[Cob] Salt in building materials

[Monica Proulx]

Interesting question about salt in cob.  I have the same concerns about
possible effects of some salts on binding properties of the clay that
William Pittman has.  There are different kinds of salts of course, but ones
with sodium (table salt/rock salt, NaCl, for example, which is probably the
cheapest and most accessible and the ones people would think to use first)
might be a problem in cob as the sodium ions (Na+) weaken the binding force
between clay particles when the clay is wet (excess sodium ions cause a
process called de-flocculation which makes clay particles repel each other,
not sure I understand the process fully but has to do with residual negative
charges on clay particles once Na+ is incorporated).

When high sodium clay soil is drying out, the sodium causes a hard concrete
like crust to form on soil surface, which doesn't allow water (air, roots or
plant shoots) in or out of soil easily.  Perhaps high sodium salts in cob
might slow drying due to same hard impervious surface layer.  Who knows,
once it finally dries, it might make a very tough cob (due to the concretion
effect), unless of course that effect is also brittle and fractures easily
at the same time.

On the other hand, calcium or magnesium salts have a different effect on
clay soil, and actually improve soil aggregation or clumping, and might have
the same or a positive effect on cob.  Not sure what the other road salts
are made of, perhaps these?

Only way to find out the effect of various salts on cob is to experiment,
make some test bricks with varying amounts of different kinds of salts (NaCl
or table salt vs. Ca or Mg salts) and some control bricks without any salts,
and see what happens.  If anyone does this, let me know, I'm curious.  Might
make an interesting high school science fair project : )  I have some
suggestions if anyone is interested, contact me.  I'm all out of high
schoolers myself.


At any rate, de-flocculation due to sodium salts weakens the binding quality
of the clay when wet, and this might also cause a nightmare of slumping or
"splooging" in wet walls.