1156 posts
Cement chemistry Question 1
What is lime used for in cement ? Many people tell me different answers. What is the ratio of lime to cement and when to use in a particular job ?
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1156 posts
Re: Cement chemistry
The simple answer is "around one per cent", but the exact response may rather depend what you mean by "lime" and what class of cement you are talking about! Portland cement is ground from a clinker produced by heat treatment of a raw mix that consists mainly of finely-ground limestone and shale. On heating, the limestone forms lime which, in due course, conbines with other substances to form calcium silicates and aluminates and there is only one per cent or so residual uncombined lime remaining. Cement sets and gains strength through the reaction of these substances with water. So, some people would say that a lot of lime goes into cement: if you analyse the clinker and do not take account of the fact that the lime is combined as aluminates and silicates, then about 65 per cent of ordinary Portland cement is "lime". Now when it comes to making mortar, we have a different story. A long time ago, lime/sand mortars were used (about 25% lime in the mix); then, to get more rapid strength development and higher final strength, people started to add cement. (Just using cement and sand leaves you with a lot af sand and not much cement and a harsh mix that is difficult to work with a trowel - and if you add more cement to overcome this then the mortar becomes too strong for general use.) So, some traditional mortar mixes are 2 parts of cement plus 1 of lime and 9 of sand ( or 2 - 2 - 12, or 2 - 6- 24 are also OK). You can buy masonry cements instead: in North America these are typically made from about 50% Portland cement interground with an inert material such as limestone and a plasticiser to aid workability and you can mix 1 part with 3 parts of sand to get a mortar roughly equivalent to one of the traditional mixes. Now, if someone says "lime" instead of "limestone" - as is common in some regions of the world - you need to change our first answer quite a bit! Concrete mix design is quite another field. Concrete is basically a mixture of aggregate - crushed stones or gravel - in which the voids have been filled by a mortar mix (sand and cement) that yields suitable flow properties when fresh and, in due course, appropriate strength. (Add too much water to get it to flow OK and you lose out on ultimate strength; add too little water and the mix will not flow into all the corners where it is needed!) Most builders' merchants, do-it-yourself outlets and suppliers of cement will give you rules of thumb as to what mixes to use for different jobs.
1156 posts
Cement chemistry Question 2
What will be the effect of clinker on cement grinding which is produced from 100 per cent petcoke. The chemistry of clinker is as equivalent as when produced with other fuels. Also nodulisation of petcoke clinker is good. Kindly advice how to improve cement grinding performance with petcoke clinker.
1156 posts
Re: Cement chemistry
It is not clear what problem you are suffering from. With petcoke clinker I expect there is a higher sulphate content in the clinker. Therefore you have to reduce the amount of gypsum addition and that will adversely affect the performance of the cement mill. I'm afrain there is no ready solution to that problem. The clinker might also be denser resulting in loss of milling efficiency. In that case addition of calcium fluoride to the kiln feed will probably help.