Main types of portland cements are: 1. Ordinary Portland Cement 2. Modified Cement 3. Rapid Hardening Cement 4. Low Heat Portland Cement (IS 12600-1989) 5. Portland Blast-Furnace Cement or Slag Cement (IS 455-1989) 6. Sulphate Resisting Cement (IS 12330-1988).
Type # 1. Ordinary Portland Cement:
This type of cement is suitable for use in general concrete construction wherever it is not exposed to sulphates in the soil or in ground water. The modern Portland cements have higher C3S contents and are of greater fineness than cements about 50 years back. Thus modern cements exhibit higher 28 days strength than cements in 1920-25. The German classification of cements is based on the 28 days strength of 1:3 mortars with a water cement ratio of 0.5 as 350, 450 & 550 kg/cm2 etc.
High Strength Ordinary Portland Cement:
This cement also is manufactured as ordinary Portland cement. This cement is useful where high strength at early age is required.
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In other details it is similar to ordinary Portland cement, but its chemical and physical requirements should be as follows as per I.S. 8112- 1976:
C3A = 2.65 (Al2O3) – 1.69(Fe2O3)
Where each symbol in brackets refers to the percent (by mass of total cement) of the oxide excluding any contained in insoluble residue.
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Physical Requirements:
1. Fineness:
Its specific surface area should not be less than 3500 cm2/gram.
2. Soundness:
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When tested by the Le Chateliar method, un-aerated cement should not have an expansion more than 10 mm.
3. Setting Time:
(a) Initial setting time not less than 30 minutes.
(b) Final setting time not more than 600 minutes.
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Compressive Strength:
The average compressive strength of at least three mortar cubes (area of face 50 cm2) composed of one part of cement and three parts of standard sand and (P/4 + 3.1 percent of combined mass of sand and cement) water, and prepared, stored and tested under standard conditions should be as follows:
After 1987 higher grade cements have been introduced in India. The original ordinary Portland cement (OPC) has been divided into three grades, namely 33 grades, 43 grades and 53 grades. These grades indicate the compressive strength at 28 days when tested as per IS 4031-1988. The compressive strength of cement should not be less than 33 N/mm2 (330 kg/cm2), 43 N/mm2 (430 kg/cm2) and 53 N/mm2 (530 kg/cm2) at 28 days.
The physical properties of these cements are shown in Table 3.2:
The production of high quality cement could be possible by using high quality lime stone, modern equipment and close control on lime constituents, finer grinding, better packing and maintaining better particle size distribution.
Advantages of High Grade Cements:
Following advantages of high grade cements have been observed:
1. They offer 10-20% saving in the consumption of cement.
2. They develop strength at faster rate, thus saving time in construction. The formwork may be released at earlier time and can be reused on other works, resulting in saving of time and money both.
The production of ordinary Portland cement is decreasing throughout the world due to the popularity of blended cement on account of lower energy consumption, environmental, economic and other technical reasons.
In advanced countries, the consumption of OPC has come down to about 40% of the total cement production. In India in the year 1998-99 the production of OPC has come down to 70% i.e., from 79 million tonnes to 57 million tones. The production of pozzolana Portland cement to 19% and slag cement to 10% of the total cement production. In times to come its use is expected to decline further, however it will remain an important type of cement for general use.
Type # 2. Modified Cement:
This cement contains about 6% C3A and 46% C3S. This cement generates heat at somewhat higher rate than that of low heat cement with a rate of gain of strength similar to that of Portland cement. This cement is recommended for use in structures where a moderately low heat generation is desirable or where moderate sulphur attack may occur. This cement is extensively used in U.S.A.
Type # 3. Rapid Hardening Cement:
This cement is very similar to the ordinary Portland cement. As the name implies, it develops strength more rapidly. Therefore it is also known as high early strength cement. Its setting time is similar to that of ordinary Portland cement. Its chemical requirements are same as that of ordinary Portland cement.
The strength developed by the rapid hardening cement at the age of 24 hours is of the order as that of 3 days strength of ordinary Portland cement and 3 day strength of rapid hardening cement is of the order of 7 day strength of Portland cement with the same water cement ratio. The increased rate of gain of strength of the rapid hardening cement is achieved by a higher C3S content of the order of about 56%. Some times C3S Content in rapid hardening cement is 70%. It is finer than ordinary Portland cement. Its specific surface area is 3250 cm2/gm of cement.
The requirement of chemical composition and soundness for rapid hardening cement are same as those for ordinary Portland cement. The density of this cement is 1280 kg per cubic metre.
Use of Rapid Hardening Cement:
The use of rapid hardening cement is recommended in the following situations:
1. Where formwork is required to be removed early for re-use elsewhere.
2. For road, air strip and bridge repairs etc.
3. In cold weather concreting.
4. In prefabricated concrete construction.
The rapid hardening cement being more fine than ordinary Portland cement needs more water for the same workability. Its shrinkage is also more than ordinary Portland cement. The rapid gain of strength means a high rate of heat development. The rate of heat development varies from 53 Cal/g to 93.2 Cal/g at 4°C to 41°C.
As the rate of heat development is high, this cement should not be used for mass concrete construction as large quantity of heat will cause cracks in the structure. On the other hand at places of low temperatures, the use of this cement is found useful, as the high rate of heat liberation will prove a satisfactory safeguard against frost damage at early age.
Extra Rapid Hardening Portland Cement:
This cement is useful for cold weather concreting or when a very high early strength is required, but it should not be used with reinforcement due to the risk of corrosion. The strength of this cement is about 25% higher than that of rapid hardening cement at 1 or 2 days and 10 to 20% higher at 7 days but the gain in strength disappears at the age of 90 days.
The setting time also of this cement is short from 5 minutes to 30 minutes depending upon the temperature at the time of placement. Hence it should be finished within 20 minutes. Its shrinkage is also higher than rapid hardening cement. It should not be stored for more than a month.
This cement can be obtained by inter-grinding calcium chloride with rapid hardening Portland cement. The quantity of calcium chloride should not exceed 2%. As calcium chloride is deliquescent, this cement should be stored under dry conditions and generally should be used within one month of dispatch from cement plant.
Ultra High Early Strength Portland Cement:
This cement is manufactured by separating fines from rapid hardening Portland cement by a cyclone air elutriator. This cement is more fine than ordinary rapid hardening Portland cement. Its specific surface area is of the order of 7000 cm2/g to 9000 cm2/gram. Due to its high fineness, it has low bulk density and deteriorates rapidly on exposure.
It’s high degree of fineness leads to a rapid hydration, which leads to high rate of heat generation at early ages and to a rapid development of strength. This cement develops strength at 16 hours of the same order as 3 days strength of rapid hardening Portland cement and at 24 hours as 7 days strength of rapid hardening Portland cement. However after 28 days, the gain in strength is very little.
As this cement contains no admixtures, therefor it is suitable for use in reinforced and pre stressed concrete. The creep and shrinkage are more or less same as those of other Portland cements. The ultra-high early strength Portland cement has its trade name as Swift Crete. This cement is used in Great Britain. However for the same mix proportion this cement results in lower workability. Gypsum content is 4%.
Very High Strength Cement:
For rapid repair of damaged concrete roads and air field pavements very high strength cements have been developed.
Some of them are as follows:
1. Magnesium Phosphate Cement (MPC):
This cement has been developed by Central Road Research Institute Delhi. This is an important development for emergency repair of airfields, launching pads, hard surface road pavements suffering damage due to enemy bombing and missile attack etc. This cement can be used for such emergency repairs.
This cement has been found to possess unique hydraulic properties, particularly a controlled rapid set and early strength development. It is a pre-packed mixture of dead burnt magnesite with fine aggregate mixed with phosphate. It sets rapidly and yields durable high strength cement mortar.
2. Pyrament Cement:
It is the trade name of a super high early strength Cement developed by cement industries in U.S.A. It is blended hydraulic cement. This cement produces a high and very early strength of mortar and concrete which can be used for emergency repair works. No chlorides are added to this cement.
The typical properties of concrete and mortar of these cements are shown in Table 3.3 below:
Speed Cement:
This cement is used in Belgium. It is somewhat less fine than ultra-high early strength Portland cement. It has specific surface area of the order of 4590 cm2/g. The compressive strength of standard vibrated 1:3 mortar cubes is 280 kg/cm2 at one day, 480 kg/cm2 at 3 days and 680 kg/cm3 at 28 days. This cement is suitable for winter concreting or for urgent jobs as well sealing or repair of roads etc.
Regulated Set Cement or Jet Cement:
This cement consists essentially of a mixture of Portland cement and calcium fluoro-aluminate (C11A7CaF2) with an appropriate retarder (usually citric acid). The setting time of this cement can vary between 1 and 30 min. The setting time is controlled at the time of manufacture of cement as the raw materials are inter-ground and burnt together. The strength development is slower with slower setting.
The early strength development is controlled by the content of calcium fluoro-aluminate. 5% calcium fluoro-aluminates develop strength of 60 kg/cm2 at 1 hour where as 50% of this material will produce strength of 200 kg/cm2 at the same time i.e. 1 hour. After wards the strength development is similar to that of parent Portland cement. This cement is widely used in Japan, Austria, and Germany.
Type # 4. Low Heat Portland Cement (IS 12600-1989):
Cement generates heat during hydration. Heat generated during hydration raises the temperature in the interior of a large concrete mass. In case the heat generated is not lost quickly, it will causes serious cracks in the concrete mass as the outer layers will cool and contract, while the inner mass is still at higher temperature.
For this reason, it is necessary to limit the rate of heat evolution of cement used for mass concrete. The rate of heat evaluation of this cement is limited to 65 cal/g at the age of 7 days and 75 cal/gram at the age of 28 days.
The limits of lime contents of this cement after correction should be as shown below:
In this cement the more rapidly hydrating components tri-cal-aluminate C3A and tri-calcium silicate C3S are kept low at 5% and 13% respectively by weight resulting increase in C2S. The rate of strength development of this cement is lower initially, but the ultimate strength is unaffected i.e., it is same as that of ordinary Portland cement. In any case to ensure a sufficient rate of gain of strength, specific surface area of this cement should not be less than 3200 cm2/g.
The setting and hardening times of this cement are nearly the same as those of ordinary Portland cement. Its strength after 7 days is 50% that of Portland cement and 66% after 28 days. Its strength becomes more or less equal to that of Portland cement after 90 days.
Hence this cement is unsuitable for ordinary work as it will require prolonged curing and keeping the form works in position. However it is very useful for mass concrete work as dams, bridges etc. This cement was developed in USA during 1930.
Type # 5. Portland Blast-Furnace Cement or Slag Cement (IS 455-1989):
This type of cement is made by inter-grinding the Portland cement clinker and granulated slag. The quantity of slag should not exceed 65% of the weight of the mixture. It may vary from 25 to 65%.
The slag is a waste product of the manufacture of pig iron. To utilize this slag which is a waste product the manufacture of this cement has been started. The development of this cement has considerably increased in recent times. Its production in 1998-99 was about 8 million tones i.e., 10% of the total production of all cements.
The slag is a mixture of lime, silica and alumina, that is, it contains same oxides that make up Portland cement, but not in the same proportions. A slag is considered satisfactory if it contains 42% lime, 30% silica, 19% alumina, 5% magnesia and 1% alkalies. Blast furnace slag varies greatly in composition and physical structure depending on the process used and on the method of cooling of the slag. Slag to be used in the cement has to be quenched (rapid cooling by water) in such a way that it solidifies as a glassy granulated slag and its crystallization is prevented. The rapid cooling by water results also in the fragmentation of the material into the granulated form.
As per I.S. 12089-1987 the Portland slag cement should have the following chemical requirements:
The composition of granulated slag should comply with one of the following requirements:
Generally following composition meets the above requirements of the granulated blast furnace slag:
The chemical composition of Blast Furnace slag (BFS) and cement clinker are similar.
The approximate chemical composition of cement-clinker, blast-furnace slag (BFS) and fly ash are shown in Table 3.5 below:
Manufacture of Slag Cement:
Slag can be used in different ways to manufacture blast furnace slag cement. In the first case, slag can be used together with lime stone as a raw material for the conventional manufacture of Portland cement. Clinker obtained from these materials often used in the manufacture of blast furnace cement. Secondly the dry granulated slag is fed with Portland cement clinker into a grinding mill for grinding. At this time, a measured quantity of gypsum is also added in order to control its setting time. Slag being harder than clinker presents some difficulties in grinding.
Blast furnace cement has been used in many countries for a number of years such as Scotland, U.S.A., Germany, France, Belgium, and Netherland. In different countries different percentage of slag has been used. In Netherland slag quantity upto 85% has been used.
Belgians have developed a new process in which wet ground granulated slag is fed in the form of slurry direct into the concrete mixer, together with Portland cement and aggregates. In this process the cost of drying the slag is saved and slag could be ground finer in wet state, resulting in saving of energy.
Portland blast furnace cement is similar in many respects to ordinary Portland cement. The requirements for setting times, soundness and compressive strength of Portland cement and blast furnace cement are same. The rate of hardening of Portland blast furnace cement is somewhat slower during the first 28 days and proper curing is essential.
Thus its 28 days strength is lower than ordinary Portland cement. The standard mortar cube 28 days strength is of the order of 340 kg/cm2 & for concrete cubes 220 kg/cm2. However after 90 days there is practically no difference in their strengths. However the exact nature of hydration of Portland blast furnace cement is not quite clear.
Fineness:
Its specific surface area should not be less than 2250 cm2/gram.
Compressive strength
At
72 ± 1h – not less than 150 kg/cm2
168 ± 2h – not less than 220 kg/cm2
The heat of hydration of Portland blast furnace cement is lower than that of ordinary Portland cement. Thus it can be used in mass concrete works with advantage. However in cold weather, the low heat of hydration and low rate of strength development can lead to frost damage. Further this cement is fairly sulphate and alkali resistant as the C3A contents in this cement is low and as such it can be used in sea water construction with advantage.
The shrinkage and modulus of elasticity of concrete made with this cement are same as those of ordinary Portland cement concrete. Recently for laying 3.5 m dia sewer line 40 m below mean sea level, in Bombay cement made out from a mixture of 70% ground granulated blast furnace slag and 30% ordinary cement clinker was used with advantage. The recent extensive research has shown that the presence of ground granulated furnace slag (GGBS) increases the intrinsic properties of fresh as well as hardened concrete.
Recently following advantages of GGBS have been recognized:
1. Increased resistance to chemical attack.
2. Reduced generation of heat of hydration.
3. Reduced permeability.
4. Refinement of pore structure.
Application of GGBS Concrete:
Recent studies have shown that combining the ground granulated blast-furnace slag (GGBS) and ordinary Portland cement at mixer gives the equivalent factory made Portland slag cement. Concrete of different properties can be made by varying the proportion of GGBS.
1. For use in mass concrete works, where the risk of early age thermal cracking is important, concrete containing 50% to 90% of GGBS can be used. Generally 70% GGBS and 30% ordinary Portland cement (OPC) may be used.
2. Resistance to chemical attack may be increased by using GGBS in concrete. The resistance to acid attack may be increased by using 70% GGBS in concrete.
3. To counter the sulphate and chloride attack 40% to 70% GGBS may be used.
4. In foundation concrete and water retaining structures the effect of aggressive water can be reduced by using GGBS.
5. The risk of ASR (alkali-salt reaction) can be minimised by the use of 50% GGBS.
Type # 6. Sulphate Resisting Cement (IS 12330-1988):
The reaction of C3A with gypsum (CaSO42H2O) forms calcium sulpho-aluminate and causes expansion or soundness in cement. This is more prominent during setting process of cement. Similarly in hardened concrete, calcium aluminate hydrate can react with a sulphate salt from outside the concrete in a similar manner as above, forming calcium sulpho-aluminate. In this case the increase in the volume of solid phase is 227%, hence disintegration of concrete takes place. This reaction or expansion in volume of concrete is known as sulphate attack.
A second type of reaction is that of Base Exchange between calcium hydroxide and sulphates, resulting in the formation of gypsum with an increase in the volume of solid phase by 124%.
The increase in volume by any type of reaction is called sulphate attack. Magnesium and sodium sulphates are more active salts. Sulphate attack is accelerated in alternate wetting and drying conditions such as in marine structures.
The sulphate attack can be minimised by keeping the C3A in cement as low as possible. The C3A content may be allowed upto 3.5%, but in no case more than 5%. The minimum fineness should be 2200 cm2g. In other respects it is similar to ordinary Portland cement.
In Portland cement C4AF varies between 6 to 12%. As often it is not feasible to reduce the AI2O3 content of the raw material, Fe2O3 may be added to reduce the C3A content. However the addition of Fe2O3 increases the contents of C4AF.
To limit the C3A and C4AF contents, IS code has suggested that the total content of C4AF and C3A should be as follows:
2 C3A + C4AF should not exceed 25%.
The low C3A and comparatively low QAF contents of sulphate resisting cement mean that it has a high silicate content, which gives the cement a high strength. However in this cement C2S is quite high of the order of 36%, thus its early strength is low. The rate of heat development of sulphate resisting cement is of the same order as that of low heat cement. Thus this cement is ideal one, but due to special requirements for the composition of the raw materials it cannot be manufactured at low cost.
Use of Sulphate Resisting Cement:
The use of sulphate resisting cement is recommended under the following conditions:
1. Concrete to be used in marine conditions.
2. Concrete to be used in foundations and basements, where soil is infested with sulphates.
3. Concrete to be used in sewage treatment works constructions.
4. Concrete to be used for the fabrication of pipes to be used under sulphate bearing soils or in marshy regions.
Super Sulphate Cement:
It is not a Portland cement. It is made by inter-grinding a mixture of 80 to 85% of granulated slag with 10 to 15% of calcium sulphate (in the form of dead burnt gypsum) and about 5% of Portland cement clinker. It is ground very fine. Usually its fineness is of the order of 4000 cm2/g to 5000 cm2/g. This cement deteriorates very rapidly in moist conditions, therefore it should be stored under very dry conditions.
The heat of hydration of this cement is low about 40 to 45 cal/g at 7 days and 45 to 50 cal/g at 28 days. Thus it is very suitable for mass concrete work, but care must be taken if used in cold weather as the rate of strength development reduces considerably at low temperatures. The rate of hardening of super sulphated cement increases with temperature upto about 50°C, but at higher temperatures abnormal behaviour has been observed.
Therefore, steam curing above 50°C should not be done without proper prior tests. Further this cement should not be mixed with Portland cement, because the lime liberated by the hydration of an excessive amount of the Portland cement will interfere with the reaction between the slag and calcium sulphate.
Wet curing for at least 95 hours after casting is essential as premature drying will result in powdery surface layer, specially in hot weather.
Super sulphated cement needs more water for hydration than ordinary Portland cement. Therefore concrete with a water/cement ratio less than 0.4 should not be made. Mixes leaner than 1:6 should not be used. The water/cement ratio law has less effect on this cement than other cements i.e. the decrease in strength with an increase in water/cement ratio has been observed less than other cements. However its early strength development depends on the type of slag used in the manufacture of cement, therefore it is advisable to determine actual strength characteristics prior to use.
This cement is highly resistant to sea water and can withstand the higher concentration of sulphates normally found in soil or ground water. It is also resistant to oils and peaty acids. Concrete with water/ cement ratio upto 0.45 has been found not to be affected in contact with weak solutions of mineral acids of PH value below 3.5.
Due to these reasons this cement can be used in construction of sewers and in contaminated ground. However it has been suggested that this cement is less resistant than sulphate resisting Portland cement when the sulphate concentration is more than 1%. Super sulphated cement has been used extensively in Belgium, France, U.K., Germany etc.
The, strength at different ages of mortar cubes and concrete cubes is shown in Table 3.6 below:
