- Cement is an extreme ground material having adhesive and cohesive properties which provide a binding medium for the discrete ingredients.
- The processes used for the manufacture of cement can be classified as dry and wet.
- In the wet process, the limestone brought from the quarries is first crushed to smaller fragments. Then, it is taken to a ball or tube mill where it is mixed with clay or shale as the case may be and ground to a fine consistency of slurry with the addition of water. The slurry is stored in tanks under constant agitation and fed into huge firebrick lined rotary kilns.
- In the dry process, the raw materials are ground, mixed and fed to the rotary kiln in the dry state.
- The identification of the major complex compounds is largely based on R.H. Bougue's work and hence these are called Bougue's compounds.
- Bogues Compounds when water is added to cement it react with the ingredients of the cement chemically & results in the formation of complex chemical compounds terms as BOGUES compounds. which are not for simultaneously.
- Tri-Calcium Aluminate (3CaO.Al2O3 or C3A) -------------------------8-12%
- Tetra Calcium Alumino Ferrate (4CaO.Al2O3.Fe2O3 or C4AF)----6-10%
- Tri-Calcium Silicate (3CaO.SiO2 or C3S)---------------------------------30-50%
- Di-Calcium Silicate (2CaO.SiO2 or C2S)----------------------------------20-45%
- Formed in 24 hrs of addition of water
- Max. evolution of heat of hydration
- check setting time of cement
2. Tetra Calcium Alumino Ferrate (4CaO.Al2O3.Fe2O3 or C4AF)
- Formed within 24 hrs of addition of water
- High heat of hydration in initial periods
- Formed within week
- Responsible for initial strength of cement
- Contribute about 50-60% of strength
- Content increase for the pre fabricated concrete construction, Cold weathering construction.
- Last compound formed during hydration of cement
- responsible for progressive later stage strength
- Structure requires later stages strength proportion of this component increase
- e.g. hydraulic structures, bridges.
Type of Cements
- Ordinary Portland Cement
- Rapid Hardening Cement – IS: 8041-1990
- Extra Rapid Hardening Cement
- Low Heat Portland Cement - IS: 12600-1989
- Portland Slag Cement – IS: 455-1989
- Portland Pozzolana Cement – IS: 1489-1991 (Part 1 and 2)
- Sulphate Resisting Portland Cement – IS: 12330-1988
- White Portland Cement – IS: 8042-1989
- Coloured Portland Cement - IS: 8042-1989
- Hydrophobic Cement - IS: 8043-1991
- High Alumina Cement - IS: 6452-1989
- Super Sulphated Cement - IS: 6909-1990
- Special Cements
- Masonry Cement
- Air Entraining Cement
- Expansive Cement
- Oil Well Cement
In addition to ordinary portland cement there are many varieties of cement. Important varieties are briefly explained below:
(i) White Cement: The cement when made free from colouring oxides of iron, maganese and chlorium results into white cement. In the manufacture of this cement, the oil fuel is used instead of coal for burning. White cement is used for the floor finishes, plastering, ornamental works etc. In swimming pools white cement is used to replace glazed tiles. It is used for fixing marbles and glazed tiles.
(ii) Coloured Cement: The cements of desired colours are produced by intimately mixing pigments with ordinary cement. The chlorium oxide gives green colour. Cobalt produce blue colour. Iron oxide with different proportion produce brown, red or yellow colour. Addition of manganese dioxide gives black or brown coloured cement. These cements are used for giving finishing touches to floors, walls, window sills, roofs etc.
(iii) Quick Setting Cement: Quick setting cement is produced by reducing the percentage of gypsum and adding a small amount of aluminium sulphate during the manufacture of cement. Finer grinding also adds to quick setting property. This cement starts setting within 5 minutes after adding water and becomes hard mass within 30 minutes. This cement is used to lay concrete under static or slowly running water.
(iv) Rapid Hardening Cement: This cement can be produced by increasing lime content and burning at high temperature while manufacturing cement. Grinding to very fine is also necessary. Though the initial and final setting time of this cement is the same as that of portland cement, it gains strength in early days. This property helps in earlier removal of form works and speed in construction activity.
(v) Low Heat Cement: In mass concrete works like construction of dams, heat produced due to hydration of cement will not get dispersed easily. This may give rise to cracks. Hence in such constructions, it is preferable to use low heat cement. This cement contains low percentage (5%) of tricalcium aluminate (C3A) and higher percentage (46%) of dicalcium silicate (C2S).
(vi) Pozzolana Cement: Pozzolana is a volcanic power found in Italy. It can be processed from shales and certain types of clay also. In this cement pozzolana material is 10 to 30 per cent. It can resist action of sulphate. It releases less heat during setting. It imparts higher degree of water tightness. Its tensile strength is high but compressive strength is low. It is used for mass concrete works. It is also used in sewage line works.
(vii) Expanding Cement: This cement expands as it sets. This property is achieved by adding expanding medium like sulpho aluminate and a stabilizing agent to ordinary cement. This is used for filling the cracks in concrete structures.
(viii) High Alumina Cement: It is manufactured by calcining a mixture of lime and bauxite. It is more resistant to sulphate and acid attack. It develops almost full strength within 24 hours of adding water. It is used for under water works.
(ix) Blast Furnace Cement: In the manufacture of pig iron, slag comes out as a waste product. By grinding clinkers of cement with about 60 to 65 per cent of slag, this cement is produced. The properties of this cement are more or less same as ordinary cement, but it is cheap, since it utilise waste product. This cement is durable but it gains the strength slowly and hence needs longer period of curing.
(x) Acid Resistant Cement: This cement is produced by adding acid resistant aggregated such as quartz, quartzite, sodium silicate or soluble glass. This cement has good resistance to action of acid and water. It is commonly used in the construction of chemical factories.
(xi) Sulphate Resistant Cement: By keeping the percentage of tricalcium aluminate C3A below five per cent in ordinary cement this cement is produced. It is used in the construction of structures which are likely to be damaged by alkaline conditions. Examples of such structures are canals, culverts etc.
(xii) Fly Ash Blended Cement: Fly ash is a byproduct in thermal stations. The particles of fly ash are very minute and they fly in the air, creating air pollution problems. Thermal power stations have to spend lot of money to arrest fly ash and dispose of safely.
It is found that one of the best way to dispose fly ash is to mix it with cement in controlled condition and derive some of the beneficiary effects on cement. Now-a-days cement factories produce the fly ash in their own thermal stations or borrow it from other thermal stations and further process it to make it suitable to blend with cement. 20 to 30% fly ash is used for blending.
Field Tests For Cement
- Colour: Grey colour with a light greenish shade.
- Physical Properties: Cement should feel smooth when touched between fingers.
- If hand is inserted in a bag or heap of cement, it should feel cool.
- If a small quantity of cement is thrown in a bucket of water, it should sink and should not float on the surface.
- Presence of lumps: Cement should be free from lumps.
- Permissible Limits for Impurities in Water
Laboratory Tests For Cement
1. Chemical Composition Test
- Ratio of percentage of lime to percentage of silica, alumina and iron oxide known as Lime Saturation Factor (LSF), when calculated by the formula shall not be greater than 1.02 and not less than 0.66.
- Ratio of percentage of alumina (al2O3) to that of iron oxide (Fe2O3) shall not be less than 0.66
- Weight of insoluble residue shall not be more than 4 per cent.
- Weight of Magnesia Shall not be more than 6 per cent
- Total loss on ignition shall not be more than 5 per cent.
- Total sulphur content calculated as sulphuric anhydride shall not be more than 2.5% when C3A is 5% or less and shall not be more than 3% when C3A is more than 5%
2. Normal Consistency Test
- The normal (standard) consistency of a cement paste is defined as that consistency which will permit a Vicat plunger having 10 mm diameter and 50 mm length to penetrate a depth of 33 to 35 mm from the top (or 5 to 7 mm from the bottom) of the mould.
- Vicat Apparatus: Vicat apparatus assembly consists of a plunger 300 gm in weight with a length of 50 mm and diameter of 10 mm and a mould which is 40 mm deep and 80 mm in diameter.
3. Initial Setting Time Test
When water is added to cement, the resulting paste starts to stiffen and gain strength and lose the consistency simultaneously. The term setting implies solidification of the plastic cement paste. Initial and final setting times may be regarded as the two stiffening states of the cement. The beginning of solidification, called the initial set, marks the point in time when the paste has become unworkable
- Initial setting time should not be less than 30 minutes for OPC and 60 minutes for low heat cement.
4. Final Setting Time Test
The final setting time is the time after which the paste becomes so hard that the angular attachment to the needle, under standard weight, fails to leave any mark on the hardened concrete. Initial and final setting times are the rheological properties of cement.
- The final setting time should not be more than 10 hours.
5. Soundness Test
- The soundness of cement is determined either by 'Le Chatelier's method' or by means of a 'Autoclave' test.
- No satisfactory test is available for deduction of soundness due to excess of calcium sulphate. But its content can be easily determined by chemical analysis.
- Le Chatelier's Method
- Autoclave Test
6. Strength Test
(a) Compressive Strength Test
- Three cubes are tested for compressive strength at 1 day, 3 day, 7 days and 28 day where the period of testing being reckoned from the completion of vibration.
- The compressive strength shall be the average of the strengths of the three cubes for each period respectively.
- The compressive strength of 33 grade OPC at 3 day and 28 day is 16 MPa, 22 MPa and 33 MPa respectively.
(b) Tensile Strength Test
- Six briquettes are tested and average tensile strength is calculated.
- A load is applied steadily and uniformly, starting from zero and increasing at the rate of 0.7 N/mm2 in 12 seconds.
- OPC should have a tensile strength of not less than 2 MPa and 2.5 MPa after 3 and 7 days respectively.
- Generally, tensile strength is 10-15% of compressive strength.
7. Fineness Test: There are three methods for testing fineness viz.
(a) Sieve Method
- 100 gm of cement sample is taken and air set lumps, if any, in the sample are broken with fingers.
- The sample is placed on a 90 micron sieve and continuously sieved for 15 minutes.
- The residue should not exceed the limits specified below:
(b) Air Permeability Method
- Fineness of cement is represented by specific surface i.e. total surface area in cm2 per gram of cement.
(c) Wagner Turbidimeter Test
- The cement is dispersed uniformly in a rectangular glass tank filled with kerosene.
- Parallel light rays are passed through the solution which strike the sensitivity plate of a photoelectric cell.
8. Heat of hydration Test
- The apparatus used to determine the heat of hydration of cement is known as calorimeter.
- The heat of hydration for low heat Portland cement should not be more than 66 and 75 cal/gm for 7 and 28 days respectively.
9. Specific Gravity Test
- The specific gravity of cement is obtained by using Le Chatelier's flask.
Uses of Cement
Cement is used widely for the construction of various structures. Some of them are listed below:
(i) A cement slurry is used for filling cracks in concrete structures.
(ii) Cement mortar is used for masonry work, plastering and pointing.
(iii) Cement concrete is used for the construction of various structures like buildings, bridges. water tanks, tunnels, docks, harbours etc.
(iv) Cement is used to manufacture lamp posts, telephone posts, railway sleepers, piles etc.
(v) For manufacturing cement pipes, garden seats, dust bins, flower pots etc. cement is commonly used.
(vi) It is useful for the construction of roads, footpaths, courts for various sports etc.
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