The following points highlight the four types of Portland Cement. The types are: 1. Portland-Pozzolana Cement (IS 1484-1991) 2. Fly Ash 3. White Cement 4. Coloured Cements.
Type # 1. Portland-Pozzolana Cement (IS 1484-1991):
An inter ground or blended mixture of Portland cement and pozzolana is called Portland pozzolana cement. This cement can be manufactured either by intimately inter grinding Portland cement clinker and Pozzolana or by intimately and uniformly blending Portland cement and fine Pozzolana.
If Portland-Pozzolana cement is manufactured by inter grinding Portland cement clinker and Pozzolana, then gypsum or calcium sulphate should be added to regulate its setting time. Grinding of the two materials together presents no difficulty, but mixing of dry powders intimately is extremely difficult. Therefore blending method should only be adopted, when grinding method is impossible or uneconomical in a particular case.
As per I.S. 1489-1976, the pozzolana constituent should not be less than 10% and not more than 25% by weight of the cement. ASTM has recommended the limits of pozzolana between 15% and d0% of the weight of the Portland-Pozzolana cement, while in U.K. the maximum limit of pozzolana has been fixed as 35%.
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Chemical Requirements:
Portland-Pozzolana cement should comply with the following chemical and physical properties:
(a) Loss on ignition = 5.0% max.
(b) Weight of magnesia (Mgo) = 6.0% max.
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(c) Sulphuric anhydide (SO3) = 2.75% max
and 3.0% when C3A is 7% or less and greater than 7% respectively.
Tri-calcium aluminate C3A is calculated by the formula.
C3A = 2.65 (Al2O2) – 1.69(Fe2O3)
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where each symbol in brackets refers to the percentage (by mass of total cement) of oxide, excluding any contained in the insoluble residue referred at S.No. (d) below.
(d) Insoluble material = [x + {2.0(100 – x)/100}] per cent max.
where x is the declared percentage of pozzolana in the given cement.
Physical Requirement:
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(a) Fineness:
Specific surface area should not be less than 3000 cm2/g
(b) Soundness:
When tested with Le Chatelier method, expansion should not be more than 10 mm.
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(c) Setting Time:
(i) Initial setting time ― 30 min.
(ii) Final setting time ― 600 min. max.
Compressive Strength:
The average strength of not less than three mortar cubes of surface area 50 cm2 composed of 1:3 cement and standard sand by weight and (P/4 + 3.0 percent of combined mass of cement and sand) water should be as follows:
At 168 ± 2 h ― 220 kg/cm2 min.
At 672 ± 4 h ― 310 kg/cm2 min.
The Portland Pozzolana cement produces less heat of hydration and offers greater resistance to the attack of aggressive waters than normal Portland cement. It also reduces the leaching of calcium hydroxide liberated during the setting and hydration of cement and also reduces the permeability of concrete. It is particularly useful in marine and hydraulic constructions and other mass concrete structures. The addition of pozzolana does not contribute to strength at early age. Generally this cement can be used where ever ordinary Portland cement is usable under normal conditions.
Pozzolana:
The history of pozzolanic materials goes back to Roman time. It is a natural or artificial material containing silica in a reactive form. As per ASTM specifications, a pozzolana is a silicious or silicious and aluminous material which itself possesses little or no cementitious properties, but in finely divided form and in the presence of moisture, it chemically reacts with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
It is essential that a pozzolana should be in a finely divided state, only then silica can combine with calcium hydroxide (liberated by hydrating Portland cement) in the presence of water to form stable calcium silicates which have cementitious properties.
Following materials may be used as pozzolana:
1. Calcined dictomaceous earths
2. Pumicite
3. Opaline, shales and cherts
4. Burnt clay
5. Volcanic ash.
6. Fly ash etc.
While considering pozzolanas in general, it should be noted that the silica should be amorphous as crystalline silica has very low reactivity.
The pozzolanic activity with cement is determined either by the measurement of a pozzolanic activity index. The pozzolanic activity index is the ratio of compressive strength of the mixture with a specified replacement of cement by pozzolana to the strength of a mix without replacement. There is also a pozzolanic activity index with lime, which determines the total activity of a pozzolana.
Pozzolanic activity is assessed by comparing the quantity of calcium hydroxide Ca(OH)2 present in a liquid phase in contact with the hydrated pozzolanic cement, with the quantity of calcium hydroxide Ca(OH)2 capable of saturating a medium of the same alkalinity. The underlying principle is that the pozzolanic activity consists of fixing Ca(OH)2 by the pozzolana, so that lower the resulting quantity of Ca(OH)2, the higher the pozzolanacity.
Type # 2. Fly Ash:
It is a waste product from coal fired power stations and Railway locomotives etc. It is the most common artificial pozzolana. The fly ash particles are spherical and of the same fineness as cement such that silica is always readily available for reaction.
The pozzolanic activity of fly ash is good, but it is essential that it has a constant fineness and constant carbon content. These two requirements are often inter dependent, since the carbon particles tend to be coarser. In fly ash usually loss on ignition should not be greater than 7%, however carbon content upto 12% is acceptable. Actually carbon is not pozzolanic, but a filler only. To maintain the uniformity of the resulting concrete, it is essential to maintain the uniformity in the properties of fly ash. A good pozzolanic fly ash should contain the following percentages of different constituents as per I.S. 3812-1981.
It has been observed that the colour of concrete is affected by the use of fly ash.
Portland pozzolana cement gains strength very slowly and thus curing is required for a longer period, but their ultimate strength is approximately the same as that of ordinary Portland cement. It has been observed that after about four months’ time the strength of fly ash pozzolanic concrete is same as that of ordinary cement concrete and after wards it is higher than that of ordinary cement concrete strength.
Pozzolanas are often cheaper than Portland cement that they replace, but their main advantage lies in slow hydration and thus low rate of heat development. This property is of great importance in mass concrete construction that is why Portland pozzolana cement or a partial replacement of Portland cement by the pozzolana is mostly used when pozzolanas are used as a partial replacement for cement, the cement and pozzolanas should be batched separately and mixed with the other ingredients in the mixer.
Partial replacement of Portland cement by pozzolana has to be carefully defined, as the specific gravity of pozzolanas is much lower than that of cement. The specific gravity of fly ash varies from 1.9 to 2.4 whereas the specific gravity of cement is 3.15. With replacement the concrete mixes have a lower early strength than when only Portland cement is used, but beyond about three months age, there is no loss of strength. With lean mixes there may even be a long term gain of strength due to the replacement.
In case equal early strength is required and pozzolana is to be used, then addition of pozzolana rather than partial replacement of cement are essential. Partial replacement of Portland cement by pozzolana reduces the permeability of concrete to a great extent due to continued filling of pores by formation of hydrates and absence of free lime, which could be leached out.
Type # 3. White Cement:
For architectural purposes white concrete is required. To achieve best results, it is advisable to use white cement with suitable fine and coarse aggregates. Further as this cement contains low contents of alkalies it is not liable to cause staining.
A typical compound composition of white cement is shown in Table 3.8:
White Portland cement is manufactured from raw materials containing no iron oxide or very little iron oxide and manganese oxide. Generally China clay is used together with lime stone or chalk free from impurities. Further to avoid contamination with coal ash in kiln, oil is used as fuel in place of pulverised coal. The elimination of iron oxide needs higher temperature in kiln to fuse the raw materials as iron oxide acts as a flux. To obtain higher temperature in kiln needs more fuel which is not economical. Hence to lower down the fusing temperature, sometimes cryolite (sodium aluminum fluoride) is added as a flux.
To prevent contamination of the cement with iron during grinding, nickel and molybdenum alloy balls are used in place of ordinary iron grinding balls. Thus the cost of grinding is higher. Due to higher grinding cost and expensive raw materials, make this cement about 3 times costlier than ordinary Portland cement.
To obtain good colour, usually white concrete of rich proportions is used. The water cement ratio usually is not adopted higher than 0.4. White cement has slightly lower specific gravity than ordinary Portland cement, generally between 3.05 to 3.10. Strength of white cement also is lower than ordinary Portland cement.
Birla White Cement:
Birla white and J.K. white cements are manufactured around Jodhpur in Rajasthan. The suitable raw material used for white cement is only available around Jodhpur in Rajasthan. The raw material used for white cement should be of high purity. Lime stone having 96% CaCO3, and iron oxide less than 0.07% is the best. Other raw materials are China clay with iron content about 0.72 to 0.8%, silica (sand) fluorspar as flux and selenite as retarder.
The fuels used are refined furnace oil or gas. Sea shells and coral can also be used as raw material for the production of white cement. The white cement is ground finer than ordinary Portland cement. The whiteness of white cement measured by ISI scale should not be less than 70%, while measured by Hunter’s scale, it should not be less than 90%. The strength of Birla white cement is much higher than given in IS Code 8042-1989.
Typical test results of Birla white cement are shown in Table 3.9 below:
Type # 4. Coloured Cements:
For the manufacture of coloured cements either gray or white Portland cement is used as base. With grey cement only red or brown colour can be given successfully, for other colours white cement clinker is used. Usually 5 to 10% pigment by weight is intimately grounded with the Portland cement clinker. The pigment used should be chemically inert with cement and should have fast colour.
Sometimes a film of free calcium hydroxide deposits on the surface of the concrete and covers the effect of pigment. It can be removed by washing the surface with dilute hydrochloric acid (HCl, 1:10) and then with plain water. The surface should be wetted with water before applying HCl. HCl should not be allowed to stay in contact with concrete surface for long.