Table of Contents
Cement may be prescribed as a material with adhesive and cohesive properties which make it capable of bonding mineral fragments into a compact whole.
For constructional uses, the definition of the word cement is restricted to the bonding materials utilized with stones, sand, bricks, building blocks, etc. The principal components of this type of cement are made of lime so that in building and civil engineering we are concerned with calcareous cement. The cement of interest in the making of concrete has the property of setting and interest in underwater under a chemical reaction with it and is therefore called hydraulic cement.
2. Classification of Cement
Cement can be broadly classified as:
1. Natural cement
2. Artificial cement
1. Natural Cement:
It is manufactured from stones containing 20 to 40 percent of clay, the remainder being carbonate of lime mixed with carbonate of magnesia. The stones are first burnt and then crushed.
It possesses a brown color and sets rapidly when mixed with water.
It doesn’t find much use in India.
2. Artificial Cement:
It may be portland cement or special cement.
Portland cement is so-called because a paste of cement with water after it sets hard, be like in color and hardness a portland stone, a limestone extracted in Dorset.
It is prepared in different varieties.
3. Properties of Cement
A good cement possesses the following properties ( which depends upon its chemical composition, thoroughness of burning, and fitness of grinding):
1. Provides strength to masonry.
2. Stiffness or hardens early.
3. Owns good plasticity.
4. An excellent building material.
5. Easily workable.
6. Good moisture-resistant
4. Uses of Cement
Following are the different uses of cement:
2. It is utilized for preparing joints for pipes, drains, etc.
3. It is utilized in concrete for placing floors, roofs, and constructing lintels, beams, stairs, pillars, etc.
4. It is utilized for manufacturing precast pipes, piles, fencing posts, etc.
5. It is utilized in the construction of important engineering structures such as bridges, culverts, dams, tunnels, lighthouses, etc.
6. It is utilized in the formation of footings, water-tight floors, footpaths, etc.
7. It is employed for the construction of wells, water tanks, tennis courts, lamp posts, telephone cabins, roads, etc.
5. Comparison between Cement and Lime
|Color||Greenish grey.||White or greyish.|
|Slaking||Do not slake when wetted with water.||Slakes when wetted with water.|
|Setting||Fixed quickly when mixed with water.||Fix lately when mixed with water.|
|Strength||Artificial cement possesses more strength.||Possesses less strength.|
|Suitability||Can be utilized for important and difficult engineering structures.||Cannot be utilized for important and difficult engineering structure.|
6. Chemical Constituent of Cement
Percentage of various ingredients for the manufacture of portland cement should be as follows:
|Iron Oxide Fe₂O₃||3%|
|Magnesium oxide MgO||2.5%|
|Sulphur trioxide SO₃||1.75%|
|Loss on ignition||1.5%|
Besides the above ingredients, calcium sulfate CaSO₄, Commonly known as Gypsum is also added by 3 to 4% during the grinding process for controlling the initial setting time and vice-versa.
7. Types of Cement
Following are the various types of cement available in the market:
1. Portland Cements:
(a) Ordinary Portland Cement
(b) Modified Portland Cement
(c) Rapid hardening Portland Cement
(d) Extra Rapid hardening Cement
(e) Low heat Portland Cement
(f) Sulphate resisting Portland cement
(g) Water-repellent Portland Cement
(h) Water-proof Portland Cement
2. Other Varieties of Cement
(a) High Alumina Cement
(b) Quick setting Cement
(c) Blast Furnace slag Cement
(d) White Cement.
(e) Colored cement
(f) Acid resistance Cement
(h) Hydrophobic cement
(i) Portland Pozoolna Cement
(j) Supersulphated Cement
(k) Masonry Cement
7.1 Portland Cements
a. Ordinary Portland Cement:
This is by far the most common cement in use.
It has a better rate of strength development and heat generation.
It has sufficient resistance to dry shrinkage and cracking, but has less resistance to chemical attack.
a. It is admirably suitable for use in general concrete construction when there is no exposure to sulphates in the soil or groundwater.
b. It is used in small structures where the heat of hydration will not cause any defect.
b. Modified Portland Cement
This cement, on the setting, develops less heat of hydration than ordinary portland cement.
Due to lower heat of hydration, it can be employed in hot climates and for construction of heavy abutments, large piers, retaining walls, etc, where sulphate content is not high.
c. Rapid Hardening Cement
It is also known as high early strength cement.
It has a huge proportion of lime and other components are the same as for Ordinary Portland Cement.
It is prepared more carefully and burnt at a higher temperature than that of ordinary Portland Cement.
The strength developed at the age of 3 days is of the same order as the 7 days strength of Ordinary Portland Cement with the same water-cement ratio.
The increased rate of gain of strength of this cement is achieved by a higher C₃S content and by finer grinding of the cement clinker.
It is ground finer and has a specific surface not less than 3250 cm²/g.
It is lighter than Ordinary Portland Cement.
The curing period is short, hence it is economical.
It is used here a rapid strength development is required.
Example: When the framework is to be removed quickly for re-use, or where sufficient strength for further construction is wanted as quickly as practicable.
d. Extra Rapid Hardening Cement
This type of cement is obtained by inter-grinding calcium chloride with rapid hardening Portland cement. The quantity of calcium chloride should not exceed 3%.
Its strength is about 25% higher than that of rapid hardening cement at 1 or 2 days and 10 to 20 % higher at 7 days.
It is suitable for cold weather concreting, or when a very high early strength is required but when it is inadvisable to use aluminous cement.
e. Low Heat Portland Cement
It contains a low percentage (about 5%) of tricalcium silicate (C₃S) which hydrates quickly and a higher percentage (about 46%) of dicalcium silicate (C₂S) which hydrates slowly.
It contains less time than ordinary cement; other materials remain the same as in the case of Ordinary Cement.
It possesses low compressive strength.
The initial setting time is about one hour and the final setting time is about 10 hours.
During the setting action of cement, a considerable amount of heat is produced, to reduce the amount of heat, this type of cement is used.
It is mainly used for mass concrete work.
f. Sulphate Resisting Portland Cement
In this cement, the percentage of tricalcium aluminate (C₃A) is kept below 5 % and it increases by resisting power against sulphates.
The heat formed by such type of cement is not much bigger than that of low heat cement.
Theoretically, it is an ideal cement but because of the special requirement for the composition of the raw materials used in its manufacture, sulphate-resisting cement cannot be easily and cheaply made.
It is used at places where sulphate action is severe.
It is employed for structures that are likely to be damaged by severe alkaline conditions such as canal linings, culverts, siphons, etc.
g. Water-repellent Portland Cement
It contains a small percentage of water-proofing materials uniformly mixed with cement and is manufactured under the name “ A quacrete“.
The cement is formed with ordinary or rapid hardening cement and white cement.
If such cement is used in concrete, considerable care is needed to avoid a reduction in strength.
It is chiefly used in water-tight concrete and water-tight renderings to check moisture penetration in basements etc and for colored rendering and stucco.
h. Water-Proof Portland Cement:
These types of cement are prepared by mixing with ordinary or rapid hardening cement, a small percentage of some metal stearate (Ca, Al, etc) at the time of grinding.
Concrete made with such cement is more resistant to penetration by water and some oils than that made from ordinary cement.
It is also adequately resistant to the corrosive action of acids and alkalies or other harmful salts usually present in industrial waters.
It is used for the construction of water retaining structures like tanks, reservoirs, retaining walls, swimming pools, dams, bridges, piers, etc.
7.2. Others Varieties of Cement
a. High Alumina Cement:
It is quick-setting cement of chocolate color.
It is manufactured from bauxite and limestone in special reverberatory furnaces.
It contains nearly 35 percent of alumina and the ratio of alumina to lime lies between 0.55 and 1.3.
1. It resists the action of acid and high temperature and doesn’t expand on the setting.
2. Its initial and final hardening times are 13/4 hours and 4 to 5 hours respectively. It, therefore, permits more time for mixing and placing operations.
3. It sets quickly and attains has higher ultimate strength in a low period. Its strength after 1 day is about 40 N/mm²and after 3 days is about 50 N/mm².
4. It is completely resistant to the action of sulphates.
5. It offers excellent resistance to fire.
6. It is not affected by frost since it evolves great heat during the setting.
7. It is not necessary to grind it to fine powder since its setting action mainly depends on the chemical reactions.
1. It is unsuitable for mass concrete construction as it develops considerable heat on the set.
2. It is much costlier than ordinary portland cement.
3. Extreme care has to be taken to ensure that it doesn’t come in contact with even traces of lime or ordinary cement.
1. Its field of application includes works in chemical plants and furnaces.
2. It is employed in colder regions having temperatures 18°C or below and during wartime emergencies.
3. It is used in underwater construction.
b. Quick Setting Cement
It contains less percentage of gypsum and is ground much fine than ordinary portland cement.
The sitting action accelerated by adding a small percentage of aluminum sulphate, during grinding.
It is very expensive as compared to ordinary portland cement.
Its initial and final hardening times are 5 minutes and 30 minutes respectively.
The setting action of such a cement starts within 5 minutes and it becomes stone-hard in less than half an hour.
Due to its quick setting property, it is used in works where concrete is to be placed underwater or in running water.
c. Blast Furnace Slag Cement
It is made by inter-grinding portland cement clinker and granulated blast-furnace slag, the proportion of the latter not exceeding 65% of the weight of the mixture.
It has lower evolution of heat.
It is more resistant to attacks by weathering agencies.
It is cheaper than ordinary portland Cement.
Its color is blackish grey.
Its initial setting time is not less than 30 minutes.
Its final setting time is not more than 10 hours.
It can be used in mass concrete structures (since its heat of hydration is lower than that of ordinary portland cement).
However, in cold weather, the low heat of hydration of blast-furnace slag cement, coupled with moderately low rate strength development can lead to frost damage.
This cement should not be used in thin RCC structures since early strength is less.
Because of its fairly high sulphate resistance, this type of cement is frequently used in seawater construction.
d. White Cement
White Portland Cement is made from raw materials containing very little iron oxide, magnesium oxide.
China clay is normally utilized together with chalk or limestone free from impurities.
Oil is utilized as a fuel in the kiln to neglect contamination by coal ash.
Since iron acts as a flux in clinkering, its absence necessitates high kiln temperatures but sometimes cryolite is added as a flux.
Contamination of the cement with iron during grinding has also to be avoided.
For this problem, rather inefficient pebble grinding is utilized in place of a usual ball mill, although nickel and molybdenum alloy balls have been considered.
The cost of grinding is expensive and this, coupled with the huge expensive raw materials, makes white cement rather expensive.
It dries quickly.
It owns high strength.
It contains superior aesthetic beauty.
It should not be set earlier than 30 minutes.
It should be carefully transported and stored in a closed container only.
1. It is used for floor finish, plasterwork, ornamental work, etc.
2. Miscellaneous applications of white cement include swimming pools ( where it replaces the use of glazed tiles with colored shades usable underwater), molding sculptures and statues, painting garden furniture, etc.
3. It is also employed for ready mixed concrete and precast concrete blocks.
e. Coloured Cement
Colored cement is prepared by adding 5 to 15% of a suitable coloring pigment before the cement is finally ground.
– Iron oxide is added to give red and yellow colors.
– Chromium oxides provide the green color.
– Cobalt oxide provides a blue color.
Colored cement is also known as “Colourcrete“.
These are much costlier than ordinary cement.
These types of cement are widely used for the finishing floors, external surfaces, artificial marble, stair treads, textured panel faces, window sill slabs, etc.
f. Acid Resistance Cement
An acid resistance type of cement contains:
1. Acid-resistant aggregates such as quartz, quartzites, etc,
2. Additive such as sodium fluosilicate (Na₂SiF₆), and
3. The aqueous solution of sodium silicate or soluble glass.
– The addition of additive sodium fluosilicate accelerates the hardening process of soluble glass and also increases the resistance of cement to acid.
– The soluble glassworks as the binding material of acid-resistant cement.
It cannot resist the action of water well.
Its water resistance can be increased by adding 0.5% of linseed oil or 2% of cresit and it is then known as acid and water-resistant cement.
It is used for acid-resistant and heat-resistant coatings of installations of the chemical industry.
g. Expanding Cement
It is formed by adding an expanding medium like sulpho-aluminate and a stabilizing agent to the ordinary cement.
This cement expands whereas other types of cement shrink.
It is used for the construction of water retaining structures.
It is also employed for repairing damaged concrete surfaces.
h. Hydrophobic Cement
It contains admixtures ( examples, acidol, naphthene soap, oxidized petrolatum, etc.) that form a thin film around the cement grains and decrease the melting ability of cement grains.
In this type of cement, in the initial stage, the gain in strength is less as hydrophobic films on cement grains prevent the interaction with water. However, after 28 days, its strength is similar to that of ordinary portland cement.
When this cement is used in the preparation of concrete, the fine pores in the concrete are uniformly distributed and thus the frost resistance and water resistance of such concrete are increased considerably.
i. Portland Pozzolana Cement
It is an interground blended mixture of cement and pozzolana.
Pozzolana is natural or artificial material containing silica and alumina in a reactive form.
Pozzolanic materials most commonly met with are volcanic ash, pumic, opaline shales and cherts, burnt clay, fly ash, etc.
In the manufacture of cement, about 25% of pozzolanic material is added to the ordinary cement clinkers, and the mix is thoroughly ground.
1. It own higher tensile strength.
2. Evolves less heat during setting.
3. Attains compressive strength.
4. Offers great resistance to expansion.
5. Imparts a higher degree of water-tightness.
6. Imparts plasticity and workability to mortar and concrete prepared from this type of cement.
7. Offers higher resistance to chemical attack and the action of seawater.
8. Not costly
1. Less compressive strength in early days.
2. Less resistance to erosion and weathering action.
1. It is mainly used for hydraulic structures such as dams, weirs, etc.
2. It can also be used for sewage works and for laying concrete underwater.
j. Supersulphate Cement
It is made by intergrinding a mixture of 80 to 85 % of granulated slag with10 to 15% of calcium sulphate and about 5% portland cement clinker and ground to a fineness of 4000 to 5000 cm²/g.
It is very much resistant to the sea-water.
It can resist the highest concentration of sulphates generally found in soil or groundwater.
It also offers resistance to peaty acids and oils.
The heat of hydration of this type of cement is low.
Its compressive strength should not be less than 15N/mm², 22 N/mm², and 30 N/mm² after 3 days, 7 days, and 28 days respectively.
This type of cement should not be mixed with other cement.
It combines chemically with more water than is needed for the hydration of portland cement, so that concrete with a water-cement ratio less than 0.5 should not be made and mixes richer than 1:6 are not approved.
It has been utilized for the undersides of bridges, over railways tracks, and for concrete sewers, carrying industrial effluents.
1. Supersulphated cement can be used in all cases where normal types of cement are used except in very hot weather.
2. It is used in a variety of aggressive conditions, for example, marine works, mass concrete jobs to resist the attack of aggressive water; reinforced concrete pipes in groundwater, concrete construction in sulphate-bearing soils.
k. Masonry Cement
It is prepared by intergrinding a mixture of portland cement clinkers with inert materials such as limestone, dolomite limestone, and dolomite gypsum and an air-entraining plasticizer in suitable proportion.
Its initial and final setting times are 90 minutes and 24 hours respectively.
Its compressive strength is 2.5 N/mm² for 7 days.
Because of its property of producing a smooth, plastic, cohesive, strong yet workable mortar when mixed with fine aggregate, masonry cement is superior to lime mortar, Lime-cement mortar, and cement mortar.
8. Storage of Cement
Cement has a great affinity for moisture and hence it should be stored carefully and should be shielded from the moisture-laiden current of air.
The exposed cement is attacked by an air setting that normally spreads: it provides rise to lumps.
The following precautions should be taken in the storage of cement:
1. Cement should be stored in a special water-tight shed with a dry damp-proof floor, water-proof walls, and leak-proof roof on the top.
2. The cement bags should be placed at a distance of 30cm from the walls. Stacks of bags should be of such a height as can be simply handled.
3. Cement bags should be arranged that they can be used on the principle of” first come first served”
4. Cement bags should be stacked close together to avoid free circulation of air.
5. Storing bags should not be placed directly on floors: planks or iron sheets may be placed underneath to guard against dampness.
6. Storing of Cement bags during the rainy season should be neglected.
7. As far as possible, the storing of cement for a long period should be neglected.
8. When cement is to be stored in bulk form, it should be store in air-tight cylindrical containers known as silos or bins.
|Read Also: Types of Concrete|
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