Cement – A Miracle Binder
Cement is a very important building material used in the construction industry. It was developed in 1824 when, Joseph Aspdin of England first patented the manufacture of a very improvised type of cementitious material for building construction. The hardened cement paste resembled the natural stone occurring at Portland in England. Hence it was named as Portland cement.
Cement is generally categorised into hydraulic and non- hydraulic binder based on the ability to set in the presence of water.
It has 2 main uses :
1) Production of mortar in masonry
2) production of concrete (cement & aggregate)
Technical knowledge of making hydraulic cement was later formalized by engineers in the 18th century. The use of concrete in construction grew rapidly from 1850 onward, and was soon the dominant use of cements.
Modern hydraulic cements began to be developed from the start of industrial revolution (around 1800), driven by needs such as, hydraulic stucco for finishing brick buildings in wet climates, hydraulic mortars for masonry construction of harbor works, development of strong concretes, etc;
Its differentiated on the basis of,
3 strength classes : ordinary, high, very high or
5 general categories and 27 common types
Other than this there are more varieties such as, sulphate resisting, low heat, calcium aluminate cements.
Some of the reasons for its popularity and universal acceptance are :
- Cement can be produced in large volumes in controlled condition, packed and transported
- Cement is several times stronger binding material than lime and clay
- It can be mixed and used at will with locally available materials at site
- When stored properly in ordinary atmosphere does not deteriorate for reasonably longer time
- When mixed with water, starts setting and acquires sufficient strength in a day or two, where as other binding materials require much longer time
- When water is added to quicklime, lot of heat is generated, but in case of cement, heat generated is unnoticeable and comparatively much lesser
- It can withstand compressive stresses well. Where tension and shear stresses occur it gives good bond to steel reinforcement and transfers excess stresses to steel
- It is produced from the materials like limestone, hematite, bauxite, clay, etc; which are abundantly available in the upper crust of the earth
Manufacturing cement :
- The quarry is the starting point
Cement plants are usually located closely either to hot spots in the market or to areas with sufficient quantities of raw materials (limestone & clay) to keep transportation costs low.
- A two-step process
Basically, cement is produced in 2 steps : first clinker is produced from raw materials in a dry, wet, semi-dry or semi-wet process according to the state and then cement is produced from cement clinker.
- Making clinker
The raw materials are delivered in bulk, crushed and homogenised mixture and fed into a rotary kiln which is an enormous rotating pipe of 60-90 m long & up to 6 m in diameter. This huge kiln is heated by a 2000°C flame inside and is slightly inclined to allow the materials to reach the other end slowly, where it is quickly cooled to 100-200°C.
There are four basic oxides in the correct proportions that make cement clinker : calcium oxide (65%), silicon oxide (20%), alumina oxide (10%) and iron oxide (5%).
These elements, mixed homogeneously (called raw meal/slurry), will combine on heating at a temperature of approximately 1450°C. The new compounds formed are : silicates, aluminates and ferrites of calcium. Hydraulic hardening of cement is due to the hydration of these compounds, the final product of this phase is called “clinker”. These solid grains are then stored in huge silos.
- From clinker to cement
The 2nd phase takes place in a cement grinding mill, located in a different place to the clinker plant. Gypsum & possibly additional cementitious (such as blast furnace slag, coal fly ash, natural pozzolans, etc.) / inert materials (limestone) are added to the clinker. All constituents are ground leading to a fine homogeneous powder, cement, which is then stored in silos before being dispatched in bulk.
Selection of cement :
|Sl no||Type of cement||End Use|
|1||OPC 33||Used for general civil construction works under normal/mild environmental conditions. OPC 33 grade is normally not used where high grade concrete is required due to limitations of its strength. Nowadays this variety is not generally produced.|
|2||OPC 43||Used widely for general construction work but is gradually being replaced by blended cements.|
|3||OPC 53||Used in RCC and prestressed concrete of higher grades, cement grouts, instant plugging mortars etc; where initial higher strength is the criteria.|
|4||Portland Slag Cement (PSC)||Provides better protection against chloride and sulphate attack. Preferred over OPC for usage in constructions where the structures are susceptible to sulphate and chloride attack. eg., Marine structure, structures near the sea, sewage disposal treatment works, water treatment plants, etc;|
|5||Portland Pozzolana Cement (PPC)||Makes concrete more impermeable & denser as compared to OPC. Produces less heat of hydration and offers greater resistance to the attack of aggressive waters than normal OPC. Can be used for all types of construction.|
|6||Sulphate Resisting Portland Cement (SRC)||In SRC, amount of C3A is restricted to lower than 5% and 2C3A + C4AF lower than 25%. Can be used for Foundation, Piles, Basements, Underground structures, sewage and water treatment plants and coastal works, where Sulphate attack due to water or soil is anticipated.|
|7||Low Heat Portland Cement||Particularly suitable for making mass concrete for dams and many other types of water retaining structures, bridge abutments, massive retaining walls, piers and slabs, etc;|
|8||Rapid Hardening Cement (RHC)||Used for repair and rehabilitation works and where speed of construction and early completion is required due to limitations of time, space or other reasons.|
|9||Hydrophobic Portland Cement||Manufactured under special requirement for high rainfall areas to improve the shelf life of cement. The cement particles are given a chemical coating during manufacturing, which provides water-repelling property, and the cement is not affected due to high humidity and can be stored for longer period. Due to abundant availability of cement in all parts of the country, this cement is very rarely produced these days.|
|10||White Cement||Made from raw materials containing very little iron oxide and manganese oxide. Burning in the kiln is done with furnace oil or gas instead of coal. Limited quantities of certain chemicals, which improve whiteness of cement, are added during manufacturing. This type of cement is generally meant for non-structural works. It is used for architectural purposes such as mosaic tiles, wall paintings, GRC and special effects.|
Classification of cement based on their use
Difference between 43 grade OPC & 53 grade OPC
Good axioms of proper inventory management of cement are :
- Buy only the quantity required for the week or for the fortnight
Please see that cement bag is not torn.
- No windows/ventilators should be allowed in godown, if unavoidable, it should be kept closed all the time.
- Do not allow workers to use hooks or sharp tools to lift the bags.
- Do not allow workers to use hooks or sharp tools to lift the bags.
- The cement should be stored in a godown built with minimum 300 mm plinth height.
- Check whether the roof of the shed leaks, if so, repair it.
- Always store cement away from the wall.
- Do not store cement directly on the floor. Use wooden planks/pallets or polythene sheets.
- Place cement bags one layer lengthwise and one layer width-wise (header and stretcher style) for better stability of the pile.
- Always adopt first in first out approach (FIFO system).
Sometimes, when the cement bag is brought down, it may appear hard (pack lumps) because of the load of the bags above it. Roll the bag two to three times before opening it.
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