Cob Lime dust

[Mark A Hoberecht]

At 09:36 AM 7/21/98 -0500, you (Jeanne Leimkuhler) wrote:
>Does anyone have information or experience with adding Limestone dust to
>concrete, mortar or cob? It is a bit like gravel and can be had for pennies
>here in southern Indiana. We are thinking that it could add strength to the
>concrete and some water resistance.
>Adding it to cob seems more problematic. It might continue to expand and
>contract if it absorbs water. Is it more like the clay or the sand?

Jeanne,

If what you are referring to is limestone "screenings" or "crusher fines,"
it is basically a replacement for the sand.  I used limestone screenings
for ALL of my cob (15 tons worth) -- no sand at all.  Because it has
particle sizes ranging from the dust that you describe up to very tiny,
sharp, angular gravel, it makes a very solid cob matrix.  Justin Idoine,
who worked out at Cob Cottage a few years back, clued me in to "screenings"
as better than sand for a very strong, structural cob.  Because the dust is
only ground stone, however, it has none of the properties that, say, a lime
mortar would have.  The dust hasn't been fired in any way.  Based on my
experience, I would say that the limestone dust performs similarly to any
other aggregate dust derived from other types of stone.  My price here in
N.E. Ohio was about $10/ton.

Best ....  Mark Hoberecht

P.S.  Say HI to Joe for me -- I met him at the Michigan thatching workshop!