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Thread: iron in pool aggregate

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    iron in pool aggregate

    Yesterday I was talking to a plasterer about different finishes and when granite came up he said there was a lot of iron in the mix. We went to his office opened up a bag and ran a magnet over the mix. To my surprise a substantial amount of material was collected with the magnet. What does this mean?

    We have pools with this finish that are a couple of years old that have not shown any visible rust issues.

    1. Could the iron be locked up in the silica to the point that it won't oxidize?
    2. Is it a matter of time before the iron oxidizes, expands and destroys the finish?
    3. Is it a matter of what % levels of iron is in the aggregate?
    4. Is it correct to assume that iron oxides used to dye the mix would not respond to the magnet?

    This is a great site! It would take a day or more of phone messages and return calls to find out answers to questions like this.

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    Join Date
    Mar 2007
    San Rafael, CA USA

    Re: iron in pool aggregate

    The iron in granite is iron oxide (in both ferric and ferrous forms), not raw iron. It is weakly magnetic, though not as strong as with iron. Iron oxide is what is used in "magnetic ink" for the automatically read account numbers at the bottom of checks.

    I suppose it is possible for the chlorine in the pool to oxidize the ferrous oxide to ferric oxide which would change the color from black to red-rust color, but the quantities are so small that it probably wouldn't be noticeable and would just be at the surface of the finish anyway.

    The real question is not whether the granite material gets oxidized, but rather whether it physically falls apart into particles. That is, it's not about dissolving (as ions) into the pool, but more about structural integrity. I can't answer that question; perhaps others with experience with granite can.

    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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    Melt In The Sun's Avatar
    Join Date
    Oct 2009
    Tucson, AZ

    Re: iron in pool aggregate

    Ha! Time for me to contribute something meaningful (I hope).

    The iron mineral you were picking up with a magnet is...magnetite (Fe3O4). Of the three iron atoms in magnetite, two are already in the ferric (3+) state. Magnetite is a fairly competent/durable mineral, though it can and will slowly oxidize to hematite (Fe203, otherwise known as rust). A true granite typically will have very little iron in it (generally <2%) and that will mostly be tied up in insoluble silicate minerals such as hornblende. Since stones with a wide variety of compositions are incorrectly marketed as "granite," many of them contain a lot more iron than that, and in much more soluble forms.

    While I can't remember all the eH/pH and other chemistry diagrams we could use to figure this out for certain, I would be very hesitant to put anything in my pool that was attracted to a magnet.
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