Cob Earth plaster & expansive clay

[Bob Bolles]

----------
> From: Robert Bolman <robtb at efn.org>
> To: coblist at deatech.com
> Subject: Cob Earth plaster & expansive clay
> Date: Wednesday, August 06, 1997 10:30 PM
> 
>  
> Martha Cather a NM geologist offered the following response to my earth
> plaster test data plea:
> 
> > I would certainly avoid using swelling clay in a plaster, unless
> > you have a climate where the humidity is going to be most regular.
> > Swelling clays cause problems in everything from slabs to plugging
> > up oil reservoirs.  One major factor is that there can be so much
> > variability from clay to clay regarding the percentage of swelling
> > clay in any given area.
-------snip-------

I was fortunate to be able to attend a seminar recently, conducted by Bruce
King (author of Building with Earth and Straw).  Although the subject was
Rammed Earth, I suspect that given the similarity of the materials, what
applies to RE would apply to plasters and cob as well.  As I recall, he
stated that as little as 1% cement in the RE eliminated the
expansion/contraction of the clay in the mix. We have not had a chance to
try it yet.
Bruce, any comment?

Bob