Competitive Exams: Cement & Glass

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Portland Cement

The approximate composition of Portland cement is :

  • Calcium oxide (CaO) - 62%

  • Silica (SiO2) - 22%

  • Alumina (Al2O3) - 7.5%

  • Magnesia (MgO) - 2.5%

  • Ferric oxide (Fe2O3) - 2.5%

The above compounds are provided by the two raw materials

(a) Limestone, (b) Clay

  • Cement containing excess amount of lime cracks during setting while cement containing less amount of lime is weak in stength. Setting of cement is an exothermic process.

  • A small amount of gypsum is added to slow down the setting of cement.

  • Cement with excess of silica shows slow setting and that having an excess of alumina shows quick setting.

  • Cement containing no iron is white but hard to burn.


  • Ordinary glass is solid mixture of silica, sodium silicate and calcium silicate.

  • Glass is a super cooled liquid hence; it has no definite crystal structure and melting point.

  • We can say that glass is a mixture, not a compound.

Types of Class

Soft Glass

  • It is soda lime silicate glass.

  • It melts at low temperature. It is used in manufacturing of bottles, test tubes and glass of windows etc.

Hard Glass

  • It is potash lime silicate.

  • It melts at high temperature in comparison to soft glass and is used in manufacturing of flask, etc.

Flint Glass

It is lead-potash silicate and is used in manufacturing of prism and lens of optical instruments.

Pyrex Glass

  • It is a mixture of sodium aluminum borosilicate.

  • It has high percentage of Silica, about 80%. It is used in manufacture of high quality equipment in laboratory because it does not melt at very high temperature.

Quartz Glass

It is obtained from pure silica. It has a low coefficient of expansion and does not break even when plunged in water while red hot.

Ground Glass

It is prepared by grinding ordinary sand (soft) glass by emery and turpentine oil.

Reinforced Glass

It has a network of wires embedded in and does not shatter easily.

Safety Glass

  • It is also known as Shatter Proof glass.

  • It is prepared by placing a layer of transparent plastic glass (usually a sheet of vinyl acetate resin) between two layers of glass by means of a suitable adhesive.

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