engineering materials 2

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Weathering of cement and

concreteDeterioration of structures

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Deterioration of structures

Weathering is due to long term exposure to corrosiveenvironmental conditions

Constituents of cement are attacked by water, salt solutionsand acids

Concrete consisting of cement mortar (a mixture ofPortland cement, fine aggregates, sand & water) withcoarse aggregate such as gravel or crushed stone is alsosusceptible to deterioration due to long term exposure

Tensile strength of concrete is increased by reinforcement

with steel rods for use in beams, pillars etc. where tensilestresses occur

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Effect of acidity & protection

Attack of cement & concrete by acidic solutions of CO2,inorganic acids increases with greater acidity

Acidic water leach out the lime and also hydrolysesaluminates and silicates and dissolve the liberated

Ca(OH)2

This attack may be impaired by coating differentprotective agents like linseed oil, chlorinated rubber orepoxy ester paints and SiF4 which forms a protective

coating of CaF2 and hydrated alumina and silica

This treatment is useful for sewage pipes

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Attack by sulphate solutions

Cement & concrete structures are susceptible to aserious deterioration due to attack by sulphatesolutions

sulphate solutions react with Ca(OH)2 to form gypsum

(Ca sulphate dihydrate)

C3A phase of hardened cement reacts with Ca sulphateand other sulphates to form Ca-sulphoaluminate with ahuge increase in volume (227%)

This volume expansion leads to disruption of cementand concrete structures

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Cement with greater resistance

Effect due to attack by sulphate solutions may be

overcome by increasing C4AF content and by

superheated steam

Superheated steam converts the free Ca(OH)2into the more hydrated mono-ca-silicate (CSH).

Cements with greater resistance to sulphate

attack are: (i)- Superheated Cement, and (ii) high

alumina cement

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Admixtures for concrete

These are added to improve the properties of concrete

These are:

Air entraining agents-Surfactants such as oleic acid, caprylic acid, p-dodecyl benzenesulphonic acid

These agents resists spalling & cracking

Water decreasing agents-lignosulphonate, melamine formaldehyderesin sulphonates etc., plasticity improves

Setting accelerators-like CaCl2 & Ca formate acclerate the hydration ofC

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S and C3

S phases (contributing to the strength of the cement) andretard the hydration of C3A.

The optimum amount of CaCl2 is about 1.5% by wt and any excesspromotes corrosion of the reinforcing steel.

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Ceramics (Gk. Keramos=burnt matter)

Ceramic materials consist of a combination of metallic &non-metallic elements

Ceramics include- Bricks, tiles, glass, abrasives, refractorieswhich are mostly produced from naturally occurringminerals of silica and alumina

Highly refractory carbides, borides, nitrides are also calledceramics

Properties-ceramics have desirable mechanical propertiessuch as good tensile and compressive strengths, resistance

to weathering, chemical attack, capability to withstand hightemp as well as good electrical resistance

7Sand, calcite (CaCO3), diamond-abrasives


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Three major components of ceramics A plastic component which imparts plasticity e.g. clay

Dry clay is not plastic but the incorporation of water makes it plastic, making itamenable to be shaped

A refractory non-plastic component- it is silica

Contributes to the mechanical strength of the products

Quartz is mostly used as the refractory component in ceramics

A flux or glassy material- which improves the workability and provides thebonding of ingredients

The flux component of the ceramic is feldspar- a constituent of igneous rock.

Naturally occuring albite (sodium feldspar-Na2O.Al2O3.6SiO2),

orthoclase (sodium feldspar-K2O.Al2O3.6SiO2)

and anorthite (lime feldspar-CaO.Al2O3.6SiO2 are used as flux material

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Method of Ceramics production

It involves following steps:

Fabrication or formation of ceramic body

Drying Firing

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Ceramics production First step-Fabrication or formation of ceramic body

By throwing soft mud (or raw material mix) on a pottershand wheel

Other methods

Stiff mud process- The mix containing 10-20% water is

forced through a steel die of the desired shape in a pug mill

Slip casting- An aqueous suspension of the mix havingpourable consistensy is poured ito a mould of PoP. PoPabsorbs the water and a uniform deposit is obtained

Dry pressing- for making floor & wall tiles (from dry mixcontaining 5-15% water)

Hot pressing- for making dense refractory oxides &carbides

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Second step is Drying in Kilns at controlled

humidity & rates of drying to prevent crackingof the product

Last step is firing of dried ceramic ware in Kilns

over a range of temp. 800-2000 C depending

on the composition of the mix

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Three Major Groups of Ceramic Materials

Three Groups

Clay Products

Glass

Refractories all of which find extensive use in a

variety of industries

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Clay Products

May be classified into three major groups

White wares or white pottery or china such as

porcelain ware, table ware or dinner ware

Stoneware such as sanitary fixtures, drainagepipes etc.

Structural clay products like tiles & bricks

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White Wares

White products with a refractory body & glossy

coating called glaze

Constituents of Glaze are: quartz, feldspar, boric oxide(B2O3) & lead oxide

The raw materials used in the manufacture of white waresare kaolin, feldspar and silica free from iron oxide and othercolored impurities

Manufacturing steps-(i)Prep of body, (ii)Glazing, (iii)Decoration

Fine powder of raw material blended in the requiredproportion is mixed with water to form a plastic mass

Desired article is shaped by hand moulding, slip casting or

pressing Now the body is carefully dried and fired in a buscuit oven

(buscuit firing)

14http://www.pillivuyt.fr/en-pillivuyt-fabrication.php


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Porous material

Biscuit

oven

Bisque ofsoft-paste porcelain

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