Setting and Hardening of cement

During burning and fusion , all the above listed ingredients undergo chemical combination and form , what are knows as Bogue compounds , in the finely ground cement . Bogue compounds and their accepted abbreviated forms are given below :

Name of Bogue compound

1. Tri-calcium silicate ( 3 Cao , SiO )
Di- calcium silicate
3. Tri - calcium aluminate
4. Tetra -calcium alumino ferrite


  The proportion of above stated Bogue compunds vary , in various types of cement . Tri-calcium silicate ( CaS ) and di-calcium silicate are the compounds mostly responsible for the strength characteristic of the cement .

  Tri-calcium hydrates first and is responsible for most of early strength of the concrete . Strength acquired during frist 7 days is mostly due to hydration of tri-calcium silicate . Di-calcium silicate reacts with water at a very slow rate and hence strength of the concete after first 7 days , is mainly due to hydration of theis compounds . Di- calcium silicate starts contributing strength after 7 days of its placement and continues for about one year , though rate of getting strength from this compounds goes on decreases with passage of time . Tetra-calcium alumino -ferrite is more or less in - active compounds and does not play any significant role in setting properties or hardening properties of tyhe cement . After addition of water in cement , tri-calcium aluminate is the first compounds which starts hydrating and is responsible for early setting of the cement . Hydration of tri-calcium aluminate does not contribute any strength to the concretye . It is mainly responsible for setting action of the cement .

  Although the processes of setting and hardening of cement take place simultaneously , yet there is difference between the two . Setting of the cement is changing plastic state of cement paste , to stiff solid state which is not strong enough and has a very small compressive strength . Hardening of the cement concrete is the rtate o gaining stength .With the passage of time as the hydration proceeds , the compressive strnegth of the product goes on inc


  Tri-calcium aluminate . As soon as wtert is added to the cement , hydrolysis of this compounds immdediately starts . This process may be delayed for about 30 to 40 minute by adding 2 to 3 percent poweder gypsum . This delay is essential to gain time for mixing water with the cement , transportation of mix to the place of use and finishing processes . It is the compounds responsible for early setting of the cement . It does not contribute any strength to the concrete . Setting effect is maximuym during first 24 hours and it does notextend beyond 28 hours . A cement havinh low percentage of crack development . It is generates low heat , resissts better sulphate action and possesss high ultimate strength .

   Tri-calcium silicate . This compounds is next to set a harden . First  7 ir 8 days stnregth , is imparted by the hydration of thjis compounds only . During the reaction again considerable heat of hydration is generated .

  Di-calcium silicate . Hydrolysis of this compounds is cimparatively slow . Hydration of this compounds starts about 7 days after mixing water in the cement concrete and continues for a bout a year .

  It is clear from above mentioned discussions that setting and hardening of cement paste is mainly due to the hydration and hydrolysis action of the above mentioned four compounds  Out of the four Tetra calcium Alumino Ferrite is pradctically inactive and does not contribute to any strength and setting of the cement . Tri-calcium silicate is the best cementing materials and greater percentage of it in the cement result in a type of cement which develops high early strength . Higher percentage of tri-calcium aluminate results in a type of cement which sets immediately . Hence in short it can be said that the rapidity of intialsetting of the cement is controlled by amount of tri-calcium silicate . Strength of cement concrete , developing after 7 or 8 days of its placement is depndent on the amount of Di-calcium silicate . As already strated earlier  , setting gypsum poweder in the cement . Percentage contents of Bogue compouns and composition of some of the important cement are given in TABLE 15.1 AND 15.2 .