[Cob] gypsum

[Lance Collins]

I've also done some experiments with gypsum and have some problems with it.

(Note: we are talking about calcined gypsum here aka plaster of Paris)

The biggest problem is the cost.   In principle it should be easier to mine 
gypsum, heat it (i.e. calcine it) and grind it up than it is to make 
cement.   But you will be lucky to find gypsum at less than 4 times the 
price of cement.

I've tried the Alker recipe and it's pretty much as Kirk says.   I doubt 
his generalization on compressive strength as the Alker technique is used 
for loadbearing walls in Turkey. (http://atlas.cc.itu.edu.tr/~isikb/)

The setting time can be delayed by adding lignum sulphate to the mix.

I have also experimented with sawdust mixes (papercrete) and found that 
eucalyptus sawdust completely negates the gypsum setting time.  My 
sawdust/gypsum/adobe blocks took several days before I could remove the 
formwork.

Compared with adobe which shrinks on drying Alker actually expands 
slightly.  The blocks I made were very precise and in a wall could easily 
pass for concrete block.  I doubt this would be an aesthetic effect popular 
on this list.

Lance



At 09:00 AM 7/11/2003, you wrote:
>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.