-----Original Message-----
From: Ken Follett <[log in to unmask]>
To: [log in to unmask]
<[log in to unmask]>
Date: June 10, 1999 8:04 AM
Subject: Re: Limestone/Iron



>
Ken said...
>I have not ruled anything out. The orange staining appears in an apparently
>random distribution of repointed joints and within a week following
>repointing.

I have come across this exact scenario before with a limestone called Tyndal
Stone.  Especially the buff stone (which is from the higher quarry beds and
oxidizes over geological time to give the buff color) rather than the grey.

Limestone commonly contains iron. Cement also contains iron and the amount
of iron is controlled precisely in the raw mix before burning clinker. (I'm
going back almost 20 years now to my days in the lab at Lafarge Canada)
There are also many different "recipes" for cement, depending on the raw
materials available.  It seems (I'm theorizing here) that if the iron
content of the cement must be brought up by an iron ore admixture, rather
than in the shale or clay, that there is an increased likelihood of a
reaction with certain limestones.  Portland cement from one plant would
cause the staining consistently on random stones while the cement from
another plant would not.  I dont know if this applies to Indiana Limestone,
but it seems like the exact same symptoms.

To alleviate the problem (in my latter masonry contracting days) we would
use white portland for both setting and pointing the stone, ensuring that
the stone does not become saturated by rain before laying.  Yes, the dryer
the mix the better, but with limestone, you can always hear those cries
"Agua!!.... more agua!!" from atop the scaffold.

I would suggest that you try a different pointing mix... preferably one
using white cement rather than grey if at all possible.  Also, in the
particular case of the Tyndal Stone, a light, dry micro-abrasive blast (now
I'm up to my present day) would easily remove the stain which seemed to
migrate right to the surface of this extremely porous stone.  It is possible
however, that the stain could come back in a lesser amount from a hard
driving rain, so a siloxane was then normally used to (cross your fingers)
prevent the re-occurence.

Good luck... let me know how it turns out.

Mark Sherman
Rust never sleeps....
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