Physical Properties of cement

 

Physical Properties of cement :

Following are the main properties of cement which are of importance because it is by these properties that the suitability of a particular type of cement is judged :

1.  Fineness

2. Setting time

3. Compressive strength

4. Soundness of cement

5. Heat of hydration

1.Fineness : The degree of grinding of cement is called fineness and hence, finer the  cement, greater is the  rate of reaction and thus early strength development.

                However excessive fineness is also undesirable because the  cost of grinding the cement to a higher  degree of fineness is high.

Fineness of cement is measured of :

I.        Percentage by weight retained after sieving it through 90 microns IS sieve or

II.      Surface area of cement particles in cm² /g.

2. Setting Time : When portland cement is mixed with water , it results in soft cement paste. Gradually it becomes less plastic and finally becomes a hard mass. When the cement paste loses its plasticity, it is sufficiently rigid to withstand a defined amount of pressure. This action of changing from fluid state to solid state is called setting and the time required for such a change is called setting time. The setting time divided into two parts, initial setting time or final setting time.

                The time in which the cement paste loses to plasticity after the addition of water is  known  as initial setting time of cement. The time corresponding to the Cement paste becoming a hard mass is known  as final setting time. 

For ordinary portland cement, the initial setting time should not be less than 30 minutes and final setting time is  not more than 600 minutes (10hrs) initial and final setting times can be founded by using vicats apparatus.

3. Compressive strength : Compressive strength is one of the most important property. Cement mortar cubes (1:3) having area of 5000m² are prepared and tested in compression testing machine. For ordinary portland cement the compression strength at 3 and 7 days curing shall not be less than 26 MPa (160kg/m³ ) and  22MPa (220kg/m³) reespectively. It is used for preparing the test block. 

4. Soundness of Cement :

     It is  very important that the cement after setting shall not any appreciable change of volume of cement. Certain cement have been found to undergo a large expansion after setting causing loss of durability. The phenomenon by virtue of which cement does not undergo large change in volume when treated with water is known as soundness. Unsoundness is due to the presence of  free lime and  magnesia in the cement. It can  be reduced by :

a)      Limiting the magnesium oxide content to less than 0.5%

b)      Thorough mixing

c)       Fine Grinding

The main tests for soundness are the Lee Chateleer and Autoclave test

5. Heat of Hydration : As mentioned earlier , the hydration of cement is an exothermic reaction i.e  heat is given out when cement react with water. The heat generated is called heat of hydration.