Competitive Exams: Cement & Glass

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.

Glass

  • 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.