exterior wall protection

[christopher j. meyer]

17 October,  96
I've seen some discussion about bentonite lately, but am not sure of the
gist behind why someone would want to use it for plaster.  There was a
study in the 30's, conducted at some ag. school in South Carolina, that
investigated materials for protecting the exterior of a rammed earth
building.  It was found that a portland cement mortar was the best with a
lime-based mortar not far behind.  Nails were driven into the walls with
the heads left protruding about a 1/4" which helped adhesion of plaster to
the wall.  The plasters were visually inspected once a year for 7 years.
After 7 yrs, the p.c. mortar showed no signs of cracking or spalling and
the lime mortar showed small cracks, but nothing significant.  These days
we can do even better.  With the introduction of fibers to p.c. mortar, the
tensile strength of the hardened product can be increased;  meaning more
resistance to cracking.  Concrete does shrink and most cracks you see in it
are shrinkage cracks.  You can purchase bags of what's known as fibrillated
polypropylene fibers to add to your plaster mix.  The addition of poly.
doesn't have much affect on the workability  of your mix.  So if you're
talking about exterior plasters, I think p.c., a little sand, water, and
some form of fiber should be tried.  I may be missing the point of the
discussion in which I noticed some concern for wall breathability.  If a
wall is dry, what harm is there in sealing it?  If freezing is a concern
there are certain ways of introducing entrained air to the plaster mix.
Also, coloring can be added to the p.c. mortar to help with aesthetics.  I
don't have any hands-on  experience so my approach from a purely academic
background may expose my ignorance.  Adios, Chris.