TYPES OF CEMENT
by Vishnu Sankar on Thursday, October 6, 2011 at 4:20pm
Cement Types
Some of the different cement types are listed below,
- 1.Ordinary Portland Cement
- 2.Rapid Hardening Cement
- 3.Extra Rapid Hardening Cement
- 4.Sulphate Resisting Cement
- 5.Quick Setting Cement
- 6.Super Sulphated Cement
- 7.Low Heat Cement
- 8.Portland Pozzolana Cement
- 9.Air-Entraining Cement
- 10.Coloured Cement
- 11.Hydrophobic cement
- 12.Masonry Cement
- 13.Expansive Cement
- 14.IRS-T 40 Special Grade Cement
- 15.Oil-Well Cement
- 16.Rediset Cement
- 17High Alumina Cement
- 18High Early Strength Cement
Ordinary Portland Cement
- Ordinary Portland cement (OPC) is the most important type of cement.
- The OPC was classified into three grades, namely 33 grade, 43 grade and53 grade depending upon the strength of the cement at 28 days when tested as per IS 4031-1988. If the 28 days strength is not less than 33N/mm2, it is called 33 grade cement, if the strength is not less than 43N/mm2, it is called 43 grade cement, and if the strength is not less than 53 N/mm2, it is called 53 grade cement.
- But the actual strength obtained by these cements at the factory are much higher than the specifications.
- The manufacture of OPC is decreasing all over the world in view of the popularity of blended cement on account of lower energy consumption, environmental pollution, economic and other technical reasons.
- In the years to come the use of OPC may still come down, but all the same the OPC will remain as an important type for general construction.
- This cement is similar to ordinary Portland cement. As the name indicates it develops strength rapidly and as such it may be more appropriate to call it as high early strength cement.
- Rapid hardening cement which develops higher rate of development of strength should not be confused with quick-setting cement which only sets quickly.
- Rapid hardening cement develops at the age of three days, the same strength as that is expected of ordinary Portland cement at seven days.
- The rapid rate of development of strength is attributed to the higher fineness of grinding and higher C3S and lower C2S content.
- The higher fineness of cement particles expose greater surface area for action of water and also higher proportion of C3S results in quicker hydration.
- Therefore, rapid hardening cement should not be used in mass concrete construction.
- In pre-fabricated concrete construction.
- Where formwork is required to be removed early for reuse.
- Road repair works.
- In cold weather concrete where the rapid rate of development of strength reduces the vulnerability of concrete to the frost damage.
- Extra rapid hardening cement is obtained by intergrinding calcium chloride with rapid hardening Portland cement.
- The normal addition of calcium chloride should not exceed 2 percent by weight of the rapid hardening cement.
- It is necessary that the concrete made by using extra rapid hardening cement should be transported, placed and compacted and finished within about 20 minutes.
- It is also necessary that this cement should not be stored for more than a month.
- Extra rapid hardening cement accelerates the setting and hardening process.
- A large quantity of heat is evolved in a very short time after placing.
- The acceleration of setting, hardening and evolution of this large quantity of heat in the early period of hydration makes the cement very suitable for concreting in cold weather.
- The strength of extra rapid hardening cement is about 25 per cent higher than that of rapid hardening cement at one or two days and 10–20 per cent higher at 7 days.
- The gain of strength will disappear with age and at 90 days the strength of extra rapid hardening cement or the ordinary portland cement may be nearly the same.
- There is small amount of initial corrosion of reinforcement when extra rapid hardening cement is used, but in general, this effect does not appear to be progressive and as such there is no harm in using extra rapid hardening cement in reinforced concrete work. However, its use in prestress concrete construction is prohibited.
- Ordinary Portland cement is susceptible to the attack of sulphates, in particular to the action of magnesium sulphate. Sulphates react both with the free calcium hydroxide in set cement to form calcium sulphate and with hydrate of calcium aluminate to form calcium sulphoaluminate, the volume of which is approximately 227% of the volume of the original aluminates.
- Their expansion within the frame work of hardened cement paste results in cracks and subsequent disruption.
- Solid sulphate do not attack the cement compound. Sulphates in solution permeate into hardened concrete and attack calcium hydroxide, hydrated calcium aluminate and even hydrated silicates.
- The above is known as sulphate attack. Sulphate attack is greatly accelerated if accompanied by alternate wetting and drying which normally takes place in marine structures in the zone of tidal variations.
- To remedy the sulphate attack, the use of cement with low C3A content is found to be effective. Such cement with low C3A and comparatively low C4AF content is known as Sulphate Resisting Cement. In other words, this cement has a high silicate content. The specification generally limits the C3A content to 5 per cent.
- Tetracalcium Alumino Ferrite (C3AF) varies in Normal Portland Cement between to 6 to 12%. Since it is often not feasible to reduce the Al2O3 content of the raw material, Fe2O3 may be added to the mix so that the C4AF content increases at the expense of C3A. IS code limits the total content of C4AF and C3A, as follows.
2C3A + C4AF should not exceed 25%
- Concrete to be used in marine condition;
- Concrete to be used in foundation and basement, where soil is infested with sulphates;
- Concrete used for fabrication of pipes which are likely to be buried in marshy region or sulphate bearing soils;
- Concrete to be used in the construction of sewage treatment works.
Quick Setting Cement
- This cement as the name indicates sets very early.
- The early setting property is brought out by reducing the gypsum content at the time of clinker grinding.
- This cement is required to be mixed, placed and compacted very early.
- It is used mostly in under water construction where pumping is involved.
- Use of quick setting cement in such conditions reduces the pumping time and makes it economical. Quick setting cement may also find its use in some typical grouting operations.
- Super sulphated cement is manufactured by grinding together a mixture of 80-85 percent granulated slag, 10-15 per cent hard burnt gypsum, and about 5 per cent Portland cement clinker. The product is ground finer than that of Portland cement.
- Specific surface must not be less than 4000 sq.cm per gm.
- This cement is rather more sensitive to deterioration during storage than Portland cement.
- This cement has high sulphate resistance. Because of this property this cement is particularly recommended for use in foundation, where chemically aggressive conditions exist.
- As super-sulphated cement has more resistance than Portland blast furnace slag cement to attack by sea water, it is also used in the marine works. Other areas where super-sulphated cement is recommended include the fabrication of reinforced concrete pipes which are likely to be buried in sulphate bearing soils. The substitution of granulated slag is responsible for better resistance to sulphate attack.
- When we use super sulphated cement the water/ cement ratio should not be less than
0.5. A mix leaner than about 1:6 is also not recommended.
- It is well known that hydration of cement is anexothermic action which produces large quantity of heat during hydration.
- Formation of cracks in large bodyof concrete due to heat of hydration has focussed the attention of the concrete technologists to produce a kind of cement which produces less heat or the same amount of heat, at a low rate during the hydration process.
- Cement having this property was developed in U.S.A. during 1930 fo r use in mass concrete construction, such as dams, where temperature rise by the heat of hydration can become excessively large.
- A low-heat evolution is achieved by reducing the contents of C3S and C3A which are the compounds evolving the maximum heat of hydration and increasing C2S.
- A reduction of temperature will retard the chemical action of hardening and so further restrict the rate of evolution of heat. The rate of evolution of heat will, therefore, be less and evolution of heat will extend over a longer period.
- The specific surface of low heat cement as found out by air-permeability method is not less than 3200 sq. cm/gm. The 7 days strength of low heat cement is not less than 16 MPa in contrast to 22 MPa in the case of ordinary Portland cement. Other properties, such as setting time and soundness are same as that of ordinary Portland cement.
- The history of pozzolanic material goes back to Roman’s time. The descriptions and details of pozzolanic material will be dealt separately under the chapter ‘Admixtures’.
- Portland Pozzolana cement (PPC) is manufactured by the intergrinding of OPC clinker with 10 to 25 per cent of pozzolanic material (as per the latest amendment, it is 15 to 35%).
- A pozzolanic material is essentially a silicious or aluminous material which while in itself possessing no cementitious properties, which will, in finely divided form and in the presence of water, react with calcium hydroxide, liberated in the hydration process, at ordinary temperature, to form compounds possessing cementitious properties.
- The pozzolanic materials generally used for manufacture of PPC are calcined clay or fly ash.
- The pozzolanic action is shown below:
Calcium hydroxide + Pozzolana + water ----> C – S – H (gel) - Portland pozzolana cement produces less heat of hydration and offers greater resistance to the attack of aggressive waters than ordinary Portland cement. Moreover, it reduces the leaching of calcium hydroxide when used in hydraulic structures. It is particularly useful in marine and hydraulic construction and other mass concrete constructions.
- For hydraulic structures;
- For mass concrete structures like dam, bridge piers and thick foundation;
- For marine structures;
- For sewers and sewage disposal works.
- Air-Entraining Cement is made by mixing a small amount of an air-entraining agent with ordinary Portland cement clinker at the time of grinding.
- The following types of air-entraining agents could be used:
- Alkali salts of wood resins.
- Synthetic detergents of the alkyl-aryl sulphonate type.
- Calcium lignosulphate derived from the sulphite process in paper making.
- Calcium salts of glues and other proteins obtained in the treatment of animal hides.
- These agents in powder, or in liquid forms are added to the extent of 0.025–0.1 per cent by weight of cement clinker. There are other additives including animal and vegetable fats, oil and their acids could be used.
- Wetting agents, aluminium powder, hydrogen peroxide could also be used. Air-entraining cement will produce at the time of mixing, tough, tiny, discrete non-coalesceing air bubbles in the body of the concrete which will modify the properties of plastic concrete with respect to workability, segregation and bleeding. It will modify the properties of hardened concrete with respect to its resistance to frost action. Airentraining agent can also be added at the time of mixing ordinary Portland cement with rest of the ingredients.
Coloured Cement
- For manufacturing various colored cements, either white cement or grey Portland cement is used as a base.
- The use of white cement as a base is costly. With the use of grey cement, only red or brown cement can be produced.
- Coloured cement consists of Portland cement with 5-10% of pigment.
- The pigment cannot be satisfactorily distributed throughout the cement by mixing, and hence, it is usual to grind the cement & pigment together.
- Chromium oxide – green colour
- Cobalt – blue colour
- Iron oxide – brown colour
- The raw materials used for white cement are:
- High purity limestone (96% CaCO3 & less than 0.07% iron oxide)
- China clay (0.72-0.8% of iron oxide, silica sand, fluorspar as flux and selenite as retarder)
- Grey colour of OPC is due to the iron oxide present.
- Hydrophobic cement is obtained by grinding ordinary Portland cement clinker with water repellant film-forming substance such as oleic acid, and stearic acid.
- The water-repellant film formed around each grain of cement, reduces the rate of deterioration of the cement during long storage, transport, or under unfavourable conditions.
- The film is broken out when the cement and aggregate are mixed together at the mixer exposing the cement particles for normal hydration. The film forming water-repellant material will entrain certain amount of air in the body of the concrete which incidentally will improve the workability of concrete.
- Some places get plenty of rainfall in the rainy season and have high humidity in other seasons.The transportation and storage of cement in such places cause deterioration in the quality of cement. In such far off places with poor communication system, cement perforce requires to be stored for long time.
- Ordinary cement gets deteriorated and loses some if its strength, whereas the hydrophobic cement which does not lose strength is an answer for such situations.
- The properties of hydrophobic cement is nearly the same as that ordinary Portland
cement except that it entrains a small quantity of air bubbles. The hydrophobic cement is made actually from ordinary Portland cement clinker. After grinding, the cement particle is sprayed in one direction and film forming materials such as oleic acid, or stearic acid, or pentachlorophenol, or calcium oleate are sprayed from another direction such that every particle of cement is coated with a very fine film of this water repellant material which protects them from the bad effect of moisture during storage and transporation. The cost of this cement is nominally higher than ordinary Portland cement.
- Ordinary cement mortar, though good when compared to lime mortar with respect to strength and setting properties, is inferior to lime mortar with respect to workability, water retentivity,shrinkage property and extensibility.
- Masonry cement is a type of cement which is particularly made with such combination of materials, which when used for making mortar, incorporates all the good properties of lime mortar and discards all the not so ideal properties of cement mortar.
- This kind of cement is mostly used, as the name indicates, for masonry construction.
- It contains certain amount of air-entraining agent and mineral admixtures to improve the plasticity and water retentivity.
- Concrete made with ordinary Portland cement shrinks while setting due to loss of free water. Concrete also shrinks continuously for long time. This is known as drying shrinkage.
- Cement used for grouting anchor bolts or grouting machine foundations or the cement used in grouting the prestress concrete ducts, if shrinks, the purpose for which the grout is used will be to some extent defeated. There has been a search for such type of cement which will not shrink while hardening and thereafter. As a matter of fact, a slight expansion with time will prove to be advantageous for grouting purpose. This type of cement which suffers no overall change in volume on drying is known as expansive cement.
- Cement of this type has been developed by using an expanding agent and a stabilizer very carefully. Proper material and controlled proportioning are necessary in order to obtain the desired expansion.
- Generally, about 8-20 parts of the sulphoaluminate clinker are mixed with 100 parts of the
Portland cement and 15 parts of the stabilizer. Since expansion takes place only so long as concrete is moist, curing must be carefully controlled. The use of expanding cement requires skill and experience. - One type of expansive cement is known as shrinkage compensating cement. This cement when used in concrete, with restrained expansion, induces compressive stresses which approximately offset the tensile stress induced by shrinkage.
- Another similar type of cement is known as Self Stressing cement. This cement when used in concrete induces significant compressive stresses after the drying shrinkage has occurred. The induced compressive stresses not only compensate the shrinkage but also give some sort of prestressing effects in the tensile zone of a flexural member.
- IRS-T-40 special grade cement is manufactured as per specification laid down by ministry of Railways under IRST40:1985.
- It is a very finely ground cement with high C3S content designed to develop high early strength required for manufacture of concrete sleeper for Indian Railways. This cement can also be used with advantage fo r o ther applications where high early strength concrete is required. This cement can be used for prestressed concrete elements, high rise buildings, high strength concrete.
- Oil-wells are drilled through stratified sedimentary rocks through a great depth in search of oil. It is likely that if oil is struck, oil or gas may escape through the space between the steel casing and rock formation. Cement slurry is used to seal off the annular space between steel casing and rock strata and also to seal off any other fissures or cavities in the sedimentary rock layer. The cement slurry has to be pumped into position, at considerable depth where the prevailing temperature may be upto 175°C. The pressure required may go upto 1300 kg/ cm2. The slurry should remain sufficiently mobile to be able to flow under these conditions for periods upto several hours and then hardened fairly rapidly. It may also have to resist corrosive conditions from sulphur gases or waters containing dissolved salts. The type of cement suitable for the above conditions is known as Oil-well cement.
- The desired properties of Oil-well cement can be obtained in two ways: by adjusting the compound composition of cement or by adding retarders to ordinary Portland cement. Many admixtures have been patented as retarders. The commonest agents are starches or cellulose products or acids. These retarding agents prevent quick setting and retains the slurry in mobile condition to facilitate penetration to all fissures and cavities. Sometimes workability agents are also added to this cement to increase the mobility.
- Acclerating the setting and hardening of concrete by the use of admixtures is a common knowledge. Calcium chloride, lignosulfonates, and cellulose products form the base of some of admixtures. The limitations on the use of admixtures and the factors influencing the end properties are also fairly well known.
- High alumina cement, though good for early strengths, shows retrogression of strength when exposed to hot and humid conditions. A new product was needed for use in the precast concrete industry, for rapid repairs of concrete roads and pavements, and slip-forming. In brief, for all jobs where the time and strength relationship was important. In the PCA laboratories of USA, investigations were conducted for developing a cement which could yield high strengths in a matter of hours, without showing any retrogression. Regset cement was the result of investigation. Associated Cement Company of India have developed an equivalent cement by name “REDISET” Cement.
- High alumina cement is obtained by fusing or sintering a mixture, in suitable proportions, of alumina and calcareous materials and grinding the resultant product to a fine powder. The raw materials used for the manufacture of high alumina cement are limestone and bauxite.
- These raw materials with the required proportion of coke were charged into the furnace. The furnace is fired with pulverised coal or oil with a hot air blast. The fusion takes place at a temperature of about 1550-1600°C. The cement is maintained in a liquid state in the furnace. Afterwards the molten cement is run into moulds and cooled. These castings are known as pigs. After cooling the cement mass resembles a dark, fine gey compact rock resembling the structure and hardeness of basalt rock.
- The pigs of fused cement, after cooling are crushed and then ground in tube mills to a finess of about 3000 sq. cm/gm.
- Development of high early strength becomes an important factor, sometimes, for repair and emergency work. Research has been carried out in the recent past to develop rapid setting and hardening cement to give materials of very high early strength.
- Lithium salts have been effectively used as accelerators in high alumina cement. This has resulted in very high early strength in cement and a marginal reduction in later strength. Strength as high as 4 MPa has been obtained within 1 hour and 27 MPa has been obtained within 3 hours time and 49 MPa in one day.