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History of Portland cement
Joseph Aspdin, a Leeds builder and bricklayer took patent of portland cement on October 21, 1824. The name of portland was given owing to the resembalance of this hardened cement to the natural stone occuring at Portland in England. German, British and ASTM standard specifications for Portland cement were drawn in 1877, 1904 and 1904 respectively. In India also Portland cement was first manufactured in 1904.
Manufacturing of cement
The raw materials required for manufacture of Portland cement are calcareous materials, such as limestone/chalk, and argillaceous material such as shale/clay. The process of manufacture of cement consists of following stages:
1. Grinding of raw materials
2. Mixing the raw materials in certain proportions upon their purity and composition
3. Burning the raw material at a temperature of about 1300 to 1500ºC, at which the material sinters and partially fuses to form the nodular shaped clinkers
4. The clinker is cooled and ground to fine powder with addition of about 3 to 5% gypsum. The product formed using this procedure is Portland cement.
Two processes known as ‘Wet Process’ and ‘Dry Process’ depending upon whether the mixing and grinding of raw materials is done in wet or dry conditions. For the semi-dry process, the raw materials are ground dry, mixed with about 10-14% of water and burnt further to clinkering temperature. The dry process requires dry mixing of powdered materials using the compressed air. This process requires much less fuel and hence, is more popular and adopted by various companies compared to the wet process. Important stages of manufacturing of Portland cement have been summarized in Table 1:
Clinker
The mix of limestone and clay/shale after grinding when fed into the kiln undergoes a series of chemical reactions. When the mix is exposed to the hottest part of the kiln, where the temperature is around 1500ºC, about 20-30% of the materials get fused. Lime, silica and alumina get recombined. The fused mass turns into nodular form of size 3-20 mm and is known as clinker. The clinker weighs about 1100-1300 gm/litre. The litre weight of clinker indicates quality of clinker. The cooled clinker is then grounded in a ball mill with addition of 3-5% of gypsum in order to prevent flash-setting of the cement. The rate of cooling influences the degree of crystallization, the size of the crystal and the amount of amorphous materials present in the clinker. Thus, the stength properties of the cement are considerabaly influenced by the cooling rate of the clinker.
Chemical Composition of cement
Relative proportions of lime, silica, aluminum and iron oxide are responsible for influencing various properties of cement in addition to rate of cooling and fineness of grinding. Approximate oxide compositions limits of OPC are presented in Table 2. Oxides present in the raw materials when subjected to high temperature combine with each other to form complex compounds. Bogue’s compounds are presented in Table 3. The chemical requirement for OPC includes evaluation of parameters eg., Lime saturation factor, ratio of percentage of alumina to that of iron oxide, weight of insoluble residue, weight of magnesia, total sulphur content and total loss on ignition.
C3S and C2S are the most important compounds responsible for strength. Together they constitute 70-80% of cement. An increase in lime content beyond a certain value makes it difficult to combine with other compounds and free lime will exist in the clinker which causes unsoundness in cement. An increase in silica content at the expense of the content of alumina and ferric oxide will make the cement difficult to fuse and form clinker. Cements with a high total alumina and high ferric oxide content are favourable to the production of high early strengths in cement.