The raw materials used for the manufacture of cement consists mainly of lime, silica, alumina and iron oxide.
Following table shows the approximate oxide composition limits of ordinary portland cement.
Oxide
(1)Lime – CaO -60 – 67%
(2) Silica-SiO-17-25%
(3) Alumina – Al O-03-08%
(4) Iron oxide-Fe2Oa-0.5-06%
(5) Magnesia – MgO-0.1-04%
(6) Sulphur-S-01-03%
(7) Alkalies -KO Na 0. 04-13%
(8) Gypsum – Caso-4%
Chemical requirement:
(a) Ratio of % of lime to % of silica, alumina and iron oxide when calculated by CaO-0.7 SO, should not exceed 1.02
2.8 Si O, + 1.2 L-O, + 0.65 Fe; O, less than 0.65
(b) Ratio of of alumina to that of iron oxide < 0.66%
(c) Free Insoluble residue
(d) Free Magnesia <5%
(e) Total sulphur content, calculated as sulphuric anhydride < 2.75%
(1) Lime (Cao) : Important ingredients of cement and proportion is maintained Excess lime makes the cement unsound and to expand which results disintegration. Deficiency of lime decreases the strength of cement and causes cement to set quickly
(2)Silica (SiO) :- Important ingredients impart strength to due to formation of calcium and tricalcium silicates Excess silica Increases the strength of cement but setting time cement
(3) Alumina (AI2, O3):- It imparts quick setting property to cement, It acts as a flux. i.s. it lowers the clinkering temperature Excess alumina weaken the cement.
(4) Calcium Sulphate (CaS04) Gypsum :- Its main function is to increase the initial setting time of cement.
(5) Iron Oxide (Fe2O3 ) : This ingredient imparts colour, hardness and strength to cement.
(6) Magnesia (MgO) : It imparts hardness and colour to cement. A high content of magnesia makes the cement unsound.
(7) Sulphur (S) :- Very small amount is useful in making cement sound.
(8) Alkalies :- The Alkalies present in raw materials are carried away by fuel gases during heating and cement contains only a small amount of alkalies. Excess Alkalies cause a number of troubles such as alkali aggregate reaction, efflorescence and staining. When used in concrete and as mortar Harmul constituents of cement are alkali oxides.
