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Technical Cement Forum / 5-10% increase of 2D strengths of despatch cement as opposed to produced

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el quimico 14/01/2013 14:15:38

5-10% increase of 2D strengths of despatch cement as opposed to produced

Dear all, (especially Ted whose therads I read voraceously)

The phenomenon is consistent and characteristic of our cement and emphasizes with increase in CF, i.e., CEM I and clinker C3S (bogue calc) . Additionally, it relates to lower initial and final setting times by 10- 15 min. I understand that there's a mechanism that is fuelling the hydration process that of course on the other hand the finer the cement the faster it progresses. Some data are:

Cement tempreature typically 70 deg C moisture 0.3 - 0.4%. We use natural gypsum dehydrate, Clinker is typically 9 - 10.5 C3A and 60 C3S Bogue with 1.3% free lime and 0.9 - 1.2 SO3. It's always puzzling me, and I find it hard to come across some article describing our case. I would appreciate if any of you have encountered such case and what is the theroetical support to this phenomenon.

Regards

El qumico

Ted Krapkat 15/01/2013 03:00:58

Hola El Químico,

Me gusta su nombre de foro, porque soy un químico también.

Yes, I have noticed often that despatch cement seems to be 3-4% finer than production cement. Depending on the fineness difference, this might explain your setting time and strength differences. (i.e. slightly finer cement will set slightly faster and be slightly stronger) However, I also have not come across any articles that mention this phenomena.

I believe this fineness increase is caused by particle attrition due to handling after the cement mill (especially pumping).

Are the Blaine finenesses of your milled product and despatch cement different?

Saludos cordiales,

Ted.

el quimico 15/01/2013 10:36:46

Dear Ted

Muchas gracias por tu respuesta.

Thank you for your quick response. Indeed, I have noticed that there's a marginally detectable 1-2% of difference in residue between depatch and production; however this varies from year to year and it swings on either side, while the difference of 1-1.5 MPa in Rc2D and IST 10 min and EST 15-20 min is there historically regardless the residue. Could compressed air play a role here or perhaps attrition as you mention somehow "activate" the cement grains? Anyhow, it's always fun to have some mindboggling excercise to chew on. I am planning to collect two samples from airslide and subject one to 60 deg C in a closed container for a few days and compare with room storage one. Will get back to you with results.

Thank you

El quimico

Ted Krapkat 16/01/2013 01:31:41

Hello El Quimico,

Compressed air could play a role in the sense that it would promote turbulence and therefore more collisions between particles of cement which would increase the particle attrition rate. I don't think the cement particles are being "activated' because, once cement is exposed to fresh air, the only chemical change that is likely to occur is prehydration, which would tend to cause the particles to become less active.

In my experience, despatch cement (pumped ~150 metres to a 20,000t storage silo) can consistently exhibit a specific surface area of about 10-15 m2/kg higher than freshly milled cement. This would be equivalent to roughly 1-1.5Mpa compressive strength increase. So far, this looks to be the most likely explanation for your observations.

A 1-2% difference on 45um residue could certainly account for 10-15 m2/kg difference in fineness, however it is quite possible for two otherwise equivalent OPC cements to have significantly different specific surface areas but exactly the same 45um residue.

The best way to determine if there is a significant difference between the critical fineness fractions of the two cements is to have a full PSD analysis performed on each cement. The amount of <5um particles has a strong influence on the 1 - 2d strengths and setting times.

I look forward to hearing about your results.

Saludos cordiales,

Ted.

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