Cement

Cement

Cement is an important building material. It was first introduced in England in 1824 by Joseph Aspdin. It is also called Portland cement because it resembles with the natural limestone quarried in the Isle of Portland, England.

Cement is a product obtained by combining a material rich in lime, CaO with other material such as clay which contains silica, SiO₂ along with the oxides of aluminium, iron and magnesium. The average composition of Portland cement is : CaO, 50-60%; SiO₂, 20-25%;  Al₂O₃, 5-10%; MgO, 2-3%;  Fe₂O₃, 1-2% and SO₃, 1-2%. For good quality cement, the ration of silica (SiO₂) to alumina (Al₂O₃) should be between 2.5 and 4 and the ratio of lime (CaO) to the total of the oxides of silicon (SiO₂) aluminium (Al₂O₃) and iron (Fe₂O₃) should be as close as possible to 2.

The raw materials for the manufacture of cement are limestone and clay. When clay and lime are strongly heated together they fuse and react to form ‘cement clinker’. This clinker is mixed with 2-3% by weight of gypsum (CaSO₄.2H₂O) to form cement. Thus important ingredients present in Portland cement are dicalcium silicate (Ca₂SiO₄) 26%, tricalcium silicate (Ca₃SiO₅) 51% and tricalcium aluminate (Ca₃Al₂O₆) 11%.

Setting of Cement

When mixed with water the setting of cement takes place to give a hard mass. This is due to the hydration of the molecules of the constituent and their rearrangement. The purpose of adding gypsum is only to slow down the process of setting of the cement so that it gets sufficiently hardened.

Uses

Cement has become a commodity of national necessity for any country next to iron and steel. It is used in concrete and reinforced concrete, in plastering and in the construction of bridges, dams and buildings.