norew
99 posts
TimePosted 16/09/2011 09:48:13
norew says

Unsoundness vs. Fineness

In what degree is fineness of cement to unsoundness?..are they related somehow?..

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Ted Krapkat
536 posts
TimePosted 21/09/2011 06:02:04

Re: Unsoundness vs. Fineness

Hello Norew,

Yes they are related, but only when the cement is far coarser than would be acceptable in most modern cements. When H.J. Force first introduced the autoclave expansion test in 1912, he expressed the opinion that the failure of cement to pass the test was not due to either free lime or free magnesia, but to the amount of coarse (>74um or 200 mesh) particles in the cement. He found that by increasing the fineness of unsound cement, the cement became sound. Of course in those days the typical residue on 200mesh screen was over 20%!

In the 100 years since then, there have been progressive increases in cement fineness which has meant that coarse particles are no longer an unsoundness issue for modern cements. The effects of free lime and magnesia expansion now predominate, but research has shown that even these effects can be reduced by grinding the cement finer still.

 

Regards,

Ted.

 

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yadavd
31 posts
TimePosted 21/09/2011 07:20:55
yadavd says

Re: Unsoundness vs. Fineness

Dear Ted Sir What may be reason for unsoundness of coarser cement ?? I Think this is due to increase of voids in cement ,but which factor reduces effect of high free lime and MgO in finer cement because if cement is finer than rate of reaction will increase thus reaction of free lime and MgO should be faster i.e unsoundness

 

I have read somewhere that in finer cement craking phenomenon is found

 is it right ????

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Ted Krapkat
536 posts
TimePosted 23/09/2011 03:57:38

Re: Unsoundness vs. Fineness

yadavd:
Dear Ted Sir What may be reason for unsoundness of coarser cement ?? I Think this is due to increase of voids in cement

Hello Dilip,

Unsoundness is caused by a increase in volume (eg due to hydration products) which occurs after the cement paste has set.  The larger particles in coarse cement continue to hydrate well after the rest of the paste has set around them. Therefore any subsequent volume increases cannot be absorbed by the plasticity of the paste, and expansive cracking may occur.

 

yadavd:
but which factor reduces effect of high free lime and MgO in finer cement because if cement is finer than rate of reaction will increase thus reaction of free lime and MgO should be faster i.e unsoundness

As I mentioned above, unsoundness is caused by a increase in volume which occurs after the cement paste has set. As you correctly pointed out... the smaller the Free Lime and MgO particles are, the faster they hydrate. But, with respect to soundness, this is beneficial because the particles of Free Lime and MgO are more likely to fully hydrate before the paste starts to set, thus avioding unsoundness.

 

 

yadavd:
I have read somewhere that in finer cement craking phenomenon is found

is it right ????

Yes, but this is most likely due to shrinkage, not expansion. Increased fineness has an influence on water/cement ratio and heat of hydration... and therefore shrinkage.

 

Regards,

Ted.

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