Details of Cement

1. What is cement?

      Cement is a fine powder, which when mixed with water undergoes chemical change and thereafter allowed to set and harden is capable of uniting fragments or masses of solid matter together to produce a mechanically strong material. Cement can be used as binding material with water, for bonding solid particles of different sizes like bricks, stones or aggregate to form a monolith. Cements used in construction of buildings and civil engineering works contain compounds of lime, silica and alumina as their principal constituents and can be called as complex compounds.


2. Importance of cement in construction

      Today, cement is a very important building material used in the construction industry. It was in 1824 that cement, as we know it today was developed. Joseph Aspdin of England first patented the manufacture of a very improvised type of cementitious material for building construction in 1824. The hardened cement paste resembled the natural stone occurring at Portland in England. Hence it was named as Portland cement.
      Cement has replaced all other binding materials, for example, clay and lime, which ruled high for hundreds of years, in construction from last century. The reason being that, cement is the strongest binding material amongst clay, lime and cement.
      Cement is now used in many construction trade items like Plain Concrete, Reinforced Concrete, Mortar, Plaster, Grouts, Paints and in Pre-cast Elements. In an ordinary building construction the cement accounts to about 15-17% of the cost of construction. In some other major constructions like concrete roads and bridges, it is very much more.

3. Some of the reasons for its popularity and universal acceptance are listed below:

  1. Cement can be produced in large volumes in controlled condition, packed and transported
  2. Cement is several times stronger binding material than lime and clay
  3. It can be mixed and used at will with locally available materials at site
  4. When stored properly in ordinary atmosphere does not deteriorate for reasonably longer time
  5. When mixed with water, starts setting and acquires sufficient strength in a day or two, where as other binding materials require much longer time
  6. When water is added to quick lime, lot of heat is generated, but in case of cement, heat generated is unnoticeable and comparatively much lesser
  7. 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
  8. It is produced from the materials like limestone, hematite, bauxite, clay, etc which are abundantly available in the upper crust of the earth


4. How Cement Is Made





1. Mining the raw material

Limestone and clay are blasted from rock quarries by boring the rock and setting off explosives with a negligible impact of the environment, due to the modern technology employed.

2. Transporting the raw material

Once the huge rocks have been fragmented, they are transported to the plant in dump trucks or by conveyor belt.

3. Crushing

The quarry stone is delivered through chutes to the crushers, where it is reduced by crushing or pounding to chunks approximately 1 ½ inches in size.

4. Prehomogenization

Prehomogenization is the proportional mix of the different types of clay, limestones, or any other required material.

5. Raw material storage

Each of the raw materials is transported separately to silos, where it later will be added in specific amounts according to the particular type of cement being produced.

6. Raw material mill

This takes places in vertical steel mill, which grinds the material through the pressure exerted by three conical rollers. Which roll over a turning milling table. Horizontal mills, inside which the material is pulverized by means of steel balls, are also used in this phase.

7. Raw meal homogenization

This process takes place in silos equipped for obtaining a homogenous mix of the material.

8. Calcination

Calcination is the core portion of the process, in which huge rotary kilns come into play. Inside, at 1400 degrees C, the raw material is transformed into clinker: small, dark gray nodules 3-4 centimeters in diameter.

9. Cement milling

The clinker is ground by different-size steel balls while it works its way through the mill’s two chambers, with gypsum being added to extend cement setting times.

10. Cement packaging and shipping

The cement is then housed in storage silos, from where it is hydraulically or mechanically extracted and transported to facilities where it will be packaged in sacks or supplied in bulk. In either case, it can be shipped by rail car, freighter truck or ship.


5. Types of cement
There are many types of cement in the market to suit every need. Some of them which are included in the revised IS : 456-2000 are as follows:

  1. Ordinary Portland Cement 43, 53 grade (OPC), 53-S (Sleeper Cement)
  2. Portland Pozzolana Cement (PPC), both Fly Ash and Calcined Clay based
  3. Rapid Hardening Portland Cement
  4. Portland Slag Cement (PSC)
  5. Sulphate Resisting Portland Cement (SRC)
  6. Low Heat Portland Cement
  7. Hydrophobic Cement
      Even though only Ordinary Portland Cement is graded according to strength, the other cements too have to gain a particular strength. 43 and 53 grade in OPC indicates the compressive strength of cement after 28 days when tested as per IS: 4031-1988, eg 43 grade and 53 grade the 28 days compressive strength should not be less than 43 and 53 MPa respectively. 43 and 53 grade are also being introduced in PPC and PSC shortly by the Bureau of Indian Standards (BIS)

Portland Pozzolana Cement (PPC)
      PPC is manufactured by inter grinding OPC clinker with 15-35% of pozzolanic material. Pozzolanas are essentially siliceous or aluminous material, which in itself possesses no cementitious properties, which will be in finely divided form and in the presence of moisture react with calcium hydroxide, liberated in the hydration process, at ordinary temperature, to form compounds possessing cementitious properties. The pozzolanic materials generally used are fly ash or calcined clay. PPC produces less heat of hydration and offers greater resistance to attack of aggressive environment, gives long-term strength and enhances the durability of structures.

Portland Slag Cement (PSC)
      PSC is obtained by mixing blast furnace slag, cement clinker and gypsum and grinding them together to get intimately mixed cement. The quantity of slag varies from 30-70%. The gain of strength of PSC is somewhat slower than OPC. Both PPC and PSC will give more strength than that of OPC at the end of 12 months. PPC and PSC can be used in all situations where OPC is used, but are preferred in mass construction where lower heat of hydration is advantageous or in marine situations and structures near seacoast or in general for any structure where extra durability is desired.
      The other types of cement listed above are for special purpose and their nomenclature indicates the purpose for which they are preferred. The following table gives the codal requirements at a glance.