Numerous cements have been developed for special uses. Some of them are: 1. Hydrophobic Portland Cement 2. Masonry Portland Cement 3. Anti-Bacterial Portland Cement 4. Expansive Portland Cement 5. IRST 40 Special Grade Portland Cement 6. Oil Well Portland Cement 7. Rediset Portland Cement.
1. Hydrophobic Portland Cement:
This cement is obtained by grinding ordinary Portland cement clinker with 0.1 to 0.5% by weight of an additive called hydrophobic agent. This additive imparts a water repelling property to ground cement by forming a water repellant film around each particle of cement which shall be destroyed only by wet attrition such as in a concrete mixer. The hydrophobic property of cement facilitates its storage for longer periods in extremely wet climatic conditions. Some of the hydrophobic agents are Oleic acid, Naphthenic acid, stearic acid, and Pentachlorophenol etc.
The addition of these substances increases the grind-ability of the clinker, probably due to electrostatic forces resulting from a polar orientation of the acid molecules on the surface of the cement particles.
Oleic acid reacts with alkalies in cement to form calcium and sodium oleates which foam, resulting in production of air entraining. If the production of air entraining is not desired, a de-entraining agent such as tri-n-butyl phosphate has to be added during grinding.
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This cement deteriorates very little during prolonged storage under unfavorable conditions. The hydrophobic properties are produced due to the formation of a water repellent film around each particle of the cement. This film is broken during the mixing of concrete in the mixer and normal hydration takes place. The early strength of this cement is low. In appearance this cement is similar to that of ordinary Portland cement, but has a characteristic musty smell.
In handling this cement seems more fluid than other Portland cements. The hardened concrete also retains the water repellant characteristics of the cement. The properties of hydrophobic cement are nearly the same as that of ordinary Portland cement, but its cost is a little more than the ordinary Portland cement.
Physical Requirements:
As per I.S. 8043-1978:
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i. Soundness:
When tested by the Le Chateliar method, un-aerated cement should not have an expansion more than 10 mm.
ii. Fineness:
Specific surface area should not be less than 3500 cm2/gram.
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iii. Compressive Strength:
The average compressive strength of three cubes of face area 50 cm2 made of one part of cement and three parts of standard sand and water given by the relation,
Water = (P/4 + 3.0 percent of combined mass of cement and sand).
Prepared, stored and tested as per I.S. 4031-1968 should be as follows:
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72 ± 1 hour ― not less than 156.9 kg/cm2
168 ±2 hour ― not less than 215.7 kg/cm2
672 ± 4 hour ― not less than 304.0 kg/cm2
Chemical Requirement:
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As for ordinary Portland cement I.S. 260-1976.
2. Masonry Portland Cement (IS 3466-1988):
Ordinary cement, when used in masonry gives a harsh mortar and water is sucked by the masonry, which results in poor bond The mortar made of ordinary cement is inferior than lime mortar in respect of workability, water retentivity, shrinkage and extensibility. Hence to overcome this disadvantage masonry cement has been developed. Masonry cement is made by inter-grinding very finely ground Portland cement, white lime stone and air entraining agent or alternatively Portland cement and hydrated lime, granulated slag, or an inert filler and an air entraining agent.
Masonry cement is mainly used for mortar in brick work. This cement produces a more plastic mortar than ordinary Portland cement. This mortar also has greater water retaining power and lower shrinkage. The strength of masonry cement is lower than that of ordinary Portland cement, particularly if high air content is introduced. However the low strength of masonry cement has been found advantageous in brick construction, but this cement should not be used in structural concrete.
3. Anti-Bacterial Portland Cement:
It is a Portland cement inter-ground with an anti-bacterial agent, which prevents microbiological fermentation. This bacterial action is encountered in concrete floors of food processing plants where fermentation is caused by bacteria in the presence of moisture after the cement has been leached out by the action of acids. Anti-bacterial cement can also be used successfully in swimming pools, public baths or similar places where bacteria or fungi are present.
4. Expansive Portland Cement:
The cement which does not suffer over all change in volume on drying is known as expansive cement. This type of cement has been developed after long experimentation by using an expanding agent and a stabilizer very carefully. In order to obtain the desired expansion proper material and controlled proportioning is very necessary.
Need of Expansive Cement:
Concrete made with ordinary Portland cement shrinks during setting period due to loss of free water. Shrinkage of concrete continues for long, which is known as dry shrinkage. Cement used for grouting pre-stressed concrete ducks, for grouting anchor bolts or grouting machine foundations, if shrinks, the purpose for which the grout is used will be defeated to some extent. Hence to overcome this short coming of cement the necessity of expanding cement is felt. At present there are many types of expansive cements available in the market.
Generally mixing about 8 to 20 parts of the sulpho-aluminate clinker with 100 parts of the ordinary Portland cement and 15 parts of the stabilizer, expansive cement may be obtained. As the expansion takes place in the presence of moisture, curing of concrete should be carefully controlled. The use of expanding cement requires skill and experience.
One type of expansive cement is known as self-stressing cement. When this cement is used in concrete, it 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 pre-stressing effect in the tensile zone of a flexural member. Another type of expansive cement is known as shrinkage compensating cement. When this cement is used in concrete, it induces compressive stresses which approximately off set the tensile stresses induced by shrinkage.
Lossier Expansive Cement:
This type of cement for the first time was developed by H. Lossier in France. He used a mixture of Portland cement, an expanding agent, and a stabilizer.
Expanding Agent:
The expanding agent is obtained by burning a mixture of gypsum, bauxite and chalk, which form calcium sulphate and calcium aluminate (mainly C5A3). In the presence of water, these compounds react to form calcium sulpho-aluminate hydrate and the expansion of the paste takes place.
Stabilizer:
It is a blast furnace slag. It consumes the excess calcium sulphate slowly and neutralizes the expansion.
Thus in order to get a desired expansion, the proportioning of the cement ingredients should be done very carefully. Generally about 8 to 20 parts of the sulpho-aluminate clinker arc mixed with 100 parts of Portland cement and 15 parts of stabilizer.
High Energy Expanding Cement:
It is made by inter-grinding Portland cement clinker, high alumina clinker and gypsum approximately in the proportions of 65:20:15. Expansion in this cement also takes place due to the formation of sulpho-aluminate as in Lossier’s cement within two or three days after casting.
This cement is quick setting and rapid hardening. It develops strength of about 70 kg/cm2 in 6 hours and 500 kg/cm2 in 28 days. This cement has a high resistance to sulphate attack also.
Thus expansive cements are used for the following purposes:
(a) To neutralise the shrinkage of concrete made from ordinary cement in order to eliminate cracks. A small percentage of this cement in the concrete will not allow it to crack. This treatment is specially essential for hydraulic structures.
(b) In repair work, to keep the new concrete tight fit in the old concrete, expansive cement is used. Usually it takes about 15 days to expand fully, but the time can be controlled by curing. The upper limit of expansion is upto 1% but it can be adjusted to any lower value by decreasing the quantity of this cement. However this cement will not do well in contact with sea water.
5. IRST 40 Special Grade Portland Cement:
For the gauge conversion, Indian Railways needed large number of sleepers. The non-availability of high quality timber sleepers at reasonable rates, choice fell on concrete sleepers. For the manufacture of these sleepers Ministry of Railways laid down specification for the developments of cement vide its letter no IRS-T40-1985. The main requirement was the development of high early strength.
This objective is achieved by increasing the C3S content in the ordinary cement clinker and grounding the clinker very fine i.e. by increasing the fineness of cement. This cement can also be used for high rise buildings, pre-stressed concrete and for other purposes where high early strength is required.
6. Oil Well Portland Cement (IS 8229-1986):
The oil wells are drilled to a great depth, sometimes upto 6000 m through stratified sedimentary rocks. Oil or gas may escape through the annular space between the steel casing and the rock formation. To safe guard against this possibility cement slurry is injected to seal off the annular space between the steel casing and rock formation and also to seal off the cavities or fissures in the sedimentary rock layers at considerable depth, where the prevailing temperature may vary from 175°C to 200°C. To inject the cement slurry or grout at such depths, the pressure required may go upto 1300 kg/cm2.
Thus under these conditions cement slurry or grout should remain in fluid condition for 3 to 4 hours and then harden fairly rapidly i.e. its initial setting time should be sufficiently more i.e., cement slurry should not set for several hours. In addition to above, the cement slurry should be corrosive resistant due to sulphur gases or salts dissolved in water. Thus cement suitable for such conditions i.e., cement having high initial setting time, and corrosive resistant properties is known as oil well cement.
The cement suitable for above conditions may be obtained either by adjusting the compound composition of cement or adding retarders to ordinary Portland cement.
For this purpose the most commonly used retarder is calcium sulphate. In addition to gypsum, sugar, salts of acids, starches, cellulose products may be used. The quantity of sugar added for retarding purposes should be kept from 0.05 to 0.15% by weight of cement. Addition of 0.05% sugar by weight of cement retards setting time by 4 hours and 0.2% cement upto 72 hours. The use of 0.2% to 1.0% of sugar will virtually prevent the setting of cement. Some authors have suggested that the quantity of sugar more than 0.15% increases the process of setting while others have recommended this amount more than 0.2% by weight of cement.
The exact effect of sugar depends on the chemical composition of cement. Hence quantity of sugar as a retarder should be determined by trial mixes.
7. Rediset Portland Cement:
For the rapid repair of air strips i.e., airport runways, cement concrete roads and pavements, slip forming and in the precast concrete industry an early setting and hardening cement is needed. Though high alumina cement is good for early strength, but when this cement is exposed to hot and humid conditions, it starts losing its strength.
Thus to overcome this short coming experimentation started to develop a cement which could yield high strength during few hours after adding water to it, without showing any retrogression. Investigation carried out in U.S.A. developed such cement and it was given the name as Regset whereas associated cement company of India developed equivalent cement and it was named Rediset cement.
Properties of Rediset-Cement:
Following properties have been found of this cement:
1. This cement sets very early and gives only 8 to 10 minutes for its handling i.e. it should be finished within 10 minutes after adding water to it.
2. The strength developed in Rediset-cement concrete in 3 to 6 hours is equal to that of 7 days strength of ordinary cement concrete.
3. The rate of release of heat of hydration of Rediset-Cement is very high which is advantageous in winter concreting, but excess heat liberation is harmful to mass concrete.
4. Though the rate of shrinkage of Rediset is fast, but the total shrinkage is similar to that of ordinary Portland cement.
5. The sulphate resistance of this cement is very poor.
6. The comparative strength pattern of REDISET and REGSET cement is shown below in table 3.13.
Uses of RESET Cement:
It can be used for the following purposes:
1. For very high early (3 to 4 hours) strength mortar or concrete.
2. For emergency and patch repairs.
3. In the precast concrete product industry for early release of form work.
4. Construction between tides.
5. Slip formed concrete construction.
6. Palletisation of iron ore dust etc.