-----Original Message-----
From: John Mascaro <[log in to unmask]>
To: [log in to unmask]
<[log in to unmask]>
Date: June 13, 1999 5:01 PM
Subject: Re: Limestone/Iron


 Many cements
>>are made using iron ore slag - the slag recovered from the production of
>>molten iron/steel.
>>I think these are the cements that would be most likely to stain.

Good information.  This could be possible.  Some cement manufacturers
consider the source of iron ore (whether it be true iron ore or mill
scale/slag) interchangeable based on availability and price.   They would
use the slag type iron whenever it was available from a local rolling mill
or whatever other source.   However, the total iron slag added  is a very
small % of the end product (at least in my experience) and when the raw mix
is burnt to make clinker at 2,500 to 3,000ºF, the mix in essence becomes
slag anyways.  I can't say how much slag could increase the soluble salts,
but I would expect that clay could be a more significant source of soluble
salts since it is a much greater propotion of the mix (if the factory
happens to be sitting on a clay deposit).

The irony of iron... is that it is not necessary for the end product (ie:
white cement) but acts as a flux to help the silica and alumina combine at
lower temperatures. There are 3 types of iron in cement, the relatively
stable FeO and Fe, and the most reactive and "free" Fe2O3 (rust).  The nasty
rust can be found in not only the iron ore or slag, but also in lesser
amounts in clay, shale, silica sand or dolomitic limestone (dolomite is not
only a major part of the raw mix to make clinker, but is also added raw to
the clinker at a rate of about 30% to grind masonry cement).

To make it more difficult yet, some manufacturers purchase product from
others and bag it under their own name so you never know what you may get in
that bag of grey powder.  I assume this unpredictability was enough to cause
the stone supplier I mentioned before to shy away from the use of grey
cement entirely and write white portland into their specifications.

I am interested to get to the bottom of this... salt... or rust... and am
going to do a little digging to find out.

Titration test kits anyone?

Mark Sherman