[Cob] gypsum

[Scott Van Kirk]

I happen to know a bit about the Alker recipe.  It is a combination of 
gypsum,
lime, sand and clay.  The interesting properties that the gypsum provides 
are a
quick set time and low shrinkage.   This stuff will harden in 15 minutes.  
Consequently you can make a very wet mix and pour it into a form and remove 
it from the form quickly and use it in your building.   I saw a 
demonstration of this down in New Mexico by the folks with the SW Desert 
Sustainability Project.
One problem with it is that the mix as given is not very strong.  The 
blocks
did not have the compressive strength of traditional unstabilized adobe
and thus could not be used for loadbearing construction.  Secondly, the
blocks do not have good resistance to water.   The gypsum will dissolve in
the presences of water.  The SWDSP people are working on a slightly 
different
formula which involves changing the proportions somewhat and adding paper 
fibre to the mix which increases its strength and its resistance to water 
erosion.  I don't believe have it all worked out as yet but it looks very 
promising.

you can see a bit more on this at their site:

http://www.angelfire.com/nm2/swdesert/


As far as gypsum goes, it is very usefull in interior work.  Wallboard is
basically gypsum covered in paper.   It also makes a very nice natural 
plaster.


-Scott


On Thu, 6 Nov 2003 12:53:44 -0800, Jilly <JILLPRUETT at peoplepc.com> wrote:

> If you plaster with gypsum after you build the cob wall, for the purpose 
> of making it weather resistant, why wouldn't you just add that to the mix 
> from the start?
>
> http://atlas.cc.itu.edu.tr/~isikb/Tech1.htm
>
> This link talks about it, though does not use the straw that cob does. I 
> am not sure why / why not? Does the straw we add to make cob provide 
> strength or insulation? Both? What if it was added to the same mix as in 
> the link?
>
> Thoughts?...
>
>
> jilly
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-- 
-Scott