Manufacturing of Cement-Construction Material

Construction Material-Portland Cement

Manufacturing of Cement-Construction Material

Cement

Materials, which exhibit characteristic properties of setting and hardening when mixed with water to form a paste, are called as cement.  Cement may be broadly described as a material possessing adhesive and cohesive properties and capable of bonding material. The principal constituents of cement used are compounds of Ca (calcareous) and Al + Si (argillaceous).

Portland cement:  An extremely finely ground product obtained by calcining together argillaceous and calcareous raw materials, with the addition of gypsum as a retarder.

Raw materials for Portland cement

Calcareous materials mainly supply calcium oxide (CaO) (e.g. Limestone, cement rock, marine shell, chalk etc.) and MgO (less than 5 %).

Argillaceous materials which supply mainly silica (SiO₂) and alumina (Al₂O₃) in combined form e.g.: clay, slate, shale, slag, ashes.

Retarder: 2-3% of gypsum is added to cement clinkers during grinding.

Fuel: Powdered coal or fuel oil or natural gas can be used as fuels.

Functions of Ingredients

1.         Lime(CaO): It is the principal constituent of cement its lesser amount reduces strength and quick setting while higher amount again reduces strength as it responsible for the expansion of cement.

2.     Silica:      It imparts strength to cement

3.     Alumina: It helps cement for quick-setting

4.     Gypsum: It helps to retard setting action of cement.

5.     Iron-oxide: It imparts colour, strength, and hardness.

6.     Sulpher trioxide (in a small proportion): It imparts soundness to cement.

Manufacture of Portland cement

Following steps can be used for manufacturing of Portland cement

1.         Selecting of raw materials

2.         Crushing and grinding

3.         Storage of the slurry

4.         Burning the ground mix to clinker in a rotary kiln

5.         Cooling of hot clinker

6.         Grinding the clinker with gypsum

7.         Storage and packing

 

1.         Selection of raw materials

The primary calcareous material used is lime (limestone, chalk, marine shells, marl, etc.).

The argillaceous materials, Al₂O₃, and SiO₂ (such as clay, shell, slate, and other residual clay like bauxite, laterite, moorum)

Gypsum (CaSO₄.2H₂O) and water (for the wet process) are the other raw materials needed for cement manufacture. The composition of varies constituents of typical Portland cement is:

Component	Percentage
Lime (CaO)	60-69
Silica (SiO2)	17-25
Alumina (Al2O3)	3-9
Iron oxide (Fe2O3)	2-4
Magnesium oxide (MgO)	1-5
Gypsum (CaSO4.2H2O)	2-3
other	1-3

Coal, furnace oil and natural gas can be used as fuel.

2.         Crushing and grinding

The raw materials are crushed in large jaw crushers or coarse gyratory crushers. Then, they are ground in the raw mill. In dry milling, the raw materials are dried with hot air and pulverized. In wet milling, the raw materials are ground with 30 – 40% of water in tube mills. The finer the particles the better is the cement produced.

3. Mixing of raw materials

Can be done by the dry process or wet process

Dry process:    After grinding the calcareous and argillaceous materials they are dried and mixed in calculated proportion. Then its fine powdered is formed in tube mills. Homogenized the powder and stored in storage bins (silos) and kept ready to feed in a rotary kiln.

Wet process:  The calcareous raw materials are crushed, powdered and stored in a big tank. The argillaceous materials in thoroughly mixed with water. The raw materials are finely ground and mixed in the desired proportion and 30-40% of H₂O added to form a flowing slurry. The slurry finally stored in a storage tank and kept ready for feeding to the rotary kiln.

Rotary Kiln

4.         Storage of the slurry

The ground raw materials are stored in concrete silos where they are kept agitated with compressed air.

5.         Burning the ground mix to clinker in a rotary kiln

The rotary kiln is a long horizontal steel cylinder lined with refractory bricks and rotating at a speed of 0.5 to 2 rotations per minute. The slurry of the raw materials enters from the upper end of the rotary kiln while the burning fuel (pulverized coal, oil or natural gas) and air are induced from the lower end of the kiln. The slurry gradually descends in the kiln into different zones of increasing temperature.

Drying zone: Upper part of the kiln, where the temperature is about 400ºC; in this zone entire water of the slurry gets evaporated.

Pre-heating zone: the Middle zone where the temperature is about 400-700 ºC. In this zone, clay and MgCO₃ decomposes.

Calcining or Decarbonate zone: It is a lower middle zone where the temperature is about 700-1000 ºC. In the zone, limestone decomposes to form CaO and CO₂.

Burning or clinkering zone

It is the hottest zone, (1350-1600 ºC). In this zone lime clay combines each other to form calcium silicates, calcium aluminates, and calcium alumino ferrite:

     

The compounds then combine together to form small, hard, grayish pellets called cement clinkers. C₂S, C₃S, C₃A and C₄AF are called microscopic constituents of cement.

6.         Cooling of hot clinker

The hot clinker emerging from the kiln is cooled by various systems such as rotary coolers, planetary coolers or air quench type coolers.

7.         Grinding the clinker with gypsum

The cooled clinker is then finely pulverized together with 2 to 6 % gypsum in long tube mills.

8.         Storage and packing

The cement coming out of the grinding mills is stored in concrete storage silos. Moisture-free compressed air is used to agitate the cement.

 Comparison of dry and wet process

S. No.	Dry Process	Wet process
1	Cost of grinding raw material is higher	Lower cost of grinding in the wet process
2	Less long kiln used	Longer kiln necessary.
3	Lesser fuel required	More fuel required to evaporate water
4	Dry process preferred when raw material is in very hard form	Preferred process when the raw material is soft
5	Composition of raw materials is not controlled accurately.	Composition control and homogeneity are easier.
6	Inferior quality cement formed	Superior quality cement formed
7	Process is slower	Process is faster.

Setting and hardening of Portland cement

Setting is the process of solidification of cement paste with some early strength and takes the shape given. Hardening is meaning by development of high strength in the set mass of cement. The setting and hardening of cement are mainly due to hydration and hydrolysis reactions taking place when the different microscopic constituents of cement interact with water. Setting and hardening is taking place in three stages.

1.      Initial setting or flash setting.

2.      Final setting or initial hardening

3.      Final hardening

1.      Initial (flash) setting

When water is added to the cement powder, it forms a plastic, semisolid paste like mass, which can be given shape easily while working. This initial setting is known as flash set. The time for initial setting is generally half hour. Initial setting involves the hydration of tricalcium aluminate (C₃A) to form crystals of tricalcium aluminate hydrate.

This reaction is very fast and gets completed in about 15 minutes and it is highly exothermic. The crystals formed in above reaction prevent the hydration reactions of other constituents forming barrier on them.  Gypsum present in the cement reacts with C₃A in presence of water and form insoluble sulfo aluminate complex. This complex increases the flash set time.

Tetra calcium alumino ferrite (C₄AF) also gets hydrolyzed to form both gels and crystalline compounds.

These gels shrink with time and leave some capillaries. Through these capillaries water enter inside and reacts with dicalcium silicate and tricalcium silicate.

2.      Final setting or initial hardening

Hydration of C₃A and C₄AF do not result in developing any appreciable strength. Initial hardening is gained by hydrolysis of tricalcium silicate.  Hydrolysis of C₃S is slow and it requires 7 days to complete. This reaction gives strength.

3.      Final hardening

The development of ultimate stiffness or strength in the cement mass is knows final hardening. For this stage to complete, sufficient water must be provided (curing). Final hardening is due to mainly two chemical reactions i.e. continued hydrolysis of C₃S and hydration of C₂S.

The tobermonite gel formed has strongly binding character. This stage produces upto 99% of the total strength, if properly cured upto 28 days. The relative strengths developed due to reactions of compounds in cement with water during setting and hardening.

Heat of hydration

When water is mixed with Portland cement, some amount of heat is liberated due to hydration and hydrolysis reactions to leading setting and hardening of cement. The heat of hydration of the different constitutional compounds is in the following order:

C₃A > C₃S > C₄AF > C₂S

                                               kJ/kg     878    502       418    251

Therefore, when the large masses of concrete are poured into the construction point (such as construction of dams), the quickly dissipation the heat generated during hydration is necessary. This voids the formation of shrinkage cracks on setting and hardening.

Soundness

If cement on hydration produces only very small volume changes and that such volume changes are will within tolerance limits laid down in the specification, the cement is said to be “sound”. Presence of excessive quantities of crystalline magnesia contributes to delayed expansion or unsoundness. The soundness is determined by Le Chatlier`s test in which the expansion of a test piece in boiling water for 3 to 5 hours is measured. Recently this test is replaced by the “Autoclave test”.     

Function of gypsum

Tricalcium aluminate C₃A undergoes hydration quickly and the reaction being highly exothermic (880kJ/kg) the suddenly liberated. Due to exothermic reaction temperature of the mass increases and causing minor cracks while setting. It will affect the strength. Therefore, the hydration of C₃A should be prolonged or delayed.

The hydration of C₃A can be delayed by gypsum present in cement. As soon as water is added to the cement powder, most of the C₃A reacts with gypsum, producing calcium sulphoaluminate complex. This complex is responsible for delaying the hydration of C₃A.

This product slowly disintegrates into C₃A.6H₂O and thus sudden heat liberation is prolonged by gypsum upto about half hour.

Important parameters for manufacturing good cement clinkers

Indian standards (I.S. 269-1967) have given the specifications regarding the raw material composition to get better quality cement as follow:

1.      The lime saturation factor:  

This will ensure the formation of  which are responsible for giving strength.

2.         Silica modulus:  .

3.         Alumina modulus: .

4.      Burn ability index: , the composition of clinker depends on burn ability index.

5.         Magnesia  should be below 5%.

6.         Total sulphur content  should be below 2.75%.

7.         Fine grinding of the raw materials which helps the kinetics of reaction.

8.         Maintaining alkali chloride within the specified limits.

Microscopic constituents of Cement

The properties of cement depend upon the relative proportions of the constitutional compounds present. These constitutional compounds are also called microscopic constituents.

1.      Tricalcium aluminate

a.       It goes hydration at a very fast rate

b.      It is responsible for initial set or flash set.

c.       Setting time is 1 day.

d.      Early strength is good but the ultimate strength is quite low.

e.       Heat of hydration is about 880 kJ/kg

f.       The rate of hydration is 82.5 %.

2.      Tetracalcium aluminoferrite

a.       Hydration of C₄AF comparatively slower than C₃A.

b.      Setting time is 1 day.

c.       Early and ultimate strength is poor.

d.      The rate of hydration is slow (57%).

e.       The heat of hydration is 418.4 kJ/kg.

3.      Tricalcium silicate

a.       It develops very high strength quite early and ultimate strength is also highest.

b.      Setting time is 7 day.

c.       The rate of hydration is medium.

d.      The rate of hydration is 73.5%.

e.       The heat of hydration is 502 kJ/kg.

4.      Dicalcium silicate

a.       This hydrate very slowly

b.      Rate of hydration is 37.5%

c.       Setting time is 28 day.

d.      The heat of hydration is the lowest about 251 kJ/kg

e.       Early strength is low but ultimate strength is very high.

Comparative study of microscopic constituents of cement

	C3A	C4AF	C3S	C2S
Hydration	Fast	Fast	slow	Slow
Rate of hydration	82.5%	57.0%	73.5%	37.5%
Heat of hydration	880 kJ/kg	418.4 kJ/kg	502 kJ/kg	251 kJ/kg
Setting Time	1 day	1 day	7 days	28 days
Strength	Early strength is high but ultimate strength is poor	Early and ultimate strength is poor	Early and  ultimate strength is high	Early strength is low but ultimate strength is high

Additives for Cement

Certain compounds can be mixed in cement to develop certain character in cement and such compound is known as additives. The mixing of such compound is done at the time of grinding the cement clinker, during the cement manufacture. Some of the additives are given below:

1.      Accelerators: These are added in cement to develop early strength. Such compounds are common salt, , some organic compounds such as triethanol amine, some water soluble carbonates, silicates and flurosilicates. Generally 2% of  is used for the increase of early strength in setting of portland mass. in small quantities is important to develop strength and durability. Excess addition of accelerator has lowered the strength and corrosive to the reinforcement metal.

2.      Retarders: These additives prevent the structure from temperature variation and hot water flow in grouting. These additives prevent the premature stiffness of cement mass. During grouting (filling the cracks in rocks of tunnel, filling the cracks in dam walls, etc.) operation, the slurry of cement is pumped from a distance and it is necessary to retard the setting time, otherwise the slurry will set in pipe itself. The retarders are added in small quantity. Admixture of very small quantities of carbohydrate derivatives and calcium lignosulfonate are the more commonly used retarders.

3.      Air-entrainment agents: These are the additives have similar function like foaming or foam stabilizing agents. Vinsol resin and Darex are the commonly used commercial air-entrainment agents; it mixed with clinkers at the time of grinding. They offer pavement durability against the action of alternating cycles of cold weather or hot weather. If the cement mass has not foamy or porous in nature than the mass suffers cracks development due to contraction action of cold weather and expansion action of hot weather. The air entrainment agents make the mass of cement to have various air pockets and make the cement mass durable to action of very cold or hot conditions. Other organic compounds such as the salt of natural resins, fats and oils are used as air-entrainment agents.

4.      Water-repelling agents: The waterproof cement contains a water-repelling agent added 0.1 to 0.2 %. Chemically the water-repelling agent is the calcium/ magnesium/aluminum salts of fatty acid e.g. Calcium stearate, magnesium palmitate, etc. These generally add in cement at the time of making mortar. Cement with water-repelling agents is used for plastering the walls which are in contact of moist soils, water. Water-repelling agents increase the workability of the fresh concrete and decrease the absorption by capillarity.

Types of cement

There are various types of cement. The ordinary Portland cement is used for common constructions but other types of cement are used for specific purposes. Few of the types of special cement are given below

1)            White cement

·   The major constituents of white cement are limestone and clay.

·   This cement is manufactured without using.

·   It has a white color.

·   It is used for joining tile, marble work, marble pillars, and blocks.

·   White cement is also used for preparing cement paints (by mixing colored inorganic matter). White cement is costlier.

2)      High alumina cement

·   It is manufactured from bauxite and limestone.

·   Typical composition used is

               -------              35 - 55 %

                --------              35 - 40 %

                --------              5 - 10%

              --------              5 - 15 %

·   The important constituents of high alumina cement are along with C₃A₅, C₄AF, and CA

·   On hydration form stable crystalline hydrates and gel of . They are responsible for an initial setting time similar to ordinary Portland cement but hardening is rapid.

·   This cement construction has high-temperature stability but the construction has lower final strength.

·   This cement has better chemical resistance to seawater, but it attacked more easily by acids.

·   It liberates a larger amount of heat during the initial setting, therefore it is better used for constructions under low atmospheric temperatures.

3)            Low heat cement

·   This cement contains a lower percentage of C₃A, C₃S and a higher percentage of C₂S, C₄AF.

·   The heat of hydration of this cement is almost half of the Portland cement, therefore shrinkage cracks formation on the construction surface, are negligible.

·   Typical composition of this cement is

  

·   This cement is specially used for mass concrete works where low hydration heat liberation is desirable, e.g. dams, monolithic works.

4)            Rapid hardening cement

·   This cement is manufactured by using a higher lime saturation factor and therefore it contains higher  for rapid hardening and gaining faster strength.

·   The typical composition of this cement is (with free lime, magnesia and gypsum).

·   Rapid hardening cement is used for the constructions where high early strength is required to develop and for used in pre-stressed concrete constructions.

·   C₃S Hydrolysis causes the development of early strength.

5)            Pozzolanic cement

·   Pozzolanic cement is the natural materials which on mixing with lime, without heating, form hydraulic cementing material.

·    Pozzolana contains silicates of aluminum, calcium, and iron. The natural Pozzolana (deposits of volcanic ash) is simply mixed with lime (CaO) and then finely ground.

They are used for low-cost constructions but nowadays; they are used as the blend of Pozzolana cement with Portland cement because the only Pozzolana cement does not produce high strength 
constructions.

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