COMPOSITION OF PORTLAND CEMENT
The principle raw materials used in the manufacture of cement are:
a) Argillaceous or silicates of alumina in the form of clays and shales.
b) Calcareous or calcium carbonate, in the form of lime stone, chalk and marl which is a mixture of clay and calcium carbonate.
The ingredients are mixed in the proportion of about two parts of calcareous materials to one part of argillaceous materials and then crushed and ground in ball mills in a dry state or mixed in wet state. The dry powder or the wet slurry is then burnt in a rotary kiln at a temperature between 1400 degree C to 1500 degree C. the clinker obtained from the kiln is first cooled and then passed on to ball mills where gypsum is added and it is ground to the requisite fineness according to the class of product.
The chief chemical constituents of Portland cement are as follows:
60 to 67%
17 to 25%
3 to 8%
Iron oxide (Fe2O3)
0.5 to 6%
0.1 to 4%
Sulphur trioxide (SO3)
1 to 3%
Soda and/or Potash (Na2O+K2O)
0.5 to 1.3%
The above constituents forming the raw materials undergo chemical reactions during burning and fusion, and combine to form the following compounds called BOGUE COMPOUNDS.
Tricalcium silicate (3CaO.SiO2)
Dicalcium silicate (2CaO.SiO2)
Tricalcium aluminate (3CaO.Al2O3)
Tetracalcium alumino-ferrite (4CaO.Al2O3.Fe2O3)
The proportions of the above four compounds vary in the various Portland cements. Tricalcium silicate and dicalcium silicates contribute most to the eventual strength. Initial setting of Portland cement is due to tricalcium aluminate. Tricalcium silicate hydrates quickly and contributes more to the early strength. The contribution of dicalcium silicate takes place after 7 days and may continue for up to 1 year. Tricalcium aluminate hydrates quickly, generates much heat and makes only a small contribution to the strength within the first 24 hours. Tetracalcium alumino-ferrite is comparatively inactive. All the four compounds generate heat when mixed with water, the aluminate generating the maximum heat and the dicalcium silicate generating the minimum. Due to this, tricalcium aluminate is responsible for the most of the undesirable properties of concrete. Cement having less C3A will have higher ultimate strength, less generation of heat and less cracking. Table below gives the composition and percentage of found compounds for normal and rapid hardening and low heat Portland cement.
Composition and compound content of Portland Cement:
(a) Composition: Percent
(b) Compound: Percent