Cement

CEMENT IST SEMESTER

Cement and lime • Following points of differences may be noted between ordinary cement and lime: • (1) The cement can be used under conditions and circumstances which are not favourable for lime.

• (2) The cement, when converted into a paste form, sets quickly.

(3) The colours of cement and lime are different.

• (4) When water is added to the cement, no heat is produced and there is no slaking action.

Composition of ordinary cement • The ordinary cement contains two basic ingredients, namely, argillaceous and calcareous. In argillaceous materials, the clay predominates and in calcareous materials, the calcium carbonate predominates. A typical chemical analysis of a good ordinary cement along with the desired range is as follows:

Composition of ordinary cement

Properties of cement • Following are the important properties of a good cement which primarily depend upon its chemical composition, thoroughness of burning and fineness of grinding:

Properties of cement • (1) It gives strength to the masonry.

(2) It is an excellent binding material (3) It is easily workable.

• (4) It offers good resistance to the moisture.

(5) It possesses a good plasticity.

• (6) It stiffens or hardens early.

Functions of cement ingredients • (1) Lime (CaO): This is the important ingredient of cement and its proportion is to be carefully maintained. The lime in excess makes the cement unsound and causes the cement to expand and disintegrate.

On the other hand, if lime is in deficiency, the strength of cement is decreased and it causes cement to set quickly.

Functions of cement ingredients • (2) Silica (Si02): This is also an important ingredient of cement and it gives or imparts strength to the cement due to the formation of dicalcium and tricalcium silicates. If silica is present in excess quantity, the strength of cement increases but at the ame time, its setting time is prolonged.

Functions of cement ingredients • (3) Alumina (AI203): This ingredient imparts quick setting property to the cement. It acts as a flux and it lowers the clinkering temperature.

However the high temperature is essential for the formation of a suitable type of cement and hence the alumina should not be present in excess amount as it weakens the cement.

Functions of cement ingredients • (4) Calcium sulphate (CaS04): This ingredient is in the form of gypsum and its function is to increase the initial setting time of cement. • (5) Iron oxide (Fe203): This ingredient imparts colour, hardness and strength to the cement. . • (6) Magnesia (MgO): This ingredient, if present in small amount, imparts hardness and colour to the cement. A high content of magnesia makes the cement unsound.

Functions of cement ingredients • (7) Sulphur (S): A very small amount of sulphur is useful in making sound cement. If it is in excess, it causes cement to become unsound.

• (8) Alkalies: The most of the alkalies present in raw materials are carried away by the flue gases during heating and the cement contains only a small amount of alkalies. If they are in excess in cement, they cause a number of troubles such as alkali-aggregate reaction, efflorescence and staining when used in concrete, brickwork or masonry mortar.

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Harmful constituents of cement • The presence of the following two oxides adversely affects the quality of cement: • (1) Alkali oxides K20 and Na20 (2) Magnesium oxide MgO. • If the amount of alkali oxides exceeds 1 per cent, it leads to the failure of concrete made from that cement. Similarly, if the content of magnesium oxide exceeds 5 per cent, it causes cracks after mortar or concrete hardens. It is due to the fact that magnesium oxide, burned at a temperature of about 1500°C, slakes very slowly, when mixed with water.

Setting action of cement • When water is added to cement, the ingredients of cement react chemically with water and form various complicated chemical compounds.

The formation of these compounds is not simultaneous. But setting action of cement continues for a long time. The mixing of cement and water results in a sticky cement paste and it goes on gradually thickening till it achieves a rock-like state. • It is found that ordinary cement achieves about 70% of its final strength in 28 days and about 90% of its final strength in one year or so.

Setting action of cement • Following are the important compounds formed during the setting action of cement: • (1) Tricalcium aluminate (3CaO, A1203): This compound is formed within about 24 hours after addition of water to the cement.

• (2) Tetra-calcium alumino-ferrite (4CaO, A1203, Fe203): This compound is also formed within about 24 hours after addition of water to the cement.

• (3) Tricalcium silicate (3CaO, Si02): This compound is formed within a week or so after addition of water to the cement and it is mainly responsible for imparting strength to the cement in early period of setting.

Setting action of cement • (4) Di-calcium silicate (2CaO, S102): This compound Is formed very slowly and hence it is responsible for giving progressive strength to the cement. • The above four principle minerals in ordinary Portland cement are designated in short as C3A, C4AF, C3S and C2S respectively and their relative proportions. expressed as percentages are as follows:

Setting action of cement

Setting action of cement • When water is added to the cement, the quickest to react with water is C3A and in order of decreasing rate are C4AF, C3S and C2S. • During the initial period of hardening, the gain in strength of C2S is small and it is about 15 per cent of that of C3S. After 28 days, the hydration of C3S comes practically to an end and the hydration of C2S only really begins at that time. Hence; when a high-strength concrete is required within a short period of time, the cement is made with a high content of C3S. On the other hand, if a highstrength concrete is required at a later stage, as in case of hydraulic engineering constructions, the cement is made with a high content of C2S.

Setting action of cement • The hardening of concrete is greatly speeded up by C3A and hence this property of C3A is utilized for producing quick-hardening Portland cement. • Depending upon the mineralogical composition of clinker in percentage, the Portland cement can be subdivided conventionally as follows:

Setting action of cement • Alit, containing C3S more than 50 to 60% • Aluminate, containing C3A more than 12% • Alumoferrite, containing C3A less than 2% and C4AF more than 18% • Belit, containing C2S more than 35% • High-alit, containing C3S more than 60%.

Storage of cement • The cement should be stored carefully.'

Otherwise it may absorb moisture from the atmosphere and may become useless for the structural work. Following precautions are to be taken for the storage of cement:

Storage of cement • (1) Moisture: If moisture is kept away from cement, it is found that cement will maintain its quality for indefinite period. An absorption of one to two per cent of moisture has no appreciable effect on quality of cement. But if moisture absorption exceeds 5 per cent, the cement becomes totally useless. Hence, when cement is to be stored for a long period, it should be stored in air-tight containers.

Storage of cement • (2) Period of storage: The loose cement may be stored indefinitely in air-tight containers.

But it is advisable to avoid storing of cement in jute bags for a period longer than 3 months.

If it is unavoidable, the cement should be tested to ascertain its properties.

Storage of cement • (3) Piles: The cement bags are stacked in piles.

It is economical to form a pile of 10 bags of cement. A distance of about 300 mm should be kept between the piles of cement bags and exterior walls of building. The passages of width about 900 mm should be provided between the piles. For long storage, the top and bottom of piles should be covered with tarpaulins or water-proof paper.

Storage of cement • (4) Quality of cement: The cement which is finely ground is more active and consequently, it absorbs moisture rapidly from the atmosphere. Hence extraordinary precautions should be taken to store finely ground cement.

Storage of cement • (5) Removal of cement: When cement bags are to be removed from piles of sufficient height, the steps should be formed by taking out two or three bags from front piles. It is also advisable to remove cement in order of its storage period i.e. cement which ¡is stored previously should be taken out first. In other words, the rule of first in, first out should be followed.

Storage of cement • (6) Storage sheds: For storing cement for a sufficiently long period, the storage sheds of special design should be constructed. The walls, roof and floor of such sheds should be of water-proof construction.

Few small windows should be provided and they should be kept tightly shut. The floor should be above ground. If necessary, the drainage should be provided to drain water collected in vicinity of such shed.

For determining the size of storage shed, it is found that 20 bags or 10 kN of cement will require about 1 m 3 of space.

Storage of cement • It should be noted that cement, even if stored in the-most favourable conditions, loses its activity when stored for a long time. For instance, the storage durations of 3 months and 12 months will cause a reduction in the activity of cement to the extent of about 20% and 40% respectively.

Hence it is advisable to reactivate the cement stored for prolonged period. The most effective method of reacting such cement consists in vibro-grinding which ensures greater fineness of cement and makes cement fit for use.

Uses of cement • At present, the cement is widely used in the construction of various engineering structures. It has proved to be one of the leading engineering material of modern times and has no rivals in production and applications. Following are various possible uses of cement:

Uses of cement • (1) Cement mortar for masonry work, plaster, pointing, etc.

• (2) Concrete for laying floors, roofs and constructing lintels, beams, weather sheds, stairs, pillars, etc.

• (3) Construction of important engineering structures such as bridges, culverts, dams, tunnels, storage reservoirs, light houses,. docks, etc.

• (4) Construction of water tanks, wells, tennis courts, septic tanks, lamp posts, roads, telephone cabins, etc.

• (5) Making joints for drains, pipes, etc.

Uses of cement • (6) Manufacture of precast pipes, piles, garden seats, artistically designed urns, flower pots, etc., dust bins, fencing posts, etc. • (7) Preparation of foundations, watertight floors, footpaths, etc.

Varieties of cement • In addition to ordinary cement, the following are the other important varieties of cement: • (1) Acid-resistant cement (2) Blast furnace cement (3) Coloured cement • (4) Expanding cement • (5) High alumina cement (6) Hydrophobic cement

Varieties of cement • (7) Low heat cement • (8) Pozzuolana cement (9) Quick setting cement • (10) Rapid hardening cement (11) Sulphate resisting cement (12) White cement.

Varieties of cement • (1) Acid-resistant cement: An acid-resistant cement is composed of the following: • (1) acid-resistant aggregates such as quartz, quartzites, etc.; (2) additive such' as sodium fluosilicate Na2 SiF6; and • (3) aqueous solution of sodium silicate or soluble glass.

• The addition of additive sodium fluosilicate accelerates the hardening process of soluble glass and it also increases the resistance of cement to acid and water.

Varieties of cement • The binding material of acid-resistant cement is soluble glass which is a water solution of sodium silicate, Na20.nSi02 or potassium silicate, K20.nSi02, where n is the glass modulus. • The term glass modulus is used to indicate the ratio of the number of silica molecules to that of alkali oxide molecules and its value in soluble glass varies from 2.50 to 3.50.

Varieties of cement • The acid-resistant cement is used for acidresistant and heat-resistant coatings of installations of chemical industry. It is not water-resistant and it fails when attacked by water or weak acids.

By adding 0.50 per cent of linseed oil or 2 per cent of ceresit, its resistance to the water is increased and it is then known as the acid and water resistant cement.

Varieties of cement • (2) Blast furnace cement: For this cement, the slag as obtained from blast furnace is used.

The slag is a waste product in the manufacturing process of pig-iron and it contains the basic elements of cement, namely, alumina, lime and silica. The clinkers of cement are ground with about 60 to 65 per cent of slag.

Varieties of cement • The properties of this cement are more or less the same as those of ordinary cement. Its strength in early days is less and hence it requires longer curing period. It proves to be economical as slag, which is a waste product, is used in its manufacture. This cement is durable, but not suitable for use in dry arid zones.

Varieties of cement • (3) Coloured cement: The cement of desired colour may be obtained by intimately mixing mineral pigments with ordinary cement. The amount of colouring material may vary from 5 to 10 per cent. If this percentage exceeds 10 per cent, the strength of cement is affected.

Varieties of cement • The chromium oxide gives green colour. The cobalt imparts blue colour. The iron oxide in different proportions gives brown, red or yellow colour. The manganese dioxide is used to produce black or brown coloured cement. • The coloured cements are widely used for finishing of floors, external surfaces, artificial marble, window sill slabs, textured panel faces, stair treads, etc.

Varieties of cement • (4) Expanding cement: This type of cement is produced by adding an expanding medium like sulpho-aluminate and a stabilising agent to the ordinary cement. Hence this cement expands whereas other cements shrink. • The expanding cement is used for the construction of water retaining structures and also for repairing the damaged concrete surfaces

Varieties of cement • (5) High alumina cement: This cement is produced by grinding clinkers formed by calcining bauxite and lime. The bauxite is an aluminium- ore. It is specified that total alumina content should not be less than 32 per cent and the ratio by weight of alumina to the lime should be between 0.85 and 1.30.

Varieties of cement • (6) Hydrophobic cement: This type of cement contains admixtures which decrease the wetting ability of cement grains. The usual hydrophobic admixtures are acidol, naphthene soap, oxidized petrolatum, etc.

These substances form a thin film around cement grains.

Varieties of cement • When water is added to hydrophobic cement, the absorption films are torn off the surface and they do not in any way, prevent the normal hardening of cement. However, in initial stage, the gain in strength is less as hydrophobic films on cement grains prevent the interaction with water. However its strength after 28 days is equal to that of ordinary Portland cement.

Varieties of cement • When hydrophobic cement is used, the fine pores in concrete are uniformly distributed and thus the frost resistance and the water resistance of such concrete are considerably increased.

Varieties of cement • (7) Low heat cement: The considerable heat is produced during the setting action of cement. In order to reduce the amount of heat, this type of cement is used. It contains lower percentage of tricalcium aluminate C3A of about 5% and higher percentage of dicalciurn silicate C2S of about 46%. • This cement possesses less compressive strength. The initial setting time is about one hour and final setting time is about 10 hours.

It is mainly used for mass concrete work.

Varieties of cement • (8) Pozzuolana cement: The pozzuolana is a volcanic powder.

• It is found in Italy near Vesuvius. It resembles surkhi which is prepared by burning bricks made from ordinary soils. It can also be processed from shales and certain types of clays. The percentage of. pozzuolana material should be between 10 to 30.

• This cement is used to prepare mass concrete of lean mix and for marine structures. It is also used in sewage works and for laying concrete under water.

Varieties of cement • (9) Quick setting cement: This cement is produced by adding a small percentage of aluminium sulphate and by finely grinding the cement. The percentage of gypsum or retarder for setting action is also greatly reduced. The addition of aluminium sulphate and fineness of grinding are responsible for accelerating the setting action of cement.

The setting action of cement starts within five minutes after addition of water and it becomes hard like stone in less than 30 minutes or so.

Varieties of cement • The extreme care is to be taken when this cement is used as mixing and placing of concrete are to be completed in a very short period. This cement is used to lay concrete under static water or running water.

Varieties of cement • (10) Rapid hardening cement: The initial and final setting times of this cement are the same as those of ordinary cement. But it attains high strength in early days. It contains high percentage of tricalcium silicate C3S to the extent of about 56%.

Varieties of cement • (11) Sulphate resisting cement: In this cement, the percentage of trlcalcium aluminate C3A is kept below 5 per cent and it results in the increase in resisting power against sulphates. • This cement is used for structures which -are likely to be damaged by severe alkaline conditions such as canal linings, culverts, syphons, etc.

Varieties of cement • (12) White cement: This is just a variety of ordinary cement and it is prepared from such raw materials which are practically free from colouring oxides of iron, manganese or chromium. For burning of this cement, the oil fuel is used instead of coal. It is white in colour and it is used for floor finish, plaster work, ornamental work, etc. It should not set earlier than 30 minutes. It should be carefully transported and stored in closed containers only. It is more costly than ordinary cement because of specific requirements imposed upon the raw materials and the manufacturing process.