Building Materials, Third Edition

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These three inversions affect the use of silica as a refractory material under conditions of
rapid temperature changes where it may produce cracking or spalling.

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 ƒ Theoretically the tensile strength of ceramics is very high but in practice
it is quite low. Tensile failures of ceramics are attributed to the stress concentrations
at the pores and micro-cracks at grain corners. The modulus of elasticity ranges from 7 × 10^4 to
42 × 10^4 N/mm^2.^ Glass fibres tensile strength of the order of 700 N/mm^2.

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 ƒ The compressive strength is high and it is usual to use ceramics like
clay, cement and glass products in compression.

ƒ— ƒ Ceramics have very high shear strength with resistance to failing in a brittle
manner.

„—   ƒ is difficult to ascertain and ceramics are not used in the places where
such strength is the criteria.

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Thermal capacity, conductivity and resistance to shocks need to be considered while using a
ceramic. The specific heat of fire-clay bricks is 0.250 at 1000° C and 0.297 at 1400° C, whereas for
carbon bricks it is about 0.812 at 200° C and 0.412 at 1000° C. The specific heat for refractories
to be used in regenerator chambers should be more. The heat in ceramics is conducted by
phonon conductivity and by the interaction of lattice vibration. The ceramic materials do not
have enough free electrons to bring out electronic thermal conductivity. At high temperatures,
conduction takes place by transfer of radiant energy. The thermal conductivity of refractories
should be minimum for linings and maximum for crucibles and retorts.
Thermal shocks are developed primarily because of expansion and contraction of ceramics.
Lithium compounds are used in ceramics to improve the shock resistance. Hot pressed silicon
nitride has the best thermal shock resistance whereas steatite is the most poor.