[Cob] Wall building problems cob clay sand mixture

[Mark Thomas]

Just a follow up for the more experienced people..

My mix is pit run from a local gravel pit, a good mixture of clay, fine sand, course sand, gravel and a few stones up to 3/4".  Ed O'Kelly runs it through his separator and takes out anything 1" or larger and crushes any large clumps of clay/sand that are common in his pit.  I get a good pit run mix when delivered.  I amend with 1/4 minus and add a shovelfull or two of the crusher fines.  1 part pit run to 1.25 parts 1/4 minus/crusher fines is my current recipe.  Clay content is 10-15%.

I mix my batches on tarps totaling 15 gallons dry.  Adding a bit more sand (1 to 1.5) will work yet the test blocks lose corners and sand falls off when the block is handled.  This mix tests out about the same for compresion and modulus of elasticity as the 1 to 1.25 mix except that the corners break off more when dropped from 3'.
This mix clay content gets well below 10% at that point.  A 1 pit run to 1 course sand will also work yet only during cool weather (less than 80 degrees) and wall kept covered for 2 days to slow drying.

I have also experimented with adding a couple cups of tile clay (Kaolinite) to a 1 to 1.5 mix and have had good results.  Less shrinkage cracks and a very strong test block.  I guess the Kaolinite counteracts the expansive Montmorillinite clay.

A bit expensive since the 50lb bags of Kaolinite have to be shippped in.

Straw is what I can get.  I found some 3 string bales in Silver city that had good strong straw yet half the bale is chaff or very short pieces.   What is left after sifting is 4-12 inches long.  Unfortunately I have not been able to find a local hay grower who bales the big round bales of oat straw who  will sell me a bale.  They don't seem to be interested in selling and delivering a single bale.   The feed stores don't seem to carry the large bales.   

Kenaf fiber?  Has anyone tried this in cob?  Fiber crop that is grown in S. Texas.  The University of Mississippi sent me some seed and I currently have a dozen plants growing.  they sure do look like another plant that is of the same family (Hemp).  Hope they don't get stolen.


Mark Thomas
Somewhere south of Deming New Mexico