Engineering Chemistry S Datta

436 ENGINEERING CHEMISTRY
Q. 5. What is a hydraulic cement?
Ans. The cement sets under water, that is why the term hydraulic came.
Q. 6. Give general composition of cement.
Ans. See text page 418.
Q. 7. What is glass?
Ans. Glass may be defined
• Physically as an amorphous, transparent or translucent, rigid supercooled liquid of
infinitely high viscosity.
• Chemically as a mixture of a number of metallic silicates, one of which is an alkali
metal.
• Glass has almost 100% elastic recovery.
Q. 8. What is vitrification?
Ans. The process of not forming crystals during cooling of glass is called vitrification.
Q. 9. What is devitrification of glass?
Ans. Glass-wares left unused for a long time gradually loose their transparency and
become opaque due to the crystallisation of some of metallic silicates of glass. That phenomenon
is known as devitrification of glass.
Q. 10. What are ceramics?
Ans. Clay materials are known as ceramics. Even at the present time ceramics remain
important in many spheres of life. Building, roads, furnaces, kitchen wares, laboratory wares
are made of ceramics.
The greatest uses of advanced ceramic products are in
• Aerospace industry due to their lighter weights compared to metal alloys
• Heat shields for space vehicles
• As bio materials like false teeth; bone replacement, artificial hip joints etc.
Q. 11. What are refractories?
Ans. Refractories are earthy ceramic materials of low thermal conductivity capable of
withstanding extremely high temperature without any change.
Q. 12. What are primary uses of refractories?
Ans. The primary uses of refractories are for lining
(i) Steel furnaces
(ii) Coke ovens
(iii) Glass wares.
Q. 13. Mention some important refractories.
Ans. (i) Fire clay (aluminium silicates), (ii) Silica, (iii) Silicon carbide, (iv) High alumina
(70-80% Al 2
O 3
), (v) Magnesite (MgO), (vi) Mullite (Clay sand), (vii) Zirconia (ZrO 2
).
Q. 14. Mention some refractory metals.
Ans. Tungsten and tantalum (W and Ta) are refractory metals capable to withstanding
high temperatures.