free lime

Hai ,

Normally free lime( calcium oxide)  is present in clinker whereas in the cement a major portion of it is present as  Calcium hydroxide. This is due to the fact that during the process of hydration it absorbs the water either from the gypsum( moisture content) or from the high temperature developed during the process of grinding.

The important requirements which a cement should fulfill can be classified into two divisions 1) Physical requirements ie., Setting Time, Workability, Strength(early and later), color and heat of hydration and 2) Chemical requirements ie, resistance to chemical attack mostly sulphates, chlorides and the Alkali-Aggregate reaction.

An increase in clinker free lime reduces the total silicates (C3S + C2S). Both C3S and C2S are involved in the hydration to produce the C-S-H gel which is the main strength forming phase in hardened Portland cement concrete. Free lime hydrates to form more of  Calcium hydroxide Ca(OH)2 crystals.
 An increase in free lime increases the proportion of C2S and decreases the proportion of C3S in the clinker. C3S is much more reactive than C2S and is a more important contributor for 28 day strength.

Considering the fact that the cement contains high free lime it will result in Unsoundness in the cement produced. It gives volume instability leading to cracking. Hence the impact of free lime on the cement and the concrete will be Faster setting time, Higher heat of hydration, Lower strength levels with a brownish grey tinge in the color.

 Regards

 Arvind

 

 

Thank you, my dear and I hope to illustrate the effect of free lime on the setting time.

Hai, you are most welcome. As stated above the initial hydration is an exothermic reaction and hence the heat of hydration will be high.To put it more lucidly as soon as cement is mixed with water considerable amount of water is lost with evolution of heat resulting in shorter setting time regards Arvind