product to a fine powder. Portland cement is most widely used non-metallic material of construction. It is also known as “Magic Powder” and is a mixture of calcium silicates and calcium aluminates with small amount of gypsum.

The name Portland cement was used because this powder, on mixing with water, sets to give a hard, stone-like mass which resembles the Portland rock. Portland cement is a type of cement and not a brand name. Every cement manufacturer makes Portland cement.

COMPOSITION OF PORTLAND CEMENT:

A good sample of Portland cement has the composition of

Calcium oxide (or) lime (CaO) = 60-70% Silica (SiO2) = 20-24% Alumina (Al2O3) = 5-7.5% Magnesia (MgO) = 2-3% Ferric oxide (Fe2O3) = 1-2.5% Sulphur trioxide (SO3) = 1-1.5% Sodium oxide (Na2O) = 1% Potassium oxide (K2O) = 1%

RAW MATERIALS:The raw materials used for the manufacturing of Portland cement are:

a) Calcareous materials: Those which supply lime eg., limestone, cement rock, chalk and waste calcium carbonate from industrial processes. Limestone high in magnesia (MgO)cannot be used, because it leads to cracking. Similarly chalk freed from flint is to be used.

b) Argillaceous materials: Those which supply silica, alumina and iron oxide. Eg: clay, blast furnace, slag ashes, shale and cement rock. Commonly used are clay and shale.

c) Gypsum: This is added during the final grinding and it controls the ratio of setting and hardening.

METHODS OF MANUFACTURING PROCESS:The manufacture of cement involves the following steps:1) Mixing of raw materials:

A mixture of finely ground limestone and clay (3:1) is made by any one of the following methods.

Dry Process: The dry process produces a find ground powder. This process is employed if the limestone and clay are hard. In this process, initially limestone is crushed into pieces and then it is mixed with clay in the proportion of 3:1. This mixture is pulverized to a fine powder and is stored in storage bins and later on it is introduced into the upper end of the rotary kiln.

Wet Process: The wet process takes place in the presence of water and usually results in a slurry formation. This process is preferred if limestone and clay are soft.

In this process, the clay is washed with water in washmills to remove any foreign material, organic material etc., Powdered limestone is then mixed with the clay paste in a proper proportion (3:1). The mixture is then finely ground and homogenized to form a slurry containing about 40% of water. This is also stored in the storage bins and can be fed into the rotary kiln when necessary.

2) Burning the mixture in a rotary kiln:The rotary kiln is an inclined steel cylinder 150-200 feet long and 10 feet

in diameter and it is lined inside with fire bricks. The kiln can be rotated at a desired speed as it is mounted on rollers. As the kiln rotates, the mixture of raw materials stored from the above two process, passes slowly from the upper to the lower end. In other words the slurry of the raw materials enters from the upper end of the rotary kiln while the burning fuel (pulverized coal, oil, or natural gas) and air are induced from the lower end of the kiln. As the mixture or slurry gradually descends, the temperature rises and infact this creates different zones

in

the rotary kiln, with increasing temperature i.e., a) the drying zone: This is present in the upper part of the kiln, where the temperature is around 4000C. In this zone most of the water in the slurry gets evaporated because of the hot gases. The clay is broken as Al2O3.SiO2 and Fe2O3 i.e.,

Al2O3 . 2SiO2 . Fe2O3 .2H2O→Al 2O3+2 SiO2+Fe2O3+2H2O

b) calcinations zone or decarbonating zone: This zone is located in the middle portion of the kiln where the temperature is of the order of 10000C. In this zone the limestone is completely decomposed into CaO (quick lime) which exists in the form of small lumps, called as nodules.

CaCO3→CaO+CO2c) burning zone or clinkering zone: This zone is at the bottom and is considered to be the hottest portion of the kiln. The temperatures over here ranges around 1400-15000C. IN this zone the mixture melts and forms little rounded pasty masses about the size of peas, which are called as clinkers. The clinkers produced are greenish black in color and have a rough texture.

In the clinkering zone, lime and clay react with each other forming aluminates and silicates.

2CaO+SiO2→2CaO .SiO2(C2 S )3CaO+SiO2→3CaO .SiO2 (C3S )3CaO+Al2O3→3CaO .Al2O3(C3 A )

4CaO+Al2O3+Fe2O3→4CaO . Al2O3 .Fe2O3(C4 AF )

3) Grinding or mixing of cement clinkers with gypsum:The clinkers are cooled and then ground to requisite fineness. The finely

ground clinkers set quite rapidly, by absorption of moisture from the atmosphere. Therefore in order to reduce the rate of setting it is mixed with 2 to 3% gypsum (CaSO4.2H2O).

SETTING AND HARDENING OF CEMENT:

Portland cement on mixing with water, changes to a plastic mass called “cement paste”, hydration reaction begins, resulting in the formation of gel and crystalline products. The interlocking of the crystals, finally bind the inert particles of the aggregates into a compact rock-like material. This process is known as setting and hardening.

Setting: “Stiffening of the original plastic mass, due to initial gel formation.” Hardening is “developing of strength, due to crystallization.”

The physical changes occurring in the setting and hardening of cement may be summarized in a flow chart as follows:

Cement +Water Setting

Metastable gel Crystalline products (colloidal) (hydrated)

HardeningStable gel Crystalline products

(coarser dimensions)

a) Crystalline theory: According to this theory, constitutional compounds after hydration form crystalline products. These crystalline products undergo interlocking which is responsible for hardening of cement.

b) Colloidal theory: According to this theory, during hydration silicate gels are formed which undergo hardening and are responsible for the hardening of cement.

REACTIONS INVOLVED IN SETTING AND HARDENING OF CEMENT:

When cement is mixed with water, the paste becomes quite rigid within a short time which is known as initial set or flash set. This is due to C3A which hydrates rapidly as follows:

3CaO.Al2O3 + 6 H2O 3CaO. Al2O3.6H2O(Crystals)

These crystals prevent the hydration reactions of other constitutional compounds forming barrier over them. In order to retard the flash set, gypsum is added during the pulverization of cement clinkers. Gypsum retards the dissolution of C3A by interacting with it forming insoluble complex of sulfoaluminate which does not have quick hydrating property.

3CaO.Al2O3 + x H2O +y CaSO4.2H2O 3CaO. Al2O3. y CaSO4.zH2O

The tetracalcium alumino ferrite (C4AF) then reacts with water forming both gels and crystalline compounds as follows:

4CaO.Al2O3.Fe2O3 + 7H2O 3CaO.Al2O3.6H2O + CaO.Fe2O3.H2OCrystals Gels

These gels shrink with passage of time and leave some capillaries for the water to come in contact with C3S and C2S to undergo further hydration and hydrolysis reactions enabling the development of greater strength over a length of time.

Final setting and hardening of cement paste is due to the formation of tobermonite gel plus crystallization of calcium hydroxide.

2CaO. SiO2 + x H2O 2CaO.SiO2. x H2O Gels

2 (3CaO.SiO2) + 6 H2O 3CaO.2SiO2. 3 H2O + 3 Ca(OH)2 Tobermonite Gel

Sequence of chemical reactions during setting and hardening of cement: When water is added to cement, its various constituents undergo hydration and crystallization at different rates. i) At first, hydration of tricalcium aluminate (C3A) and tetracalcium

aluminoferrite (C4AF) takes place.ii) Next, the hydration of tricalcium silicate (C3S) begins within 24 hours and

gets completed in 7 days.iii) The gel of aluminate begins to crystallize and at the same time, dicalcium

silicate (C2S) begins to hydrate in 7 to 28 days.

Thus the initial set of cement is due to the hydration of aluminate. The development of early-strength, between 1 to 7 days, is due to the hydration of tricalcium silicate and the further hydration of aluminate. The increase of strength, between 7 to 28 days, is due to hydration of dicalcium silicate and continued hydration of tricalcium silicate.