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Portland cement is available in different varieties suitable for different purposes. The different Portland cements available are:
- Ordinary Portland Cement
- Rapid Hardening Cement
- Extra Rapid Hardening Cement
- Low Heat Portland Cement
- Portland Blast Furnace Slag Cement
- Portland –Puzzolana Cement
- Sulphate Resisting Portland Cement
- White Portland Cement
- Coloured Portland Cement
Portland Cement:
- Argillaceous or silicates of aluminia in the form of clay and shales.
- 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 material to one part of argillaceous material and then crushed and ground in ball mills in a dry state or mixed in a wet state. The dry powder or the wet slurry is then burnt in a rotary kiln at a temperature between 1400 o c and 1500 0 c. The clinker obtained from the kilns 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
Lime ( CaO) 60 to 67%
Silica (SiO2) 17 to 25%
Alumina (Al2O3) 3 to 8%
Iron Oxide (Fe2O3) 0.5 to 6%
Magnesia (MgO) 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 four compounds (called Bogue compounds) in the finished product:
| Compound
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Abbreviated designation
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| Tricalcium solicate (3CaO.SiO2)
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C3S
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| Dicalcium Silicate (2CaO.SiO2)
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C2S
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| Tricalcium aluminate ( 3CaO.Al2O3)
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C3A
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| Tetracalcium alumino-ferrite (4CaO.Al2O3.Fe2O3)
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C4AF
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The proportion of the above four compounds vary in the various Portland cements. Tricalcium silicate and dicalcium silicates contribute most to the eventual strength.
Intial Portland cement is due to the 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 upto 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 most of the undesirable properties of concrete. Cement having less C3A will have higher ultimate strength, less generation of heat and less cracking.
- Ordinary Portland Cement ( IS:269):
The properties of various types of portland cements differ because of relative proportions of the four compounds and the fineness to which the cement clinker is ground. The Ordinary portland cement or the Setting cement is the basic Portland cement and is manufactured in larger quantities than all the others. It is suited for use in general concrete construction where there is no exposure to sulphates in the soil or in ground water.
- Rapid Hardening Portland Cement( IS:269):
This cement is also known as high-early strength cement. It is similar to Ordinary Portland Cement except that it is ground finer, possesses more C3S and less C2S than the Ordinary Portland Cement. The magnitude of the increase in strength is gauged from the fact that the strength developed at the age of 3 days is about the same as 7 days strength of ordinary Portland cement with the same water-cement-ratio. The main advantage of Rapid hardening cement is that shuttering may removed much earlier, thus saving considerable time and expenses. This is also used for road work where it is imperative to open the road to traffic with the minimum delay.
- Extra Rapid Hardening Cement:
Extra Rapid Hardening Cement is obtained by inter-grinding calcium chloride with Rapid Hardening Portland Cement. The normal addition of CaCl2 is 2% (of the commercial 70% Cacl2) by weight of the rapid hardening cement. The addition of CaCl2) also imparts quick setting properties. Hence this cement should be placed and fully compacted within 20 minutes of mixing.
- Low Heat Portland cement (IS: 269):
When concrete is poured in any structure, an increase in temperature occurs and a certain amount of heat is generated. This is due to the chemical reaction that takes place while the cement is setting and hardening. Low Heat Portland Cement is used in massive construction like abutments, retaining walls, dams, etc. where the rate at which the heat can be lost at the surface is lower than at which the heat is initially generated. The heat generated in ordinary cement at the end of 3 days may be of the order of 80 calories per gram of cement, while in low heat cement it is 50 calories per gram. It has low percentage of C3A and relatively more C2S and less C3S than ordinary Portland Cement. This is achieved by restricting the amount of calcium and increasing the silicates present in the raw materials of manufacture. Therefore, it has low rate of gain of strength, but the ultimate strength is practically the same.
- Portland Blast Furnace Slag Cement: (IS: 455):
This cement is made by intergrinding Portland Cement clinker and blast furnace slag, the proportion of the slag being not less than 25% or more than 65% by weight of cement as prescribed by IS: 455. The Slag should be granulated blast furnace slag of high lime content, which is produced by rapid quenching of molten slag obtained during the manufacture of pig iron in a blast furnace. It is usual for the Portland Cement clinker to be ground with a slag, a small percentage of gypsum being added to regulate the setting time. The blending of Portland cement clinker with slag by no means detracts from any desired property of cement. Indeed, it confers upon it some additional advantage. This is because the granulated slag itself possesses latent hydraulic properties which are tremendously activated when the slag crystallized and integrated with Portland cement clinker. In general, blast furnace cement will be found to gain strength more slowly than the ordinary Portland cement. It has less heat of hydration than ordinary Portland cement. From the point of view of a builder and structural engineer, Portland blast furnace slag cement may be used for all purpose for which ordinary Portland cement is used. In addition, in view of its low heart evolution, it can be used in mass concrete structure such as dams, retaining walls, foundations and bridge abutments.
- Portland Pozzolana Cement: (IS: 1489):
Portland Pozzolana cement is manufactured either by intergrinding Portland cement clinker and pozzolana or by intimately and uniformly blending Portland cement and fine pozzolana. While intergrinding presents no difficulty, blending tends to result in a non- uniform product . As per Indian Standards, the proportion of pozzalana may vay from 10 to 25% by weight of cement. The Pozzalana used in the manufacture of Portland Pozzalana cement in India is , at present , burnt clay or shale, or fly ash. Although pozzalanas have no cementing values themselves, they have the property of combining with the free lime to produce a stable lime pozzalana compound which has definite cementitious properties. The cement has higher resistance to chemical agencies and to attack by sea water, because of absence of free lime. Portland Pozzalana cement also has a lower heat of evolution. Portland Pozzalana cement is frequently stated to have lower rate of development of strength than ordinary Portland cement. However when the Puzzolana is selected with care and is calcined and ground with Portland cement clinker under controlled conditions, the compressive strength reached by Portland pozzolana cement are comparable with those reached by ordinary Portland cement.
- Sulphate Resisting Cement:
In Sulphate Resisting Portland Cement, the quantity of tricalcium aluminate is strictly limited. They are normally ground finer than Portland cement. The action of sulphate is to form Sulphoaluminates which have expansive properties and so cause disintegration of the concrete. Sulphate resisting cement should be allowed to harden in the air for as long as possible to allow a resistant skin to be formed through carbonation by the action of atmosphere carbon dioxide.
- White and Coloured Portland Cements:
The grayish colour of Portland cement is due to the presence of the iron oxide. White Portland cement is manufactured in such a way that the percentage of Iron oxide is limited to less 1%. To achieve this, superior raw materials, such as chalk and lime stone having low percentage of iron, and white clay (China Clay) are used. Sodium aluminum flouride (Croyolite) is added to act as flux in the absence of iron oxide. Oil fuel is used in place of pulverised coal, in the killing process in order to avoid contamination by coal ash. Coloured Portland cements are usually obtained by adding strong pigments, upto 10% to the ordinary white cement during grinding of clinker. The essential requirements of a good pigment are that it should be permanent and should be chemically inert when mixed with cement.
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