I queried the Portland Cement Association and received this response back.

I guess my theory was wrong.
I stand before the pin-head firing squad...
*pin-head salute*

-----Original Message-----
From: Melander, John <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Cc: Burg, Ron (E-mail) <[log in to unmask]>; Field, Connie
<[log in to unmask]>
Date: June 15, 1999 1:53 PM
Subject: RE: Iron stains on Limestone


>Dear Mark,
>
>I don't know of any reliable on-site method for analyzing the iron
>content of masonry cement or portland cement. The standard test methods
>are contained in ASTM C 114, Standard Test Methods for Chemical Analysis
>of Hydraulic Cement. Construction Technology Laboratories has the
>capability of performing these and other analytical procedures on a
>consulting basis. If you would like more information about their
>capabilities in this area you may contact Ron Burg at [log in to unmask]
>
>I would question the hypothesis that the staining that you are observing
>is an iron stain from the cement. The iron in cement is typically in the
>form of complex hydraulic compounds and is not readily leached from the
>hardened mortar. When iron stains occur in concrete or mortar, they are
>often the result of impurities in the aggregates.
>
>Some limestone, in particular Indiana Limestone, is prone to staining in
>the presence of alkalis. The porous limestone contains tars that are
>made soluble by an alkaline solution. As moisture moves through the
>stone the tars are deposited at the surface in the form of a brown stain
>that becomes insoluble. Cement of a very low water soluble alkali
>content, often a white cement, is sometimes used in mortars for laying
>Indiana Limestone to avoid this staining.
>
>I trust this information is helpful. Please contact us if we can be of
>further assistance.
>
>John
>