Chapter 4
Cement
Natural cement is brown in colour. It sets very quickly after the addition of water and is not as strong as artificial cement, and hence it has limited use.
It was in the eighteenth century that the most important advances in the development of cement were made, which finally led to the invention of Portland cement. In 1756, John Smeaton showed that the hydraulic lime which can resist the action of water can be obtained not only from hard lime but also from a limestone which contains a substantial proportion of clay.
In 1796, Joseph Parker found that the modules of argillaceous limestone made excellent hydraulic cement when burnt in the usual manner. Later, several experiments with several mixtures of limestone and argillaceous were carried out so that the properties of the product could be kept under more uniform and proper control by using varying lime and clay proportions. In 1824, Joseph Aspidin of Leeds in England introduced Portland cement.
4.1 PROPERTIES OF CEMENT
The properties of cement are:
- It gives strength to the masonry.
- It acts as an excellent binding material.
- It offers good resistance to moisture.
- It possesses good plasticity.
- It stiffens or hardens early.
- It is easily workable.
4.2 INGREDIENTS OF CEMENT
- Lime (CaO): The chief constituent of cement is lime. Its proportion varies from 60 to 67 per cent. The lime in excess makes the cement unsound and causes the cement to expand and disintegrate and also retards the setting property. On the other hand, if lime is in deficiency, it reduces the strength of cement.
- Silica (SiO2): It forms 17 to 25 per cent of Ordinary Portland Cement. It imparts strength to the cement due to the formation of dicalcium and tricalcium silicates. Excess of silica increases the strength of cement, but at the same time the setting time is prolonged.
- Alumina (Al2O3): It acts as a flux and lowers the clinkering temperature. It imparts quick setting property to cement. If in excess, it weakens the strength of cement.
- Calcium sulphate (CaSO4): This ingredient is in the form of gypsum. It is generally added in very small amounts (2 per cent of wt.) to cement towards the last stage of manufacture with a view of retarding the setting time of cement.
- Iron oxide (Fe2O3): This is responsible for imparting the characteristic grey colour to cement. Its percentage varies from 0.5 to 6 per cent.
- Magnesia (MgO): Magnesia varies from 0.1 to 45 per cent. Excess of magnesia reduces the soundness of cement. It imparts hardness and colour to the cement.
- Sulphur: It varies from 1 to 2.5 per cent. If it is in excess, it makes the cement unsound.
- Alkalies: Most of the alkalies present in raw materials are carried away by the flue gases during heating. If they are in excess in cement, they result in alkali-aggregate reaction, efflorescence and staining when used in masonry work.
4.2.1 Harmful constituents of cement
The presence of alkali oxides like K2O and Na2O and magnesium oxides like MgO adversely affects the quality of cement. If the amount of alkali oxides exceeds 1 per cent, it leads to the failure of concrete. If the content of magnesium oxide exceeds 5 per cent, it causes cracks after mortar and the concrete hardens. Table 4.1shows the admissible average (in %) and limits (in %) of ingredients in ordinary cement.
Table 4.1 Admissible Average (in %) and Limits (in %) of Ingredients in Ordinary Cement
| Ingredient | Limits (%) | Average (%) |
|---|---|---|
| 1. Lime | 60–66 | 63.5 |
| 2. Silica | 18–25 | 22.5 |
| 3. Alumina | 3–8 | 6 |
| 4. Iron oxide | 0.5–5 | 2.5 |
| 5. Magnesia | 1–5 | 1 |
| 6. Sodium and potassium oxides | 0.5–5 | 1 |
| 7. Sulphuric anhydride | 0.5–5 | 1 |
4.3 SETTING TIME OF CEMENT
When water is added to cement, the ingredients of cement react chemically with water and form a complicated chemical compound. The mixing of cement with water results in a sticky cement paste and it gradually goes on thickening in course of time. It is found that ordinary cement achieves 70 per cent of its final strength in 20 days and 90 per cent in 1 year or so.
The time of setting is greatly influenced by the following factors:
- The temperature at which the cement paste is allowed to set.
- The percentage of water mixed to cement in making the paste.
- The humidity at which the setting is allowed.
Setting time is distinguished into initial setting time and final setting time on the basis of the time taken by the test specimen to set to a specified minimum depth.
A Vicat needle apparatus is used for the determination of setting time (Figure 4.1).
Figure 4.1 Vicat apparatus
Apparatus
- It consists of a frame with a movable rod fitted with a cap.
- A needle of 1 mm square cross section is attached to the lower end of the rod for the determination of initial setting time. The total weight of the rod along with the needle is 300 g.
- Another needle like the above mentioned but with a hollow metallic attachment with a circular cutting edge of 5 mm diameter and having a 0.5 mm projection at the end is used to determine the final setting time.
- A standard Vicat mould in which the specimen is allowed to set.
4.3.1 Initial setting time – procedure
- Take 300 g by weight of cement and mix with 0.85 times the water required to give a paste of standard consistency.
- Start the stop watch at the instant water is added to the cement.
- Fill the Vicat mould with the cement paste and smooth the surface.
- Place the square needle of cross section 1 mm to the moving rod of the Vicat apparatus.
- Lower the needle gently bringing it in contact with the surface and quickly release allowing it to penetrate the paste.
- In the beginning the needle will completely pierce the test block. Repeat the procedure in a fresh place until the needle, when brought in contact with the test block and released, fails to pierce the block for 5 mm measured from the bottom.
The initial setting time is the interval between the addition of water to the cement and the stage when the needle fails to pierce the test block for 5 mm measured from the bottom.
4.3.2 Final setting time – procedure
- Replace the needle for initial setting time by the needle with an annular attachment for the final setting time.
- The cement shall be considered as finally set, when upon applying the needle gently to the surface of the test block, the needle makes an impression thereon while the attachment fails to do so.
The final setting time is the interval between the addition of water to the cement and the time at which the needle makes an impression while the attachment fails to make an impression on the surface of the test block. The following table shows the initial and final setting time of various grades of cements.
Table 4.2 The Initial and Final Setting Time of Various Grades of Cements
| Type of cement | Initial setting time | Final setting time |
|---|---|---|
| 1. Ordinary | It shall not be less than 30 minutes. | It shall not be more than 10 hours. |
| 2. Rapid hardening | It shall not be less than 30 minutes. | It shall not be more than 10 hours. |
| 3. Low heat | It shall not be less than 60 minutes. | It shall not be more than 10 hours. |
4.4 MANUFACTURE OF CEMENT
4.4.1 Wet process
In the earlier part of the century, from 1913 to 1960, the wet process was used for the manufacture of cement.
4.4.1.1 Mixing of raw materials
The calcareous materials such as limestones are crushed and stored in silos or storage tanks. The argillaceous materials, such as clay, are thoroughly mixed with water in a container known as wash mill and they are stored in basins. Now in correct proportions, the limestones from storage tanks and wet clay from basins are allowed to fall in a channel. This channel leads the material to grinding mills where they are brought to form a slurry. The grinding is carried out in either ball mill or tube mill or both. The slurry is lead to correcting basins where it is constantly stirred and at this stage the chemical compositions are adjusted as necessary. This corrected slurry is then stored in a different storage tank from where it is fed to the rotary kiln for burning.
4.4.1.2 Burning
The burning is carried out in the rotary kiln. The rotary kiln is formed of steel tubes whose diameter varies from 250 to 300 cm. The length varies from 90 to 120 m. It is laid at a gradient of 1 in 25 to 1 in 30. The kiln is supported at intervals by columns of masonry. A refractory lining is provided inside the kiln. It is arranged in such a way that the kiln rotates at 1–3 revolutions per minute about its longitudinal axis. The corrected slurry is charged into the rotary kiln for the wet process. Coal in finely pulverized form, fuel oil and gas are the common fuels for burning these kilns. The portion of the kiln near its upper end is known as dry zone and in this zone the water of the slurry is evaporated. As the slurry descends to the next zone, there is a rise in temperature from where the carbon dioxide from the slurry is evaporated. Small lumps known as nodules are formed at this stage. These nodules gradually pass through zones of rising temperature and ultimately reach the burning zone where temperature is around 1,500°C. In the burning zone, the calcined product is formed and nodules are converted into small, hard, dark, greenish blue balls which are known as clinkers. The size of the clinkers varies from 3 to 20 mm. Rotary kilns of small size are provided to cool down the clinkers and the cooled clinkers having temperature around 95°C are collected in containers of suitable sizes.
4.4.1.3 Grinding
The clinkers obtained from the rotary kiln are finely ground in ball mills and tube mills. During grinding, a small quantity, around 3-4 per cent, of gypsum is added. Gypsum controls the initial setting time of cement. If gypsum is not added, the cement would set as soon as water is added. After grinding, the product is stored in storage tanks and finally they are packed in bags of different types to ensure a 50 kg net weight of cement bag with ±200 g. Each bag contains 50 kg or about 0.035 m3 of cement. The bags are automatically discharged from the packer to the conveyor belt to different loading areas and are carefully stored in the right place (Figure 4.2).
4.4.2 Dry process
Nowadays the dry process of manufacture of cement is most often adopted and this improves the quality of cement produced, with less consumption of power. In this process, the raw materials which are ground to about 25 mm size in crushers are dried by passing dry air over it. They are then pulverized to a very fine powder in ball mills and tube mills. This is done separately for each raw material and then they are mixed in the correct proportion and made ready for the feed of the rotary kiln.
Figure 4.2 Schematic diagram of different processes involved in the manufacturing of cement
4.5 DIFFERENT TYPES OF CEMENT AND USES
- Ordinary Portland Cement: It derives its name from the name of a stone (Portland) which resembles its colour. It is the most commonly used building material in mortar for masonry work, in mortar for plastering and pointing and as a binding medium in cement concrete, reinforced cement concrete and prestressed cement concrete construction.
- Rapid hardening cement: The rapid hardening property is imparted to the cement primarily by burning at a higher temperature and secondly by finer grinding of the particles. The initial and final setting time of the cement is the same as ordinary cement, but it attains high strength in the early stages. It is useful in emergency situations as it develops the same strength in 4 days which ordinary cement acquires in 28 days. It is comparatively costlier than Ordinary Portland Cement. The uses and advantages of this cement are:
- It can be used when the construction has to be carried out fast.
- When the formwork of the concrete has to be removed earlier.
- It is light in weight.
- It is not damaged easily.
- The structural members constructed out of this cement can be loaded earlier.
- This cement requires short period of curing.
- It allows higher permissible stresses in the design.
- Low heat cement: It is a type of Portland cement which sets and hardens with the evolution of very low heat of hydration. It contains low percentage of tricalcium aluminate, of about 5 per cent, and higher percentage of dicalcium silicate, of about 45 per cent. This is the ideal cement for construction of dams as it reduces the development of cracks in the structure.Heat of hydration is the heat produced during the chemical action between cement and water. In mass concreting like construction of dams, this heat produced will be high and will affect the stability of the structure. So, there is a necessity to control the amount of heat produced and it is in these situations that the use of this type of cement comes into play.
- Quick setting cement: It is produced by adding a small percentage of aluminium sulphate and by finely grinding the cement. It contains very little or no retarding substances like gypsum. The setting action of the cement starts within 5 minutes after addition of water and it becomes hard in less than 30 minutes. The mixing and placing of concrete should be done in a very short time. This type of cement can be used for construction under water.
- High alumina cement: It is obtained by adding bauxite (Al2O3) of about 55 per cent and lime (CaO) of about 35–45 per cent. The advantages are:
- It is highly resistant to attack by sea water.
- It rapidly hardens.
- It does not expand while setting.
- It can stand very high temperatures.
- It resists the action of frost.
The disadvantages are:- It cannot be used for massive concrete work.
- It is much costlier.
- Extreme care should be taken to see that it does not come in contact with ordinary cement or lime as it reduces the strength.
- Coloured cement: This cement will produce a surface of desired colour and is manufactured by the addition of a small proportion of some colouring material, generally a mineral pigment to the clinker. The amount of colouring material may vary from 5 to 10 per cent. Chromium oxide gives green colour and cobalt imparts blue colour. Iron oxides in different proportions give brown, red and yellow colour and manganese dioxide produces black and brown colour.
- Expanding cement: This cement is used to neutralize the effect of shrinkage of ordinary concrete. It is produced by adding an expanding medium like sulpho-aluminate and a stabilizing agent to the ordinary cement. It is used for the construction of water-retaining structures and also for repairing damaged concrete surfaces.
- Hydrophobic cement: It contains admixtures which decrease the wetting ability of cement. The admixtures usually used are acidol, naphtenesoap, etc. These substances form a thin film around the cement grains. When water is added to this cement, the absorption films are torn off the surface and they do not in any way prevent the normal hardening of cement. However, in the initial stage the gain of strength is less as the hydrophobic films of cement grains prevent the interaction with water.
- Air entraining cement: Air content of 2–6 per cent is introduced in the cement by grinding air entraining agents with the cement clinker during the manufacture of cement. The addition of air entraining agents introduces large amount of air which results in the formation of voids and increases the workability of concrete. The weight as well as the strength of the concrete is reduced.
- White cement: White cement is manufactured from china clay and white chalk in place of limestone and clay. It is used as a decorative feature for high-quality plasterwork. The white colouring effect is due to the absence of iron oxide. The cement is about four times costlier than Ordinary Portland Cement. It has quick drying properties, high strength and superior aesthetic values. It is used in swimming pools where it replaces the use of glazed tiles with coloured shades, for moulding sculptures and statues, for painting garden furniture and for fixing marbles and glazed tiles.
- Blast furnace slag cement: The iron and steel industry produces large quantities of blast furnace slag as a by-product. The slag is a waste product in the manufacturing of pig iron and it contains the basic elements of cement, namely alumina, lime and silica. The clinkers of cement are ground with 60–65 per cent of the slag. This cement has a slow rate of hardening and less heat of hydration. It is not affected by sea water and, hence, is used for marine structures. Its strength in the early days is less and, hence, requires longer curing period.
4.6 DIFFERENT GRADES OF CEMENT
Prior to 1987, there was only one grade of Ordinary Portland Cement which was governed by IS 269-1976. After 1987 higher-grade cements were introduced to India. The Ordinary Portland Cement was classified into three grades, namely 33 grade, 43 grade and 53 grade depending upon the strength of the cement at 28 days when tested as per IS 4031-1988. If the 28-day strength is not less than 33 N/mm2, it is called 33 grade cement. If the 28-day strength is not less than 43 N/mm2, it is called 43 grade cement. If the 28-day strength is not less than 53 N/mm2, it is called 53 grade cement. But the actual strength obtained by the cement at the factory is much higher than the BIS specification. Table 4.3 shows compressive strength of different grades of cement.
Table 4.3 Compressive Strength of Different Grades of Cement
N.S. – Not specified
The compressive strength of Ordinary Portland Cement increases with time. For example, 33 grade OPC (IS 269-1989) acquires a compressive strength of 16 N/mm2 at 3 days, 22 N/mm2 at 7 days and 33 N/mm2 at 28 days.
4.7 STORAGE OF CEMENT
Cement absorbs moisture from nature and gets hardened. So suitable precautions should be taken in storing cement.
An absorption of 5 per cent moisture means the cement becoming useless and so the cement is to be stored in a moisture-free atmosphere. It is advisable not to store cement in jute bags for a period of more than 3 months. The cement bags should be stored in piles of one above the other, at a minimum distance of 300 mm from the exterior walls. Between the piles, a passage of 900 mm width should be kept. The top and bottom of the piles should be covered and waterproofed for long storage.
Storage for longer periods makes the cement weaker, even under favourable conditions.
REVIEW QUESTIONS
- What are the properties of cement?
- Briefly discuss the ingredients of cement.
- What is the setting time of cement and what are the factors affecting it?
- How is the setting time of cement determined? Explain briefly.
- What is the difference between initial and final setting time of cement?
- How is cement manufactured in the wet process?
- Draw a schematic diagram showing the different processes involved in the manufacture of cement.
- What are the different types of cement?
- Write short notes on
- Low heat cement
- Quick setting cement
- Rapid hardening cement
- What are the different grades of cement and how is it stored?
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