Limestones containing 20 to 40 percent clay may be burnt and crushed to powder to get natural cement. The best variety of natural cement is known as Roman cement.
Artificial cement is manufactured process by mixing argillaceous and calcareous materials in suitable proportions and burning at a temperature of 1400°C to 1450°C to get clinkers, which are then ground to get cement.
ordinary portland cement contains the following chemical compounds
Ordinary Portland cement (OPC) contains lime (60 – 67%), silica (17 – 25%), alumina (3 – 8%) iron oxide (0.5 – 5.0%), calcium sulphate (3 – 4%), magnesia (0.1 – 3.0%), sulphur oxide (1 – 3%) and alkalies (0.4 – 1.3%). Lime imparts strength. Silica contributes to strength by forming dicalcium and tricalcium silicates.
Excess silica prolongs the setting of lime. Alumina imparts quick setting property. Iron oxide provides color, hardness, and strength.
Calcium sulphate increases the initial setting time.
Magnesia provides hardness and colour. Sulphur makes cement unsound. Alkalies, if in excess, cause efflorescence.
1. Tricalcium silicate 3CaO◊ SiO2 (C3S) 40%
2. Dicalcium silicate 2CaO◊ SiO2 (C2S) 30%
3. Tricalcium aluminate 3CaO◊ Al2O3 (C3A) 11%
4. Tetracalcium alumino ferrite. 4CaO◊ Al2O3 Fe2O3 (C4 AF) 11%
It also contains small quantities of impurities like calcium oxide (CaO) and magnesium oxide (M2O). When water is added
• C3A is first to react and sets generating heat
• C3S hardens early and develops strength in the first 28 days.
• C2S hydrates slowly and increases strength over a year and contributes to ultimate strength.
• C4 AF is a comparatively inactive compound.
Physical requirements of ordinary portland cement are:
1. Fineness: The material retained on a 90-micron sieve should not be more than 10%. In terms of a specific surface, it should not be less than 2250 cm2 /gm [225 m2 /kg].
2. Setting time: The minimum initial setting time should be 30 minutes and the maximum final setting time should be 600 minutes.
3. Soundness: After the test is conducted in Le Chatelier mould, the indicator should not show more than 10 mm widening.
4. Compressive strength: Mortar cubes of size 70.6 mm with 1 part cement, 3 parts of standard sand with specified water should give the strength as shown in Table.
Manufacture of cement – consists of mixing, burning, and grinding processes.
1. Mixing: It may be a wet process or by dry process. In a wet process, wash mill is used which is a heavy cylinder of 2.5 to 3.0 m in diameter and 9 to 12 m in length. It is kept slightly inclined to the horizontally and can rotate at 15–20 revolutions per minute. The cylinder is provided with steel balls.
2. Burning: Burning is carried out in a rotary kiln, which is a steel tube of diameter 2.5 to 3.0 m and length 90 to 120 m, placed at an inclination 1 in 25 to 1 in 30. It rotates at a rate of 1 to 3 rotations per minute. Coal dust is injected from the lower end and the mix is fed from the top end. The temperature at the feed end is 1400°C to 1500°C.
3. Grinding: The clinkers from the rotary kiln are fed into a ball mill or tube mill. During the process of grinding about 3 to 4 percent of gypsum is added. The ball mill or tube mill contains steel balls.
Cement Manufacturing Process
Storage of cement: Cement absorbs moisture from air and hydrates, which results into loss of strength.
Hence, cement should be stored on a raised platform in a covered room. First- in-first-out rule should be used while taking out cement.
The drainage system on the roof and around the storage should be well maintained. The storage period should be as little as possible.
Types of cement
1. OPC: 33 grade, 43 grade and 53 grade (OPC)
2. Portland Pozzolana cement (PPC)
3. Fly ash cement
4. Blast furnace slag cement
5. Acid-resistant cement
6. Sulphate-resistant cement
7. High alumina cement
8. Quick setting cement
9. Rapid hardening
10. Expanding cement
11. Low-heat cement
12. Hydrophobic cement
13. White Cement
14. Coloured cement
Fine and Coarse Aggregates
Sand, gravel, and crushed stones which are the products of weathering or crushing of rocks are known as aggregates. Sources of fine aggregates on the basis of which sand is classified are sea sand, river sand, stream sand, pit sand, and manufactured sand.
On the basis of grains, size sand is classified as fine sand, coarse sand, and gravelly sand. IS code classifies sand as a grading zone.
1. Grading zone
2. Grading zone
3. and Grading zone
4. on the basis of the percentage of sand passing through different sizes.
The function of sand:
It subdivides the cement paste into thin films and allows it to spread and adhere.
It allows the carbon dioxide from the air to penetrate and improve the setting. It prevents shrinkage and adds to the density of mortar. It fills the gap between building blocks and gives a level surface to mortar.
Bulking of sand:
The increase in volume due to moisture content is known as the bulking of sand. It is due to the formation of thin films around sand particles. The increase in volume is as high as 30 – 37 percent.
At around 8% of moisture content, there is maximum bulking. The finer the sand more is the more bulking. After about 20% moisture content, thin films start breaking and volume reduction takes place.
Good sand has the following properties:
- Chemically inert
- Contains sharp and angular grains
- Free from salt, clay, and organic matter
- Well graded.
The field tests are possible to find the presence of clay, salt, and organic impurities. The size and shape of gains may be felt by touching it with fingers.
Laboratory tests may be conducted to ascertain grading, and bulking and to find fineness modulus. To determine fineness modulus sieves to be used are 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 microns, 300 microns, and 150 microns.
The cumulative percentage of weight retained on the above sieves divided by 100 gives fineness modulus (FM). If FM is between 2.20 and 2.60 it is fine sand, 2.6 to 2.9 FM indicates it is medium sand and more than 2.9 FM indicates it is coarse sand.
The coarse aggregate gives mass to concrete.
• For structures like abutments, retaining walls, and bed concrete 40 mm down size aggregates are used. For normal R.C.C. works like flooring roofing and columns 20 mm downsize aggregates are preferred. For thin members, 12.5 mm-sized aggregates are used.
• The various tests conducted on coarse aggregates are to determine:
1. Flakiness and elongation indices
2. Fineness and grain size distribution
3. Specific gravity and water absorption
4. Clay, silt, and dust content
5. Resistance to crushing
6. Resistance to impact
7. Resistance to abrasion.
Cement concrete is an intimate mixture of cement, sand, coarse aggregates, and water.
Occasionally some admixtures are added to introduce special properties.
Water lubricates aggregates, activates chemical reactions, and gives workability to mix. However, the quantity of water to be used should be determined carefully since the workability of concrete increases with the quantity of water but the strength decreases.
AcceleratorsVarious admixtures used with concrete are:
- Air entraining agents
- Coloring agents.
Important properties of concrete in the plastic stage (green concrete) are workability, segregation, and bleeding.
The important properties of hardened concrete are:
- Resistance to wear
IS code classifies concrete mix as M 20, M 25, etc. which means a mix of strength 20 N/mm2, a mix of strength 25 N/mm2, etc. at the age of 28 days.
IS: 10262–1982 and SP 23–1982 give detailed procedures to design a concrete mix of required strength.
For small works nominal mixes as shown is the Table may be used.
It involves mixing, transporting, placing, compacting, and curing.
1. Mixing: One can adopt hand mixing or machine mixing. Coarse aggregate and fine aggregates are dry mixed, then cement is mixed till uniform color is seen to the dry mix. The required quantity of
water is mixed gradually while the mixing process is kept continuous.
2. Transportation: Care should be taken to avoid segregation during transport.
3. Placing: The concrete should be placed to its final position by dropping it as close to the final position as possible, in any case, not more than 0.8 m.
4. Compacting: To remove entrapped air compacting of concrete is necessary. It may be hand compacting or by using vibrators. Over-compacting should be avoided to avoid segregation.
5. Curing: It is the process of maintaining satisfactory moisture and temperature in freshly laid concrete. Curing should be done well in the first 2 weeks and continued for another 1–2 weeks. If curing is not satisfactory shrinkage cracks may develop and durability is reduced.
methods of curing concrete are:
- Spraying water
- Covering with gunny bags
- Steam curing
- Applying curing compounds.
Different Concrete Works
1. Plain concrete
4. Precast concrete
5. Special concrete.
To meet the requirements of special situations, the following types of concrete are manufactured:
1. Fiber-reinforced concrete (FRC).
2. Polymer-impregnated concrete (PIC)
3. High-performance concrete
4. Lightweight concrete
5. Self-compacting concrete.
Tests on Concrete
To measure the workability slump test, compaction factor test or Vee-Bee consistometer tests are conducted.
To find the strength, compression tests are conducted on 150 × 150 × 150 mm cubes after 28 days.
Also, Read This
- MIX DESIGN CONCRETE PROCEDURE AS PER IS CODE, GRADES OF CONCRETE, ADVANTAGES, RATIO
- READY MIXED CONCRETE PROCEDURE – ADVANTAGES, TYPES, RMC, PLANT, DISADVANTAGES
- REINFORCED CEMENT CONCRETE – TYPES, USES, ADVANTAGES, RCC, MATERIAL, CHECKLIST, DESIGN
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