exterior wall protection

[christopher j. meyer]

>Now I have heard from the grapevine that cement over adobe will cause the
>adobe to
>disintergrate (I not sure what that means except maybe get crumbly?)

>I am wondering why the ag school in South Carolina find it ok
>and my brother in law find it ok
>but... some folks out west find trouble with some adobe buildings with
>cement coatings???


Pat, I believe there may be two possible explanations.  First, the dry clay
may be sucking enough water from the concrete mortar that the adhesion of
the concrete/clay interface, once the concrete sets, is greatly reduced.
So maybe the clay is not disintegrating but the concrete is or a
combination of the two.  At any rate the adhesion of the two materials
would be substandard provided the mortar lost enough water.  The best
masons always spray their brick with a hose or even soak them in water
over-night to make the brick less thirsty.  Second, and much more likely,
is what's known as sulfate attack.  It's about the most common form of
chemical attack on concrete.  Clay can contain very high proportions of
sulfates.  With concrete in contact with high sulfate clay you'll  have a
reactive interface.  However, by using type V cement in your concrete the
threat of sulfate attack will be minimized.  Type V is known as sulfate
resistant cement.  A drawback is that type V can be hard to locate; but if
you live in an area where the soils are high in sulfates you can most
likely get type V cement.  A good check to determine the probability of
having high sulfate soil in your area is to see if type V cement is readily
available.  Question:  Do the folks out west know if their soil is high in
sulfate content?  If so, I believe we have the answer to our question of
p.c. plaster durability.
Adios, Chris.