Raju Verma 12/08/2013 17:37:00

Product facing 28 strength lower problem

Dear expert

in existing condition

my clinker with 66% C3s,15 to 16% C2S, 1% free lime and >95% LSF in clinker,

My clinker factor in Lime stone pozzolonic cement is 58%,(Cement composition is 8% Slag,20 % limestone,10 % Pozzolona rest is gypsum). My 3 day strength 28 to 30 MpA( EN stand.) and 28 day strength is 44 MpA.

now in this situation if we go for higher mic addtion( which my national standard allow next 3% more), drop in 28 days strength very high.

can you please comments in this situation what should be my action for maintaing the later strength with more MIC %.

our cement residue in the ramge of 3 to 4% on 45 mic. blaine 4000 cm2/gm.

in microscopy we face problem very low size of alite content with very high %, please comments also what should be my action in kiln or cooler for control the size of alite in clinker as well as % of alite and belite in clinker.?

thanks

regards

raju verma

Ted Krapkat 13/08/2013 02:48:03

Hello Raju

This is a common challenge nowadays. With the constant economic pressure to increase mineral addition, cement clinker contents are continually dropping. Eventually at some point, blended cement strengths are sure to suffer.

The answer is to maximise the content and hydraulic reactivity of the C3S in your clinker. This makes the clinker more expensive to make, but it is the only way to ultimately maximise strength potential in blended cements.

Of course there is a practical limit to the amount of C3S that industrial clinkers can contain, and that limit has traditionally been determined by two things;- the clinker LSF and the maximum acceptable free lime content. In theory, the maximum LSF for clinker is obviously 100, since above this value no amount of harder burning will reduce the free lime.

In practice however, the maximum LSF that can be obtained, before free lime becomes an uncontrollable issue, is about 98. At this LSF, and using traditional clinkering methods, the maximum amount of C3S achievable in the clinker is about 70%

Higher C3S values may be obtained by the use of fluxes and mineralisers (in particular fluoride), or by manipulating the ratio of C3A to C4AF, and also by actively minimising species such as SO3 (in excess of alkalis), K2O (in excess of sulphur) and P2O5... all of which inhibit the formation of C3S. Fluoride in particular can promote the formation of fully-combined clinkers with LSF values up to 104, and C3S values of ~80%. This can be achieved at fairly low concentrations of fluoride (0.2-0.3%) in the clinker

For further information I suggest you go to the following link;- http://www.cemnet.com/Articles/story/151799/clinker-c3s-paramount.html

This excellent article by CemNet's Michael Clark fully covers this issue and suggests several ways to practically increase the C3S content and reactivity of your clinker, specifically for the purpose of improving strengths in blended cements containing a high level of mineral addition.

For more information on the use of fluoride as a mineraliser, see this link;- http://materialsknowledge.org/docs/Hansen.pdf

Regards,
Ted.

Raju Verma 16/08/2013 14:06:48

dear Ted

In cement we are adding 20 % lime stone, we replace 5% lime stone with quick lime in cement,and rest 15% is lime stone as it is for strength increase.

What your opinion is that the good idea for getting some more strength in Lime stone cement replace small quantity of lime stone by quaick lime.

because this is the process lime we think it give some benefit in cement for better strength,due to enhancing the hydration reaction (alkaline media).

but one quetion? any bad impact due to heat libration during hydration? (amount is very less 5% from all cement basis)

lab trial under process ,we'll inform you the results.

thanks

regards

raju verma

Ted Krapkat 20/08/2013 03:06:59

Hello Raju,

I have heard of quicklime being used in Autoclaved Aerated Concrete (AAC) to produce bubbles of hydrogen by reaction with powdered aluminium metal, but I have not had direct experience with adding quicklime to cement before, so I have a few questions;-

1. Does it really improve your cement strength?
2. What about expansion / unsoundness?
3. Why do you use quicklime rather than hydrated lime?
4. Have you tested this product in the market place?

Addition of quicklime would most probably improve the initial rate of the pozzolanic reaction and provide some increase in early strength. But late strengths are likely to remain the same. Since, over time, there is more than enough calcium hydroxide gernerated by the hydration reactions of the clinker minerals to satisfy the requirement of the pozzolanic reaction.

With regard to heat generation, there would certainly be some extra heat produced but, if the quicklime is ground fine enough and well dispersed in the cement, this may not be a problem, except possibly in hot weather where cracking may result.

An increase in the alkalinity of the pore liquid in setting concrete or mortar is certainly known to accelerate alite hydration, giving higher early strengths. But increased alkalinity is only achieved by the reaction of alkali sulphates with the aluminate phase. As sulphate is removed from solution to form ettringite, the loss of the sulfate ion is balanced by the creation of two hydroxyl ions producing a solution of alkali hydroxide. The pH of alkali hydroxide in solution is significantly greater than that of the calcium hydroxide which is normally produced by the hydration of the clinker minerals. However, due to the limited solubility of calcium hydroxide, simply adding extra lime would probably not increase the pore solution alkalinity any higher than that due to the substantial amounts of calcium hydroxide generated during early hydration.

I would also imagine that the addition of quicklime would increase the water demand of the concrete as well. Have you experienced any problems related to workability of this cement?

Regards,
Ted.