Free Lime

Hello Norew,

Could you please be more specific.

Regards,

Ted.

Is there sense in determining the free lime content of ground cement when limestone is present as an additive?..what is the effect of free lime to the quality of cement?

thank you.

norew:

Is there sense in determining the free lime content of ground cement when limestone is present as an additive?

Yes, CaCO3 does not dissolve in the glycol(or glycerol)/ethanol solvent used in the free lime test. However to prevent the CaCO3 from reacting with the HCl titrant, the mixture should be filtered and the residue washed with absolute ethanol before titrating with HCl.

Alternatively, if filtering is not desired, you can use a standardised solution of anhydrous ammonium acetate instead of HCl to prevent any reaction with fine CaCO3 during the titration. This is the basis of some of the so-called "Free Lime Rapid Methods".  See;-

http://books.google.com.au/books?id=oJyWkGcPsVcC&dq=isbn:080310989X

http://books.google.com.au/books?id=oJyWkGcPsVcC&dq=isbn%3A080310989X&q=free+lime

norew:

..what is the effect of free lime to the quality of cement?

Excess free Lime (>2%)  is detrimental in several ways;-

1. Progressively higher amounts of free lime result in a proportional lowering of the amount of clinker minerals present (particularly C3S) in the cement and thus a drop in cement strengths.

2. Because of the difference in hydration speeds and reaction products between clinker silicate minerals and free fime, excess free lime will cause volume instability (increase in mortar expansion) with the potential for cracking (unsoundness) and subsequently lower cement strengths.

3. Excessive free lime (under burnt clinker) may also cause a change in cement colour (lightness).

However, conversely, if free lime is too low (<0.5%) the resultant excessively hard-burned clinker may also result in low cement strengths due to a reduction in the reactivity of the clinker silicate minerals.

Regards,

Ted.