Ordinary Portland Cement

UNIT – II

Chapter - 4 (D)

CEMENT

 

ORDINARY PORTLAND CEMENT

Literally, Cement means a Binding Material. Cement is available in various types, each having different chemical compositions and used for specific applications. The word Cement usually means the Ordinary Portland Cement used in civil engineering works. [Ordinary cement resembles a type of sandstone found in Portland city in U.K.; hence the name Portland Cement.]

Cement is the product obtained by burning a well proportioned mixture of Calcareous (Limestone), Siliceous (Silica) and Argillaceous (Clay) materials and crushing the same into a fine powder. The cement-water-paste has its characteristic properties of adhesion and cohesion by which it can bond well with sand (fine aggregate) to form mortar or a strong rock like mass called concrete due to the chemical reaction between cement and water.

 

1. CONSTITUENTS OF PORTLAND CEMENT AND THEIR FUNCTIONS

1. Lime (CaO) – 62%

Lime is the major ingredient responsible for the strength characteristics of the cement. If lime is in right proportion, it makes the cement sound and strong. If in excess, it makes the cement unsound causing the cement to expand and disintegrate. If in deficiency, the strength of cement is decreased and the cement sets quickly.

2. Silica (SiO₂) – 22%

Silica imparts strength to cement due to formation of dicalcium and tricalcium silicates. If in excess, it prolongs the setting time of cement.

3. Alumina (Al2O₃) - 5%

Alumina imparts quick setting quality to cement. It acts as flux and lowers the clinkering temperature. If in excess, Alumina reduces the strength of cement.

4. Calcium Sulphate (CaSO₄) - 4%

It is present in the form of Gypsum. It helps in increasing the initial setting time of cement.

5. Iron Oxide (Fe2O₃) – 3%

Iron Oxide provides colour, hardness and strength to cement. It also helps the fusion of raw materials during the manufacture of cement. If in excess, Magnesium Oxide seriously

6. Magnesium Oxide (MgO) – 2%

It imparts hardness and colour to the cement. affects the soundness of cement.

7. Sulphur Trioxide (SO3) - 2%

It makes the cement sound.

 

2.  MANUFACTURE OF PORTLAND CEMENT

.The manufacture of Ordinary Portland Cement involves the following three stages:

1. Mixing of Raw Materials 2. Burning and 3. Grinding

 

1. Mixing of Raw Materials

Raw materials for cement are Lime Stone (Calcareous Material) and Clay (Argillaceous Material). These raw materials are mixed in dry condition or wet condition. Accordingly, the process of mixing is called Dry Process or Wet Process.

Dry Process

It is an outdated process. In this process, lime stone and clay are broken separately into small pieces by crushers. An air draught is passed over the crushed materials for drying. These are ground to a fine powder in grinding mills. Each ingredient is stored separately in separate hoppers. Then, the powdered materials are mixed in the required proportion. The raw mix is stored in the Storage Tank.

Wet Process

In this process, lime stone is crushed and stored in a tank. Wet clay is stored in a basin. The crushed lime stone from the tank and the wet clay from the basin are allowed to fall in a mixing tank in proper proportion and get mixed. The mixture is ground in a Ball Mill to form wet Slurry.

The wet slurry is conveyed into the Storage Tank. In this, the chemical composition of the slurry is adjusted. Then, the wet slurry is passed on to the Rotary Kiln for burning.

 

2. Burning in ROTARY KILN

Description of Rotary Kiln See Fig. 7.

Rotary Kiln consists of a Long Steel Cylinder supported on Rollers provided over Masonry Pillars.

Inner surface of the cylinder is lined with refractory fire bricks.

Cylinder is placed at a slope of 1 in 30 (i.e., 2.5° to horizontal).

The cylinder rotates slowly about its axis at the rate of 1 rpm.

Working of Rotary Kiln

Dry Zone

The slurry from the Storage Tank is fed at the upper end of the kiln through a Hopper. The kiln cend. Heating is done by injecting pulverized coal using hot air blast by the Blower. The slurry meets the hot gases in the Dry Zone and gets dried prior to burning.

Burning Zone

The dried slurry descends towards the Burning Zone. In this zone, Carbon Dioxide from the slurry is evaporated. On burning, slurry is converted into small and very hard lumps called Nodules. The nodules gradually roll down. Ultimately, noodles reach the burning zone at a maximum temperature of 2800°F. At this temperature, lime and clay in the slurry get chemically fused to form hard balls known as Clinkers.

The hot clinkers are collected in a Cooler. In the cooler, the temperature of clinkers is brought down by passing a current of cold air. Clinkers from the cooler are collected in Containers.

 

3. Grinding in GRINDING MILL

The grinding mills have horizontal rotating steel cylinders. The clinkers from the cooler are ground in the grinding mill with 3% of Gypsum. Gypsum controls the initial setting time of cement. If gypsum is not added, cement would set as soon as water is added.

The finely ground cement powder is screened through standard mesh. It is conveyed to large silos (storage tanks), where it is stored, packed and kept ready for marketing.

 

3. PROPERTIES OF CEMENT

1. Physical Properties

1. Good Binding Property: Cement has good binding property as mortar and concrete.

2. High Strength to Masonry: Cement provides high strength to masonry. Compression strength of 1:3 mix after 7 days curing should not be less than 22 N/mm² and after 28 days curing, it should not be less than 33 N/mm². Tensile strength after 3 days should be 2.0 N/mm² and after 7 days, it should be 2.5 N/mm².

3. Setting Time: When water is added, ingredients of cement react chemically with water. Chemical compounds that are formed, impart strength to cement. This is called Setting Action or Hardening of Cement. Initial setting time should not be less than 30 minutes and final setting not more than 10 hours.

4. Good Plasticity: Cement has good plasticity.

5. Resistance to Atmospheric Effects: Cement has high resistance to atmospheric effects.

6. Easily Workable: Cement has to be easily workable.

7. Fineness: Cement should be fine powder free from lumps.

8. Colour: Colour of cement should be uniform.

 

2.  Chemical Properties

1. Ratio of Percentage of Alumina to Iron Oxide: It should not be less than 0.65%.

2. Sulphur Content: Total sulphur content should not be greater than 2.75%.

3. Insoluble Residue: Weight of insoluble residue in cement should not exceed 1.5%.

4. Weight of Magnesia: It should not exceed 5% in cement.

 

3. Setting Action of Cement or Hardening of Cement

When water is added to cement, the ingredients of cement react chemically with water. Various chemical compounds are formed which give strength to the cement. This phenomenon is called Setting Action of Cement. Cement attains about 70% of its final strength in 28 days and about 90% of its final strength in one year or so.

 

4. USES OF CEMENT

1. Brick and Stone Masonries

Cement mortar is a mixture of cement and sand. It is used for brick and stone masonry works for walls and their plastering.

2. Floor and Floor Finishes

Cement mortar is used for floors and floor finishes.

3. Pointing

Cement mortar is used for Pointing of brick or stone masonry. [Pointing means finishing the mortar joints of the walls or similar structure with cement mortar to protect the joints from atmospheric agencies and to improve the appearance.]

4. Joints of Pipes:

Cement mortar is used in the joints of pipes, drains, etc.

5. Decorative Works

All decorative and aesthetic works in a building involve the use of cement mortar.

6. Cement Concrete

It is used for laying floors, roofs, etc. R.C.C. is used for constructing lintels, beams, pillars and columns, pile and mat foundations, stairs, sumps, septic tanks, etc.

7. Heavy Engineering Structures

Cement concrete is used in engineering structures such as Roads, Bridges, Dams, water tanks, culverts, tunnels, storage reservoirs, docks, lighthouses, railway sleepers, etc.

8. Cement Products

Cement is used in the manufacture of cement products like pipes, posts, pre-cast piles, asbestos cement sheets, hollow blocks, concrete blocks, etc.

 

5. TESTING OF PORTLAND CEMENT

1. Field Tests to ascertain the Quality of Cement roughly

It is not possible all the time and in all cases to test the quality of cement in a laboratory. The following are a few rough and ready methods of testing cement in the field:

• Cement should be cool to the feel when the hand is introduced into a bag of cement.

• When a small sample of cement is rubbed between fingers, it should feel smooth. If it is felt rough, it indicates adulteration of cement with ash.

• A handful of cement thrown into a bucket of water should gradually sink. Floating stuff indicates adulteration chiefly with ashes. Lumps: There should be no lumps in cement. Lumps indicate cement has already set.,

• Colour: A good cement should be of uniform colour, that is, greenish grey. Reddish or blackish colour of cement will indicate adulteration of foreign materials.

• A thick paste of cement with water on a glass plate, kept under water for 24 hours should not crack, but set.

• Initial setting time may be observed in the field by preparing a thin cement paste in a ware and inserting a very thin stick or wire into it after 30 minutes. If the paste resists penetration of the stick or the wire, the quality of cement may be ensured.

2. Fineness Test: Sieve Test should be conducted for finding the fineness of cement. By BIS 90 micron sieve, residue by weight should not exceed 10%.

3. Specific Gravity Test: This test is valuable for detecting adulteration.

4. Setting Time Test: This test is conducted to check the initial and final setting times of cement. Initial setting time is determined as to give sufficient time for various operations such as mixing, transportation, placing and compaction of cement mortar or concrete. Final setting time is determined to find that after laying the mortar or concrete, the hardening should be rapid so that the structure may be used as early as possible. Time required for setting of cement is tested by Vicat’s Needle Apparatus.

5. Soundness Test: This test is performed to detect the presence of uncombined lime and magnesia in cement. This test is conducted using Le Chatelier apparatus.

6. Compressive Strength Test: Cubes of cement and sand (1 : 3) are cured for 7 days under : water and tested in Compression Testing Machine. This test finds the suitability of cement for developing required compressive strength of cement mortar and concrete.

7. Tensile Strength Test: This test is carried out to find the suitability of cement for developing required tensile strength of cement mortar and concrete.

 

6. TYPES OF CEMENT AND THEIR USES

There are various kinds of cement in use and they are as follows:

1. Normal Setting Portland Cement

Uses: It is used for masonry work, plastering, etc., in the form of cement mortar. Cement concrete is used for constructing beams, columns, lintels, stairs, pre-cast piles, etc.

2. Quick Setting Cement

It contains less percentage of gypsum. It is finer than Portland cement. Setting action is accelerated by adding a small percentage of Aluminium Sulphate during grinding.

Uses: This cement is used for concreting under-water or in running water in bridges.

3. White Cement

The trade name for white cement is Snowcrete. This cement does not contain colouring ingredients such as Iron Oxide and Manganese Oxide.

Uses: It is used as a base for coloured cement. Due to its white colour, it is used for interior and exterior decorative works like flooring, plastering, pointing of masonry, manufacture of pre-cast stones, runway markings, architectural and ornamental works.

4. Coloured Cement

The required colour for the cement is obtained by initially mixing 5 – 10% of colouring materials such as Iron Oxide for brown colour, Chromium Oxide for green, Cobalt for blue, etc.

Uses: It is used for external finishing of walls and floors, for garden paths, swimming pools, tennis courts, aesthetic purposes, etc.

5. Acid Resisting Cement

In this type, acid resisting ingredients like Quartz, Sodium Silicate, etc., are added to the cement to attain the acid-resistant quality.

Uses: It is used for acid-resistant coatings of installations of chemical industries.

6. Rapid Hardening Cement or High Strength Cement

This is produced by burning the raw material at high temperature, by increasing the lime content and by very fine grinding. As the name suggests, it develops strength rapidly.

Uses: It is used in the works wherein speed of construction is needed such as repair works, early removal of frame work for re-use, etc.

7. Expanding Cement

It is produced by adding an expanding medium like Sulpho-Aluminate to ordinary cement. This cement expands in concrete during curing, whereas other cements shrink. Due to expansion, cracks due to shrinkage of concrete are avoided.

Uses: This cement is used for filling cracks and to overcome crack formation in R.C.C. structures. It is also used for repairing the damaged concrete surfaces.

8. Portland Pozzolana Cement

Pozzolana is a volcanic ash powder. It is siliceous material. Pozzolana cement is produced by grinding Portland cement clinker and Pozzolana with gypsum. It offers great resistance to the attack of aggressive water.

Uses: It is widely used for hydraulic structures (mass concrete works) such as dams, etc.

9. Low Heat Cement

Heat generated during setting of this cement is very less.

Uses: Heat generated is very high during setting of cement in mass concreting of dams. It takes very long time for heat dissipation. Hence, low heat cement is used for dams.

10. High Alumina Cement

It is a rapid hardening cement of chocolate colour. It generates high heat while reacting with water and causes high and early strength development.

Uses: It is used for furnace insulation and in colder regions.