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GSBN:Jeff Ruppert and clay tests

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Many thanks for this test.  We use clay and lime plasters
almost exclusively now, but I am still uncomfortable promoting them
without solid data.  Also, without any real sense of the tensile
and compressive strengths of these materials, it is difficult to rely
on them for any portion of the structural component of the buildings,
even though it is plum obvious that they are capable of carrying at
least<i> some</i>  of the loads....
Anyway, I hope that you and others on this list will take some
interest in the work we have been doing with lime stabilized clay
plasters.  At least in New England, I have found them to be far
superior to straight clay plasters in three basic areas:  mold
prevention, weather resistance, and shrinkage.  They also clearly
have a higher compressive strength.
Here's what I've found:
<font color="#000000">We learned about this in a big way in the
summer of 1999, which was very dry, with no rain from April until the
third week in August.  When were we installing bales and applying
clay/straw plaster?  You guessed it- the exact weekend the rains
began.  I'm convinced that every mold spore in the state of
Massachusetts woke from dormancy that weekend, and the freshly
plastered walls erupted in a fur of pink, green, and beige mold, which
made me feel rather ill and spoke badly of straw bale construction to
every person who came nearby.  To make matters worse, on that
weekend the weather changed from hot and dry to cool and wet, and the
plaster took a very long time to dry out.  I tried hydrogen
peroxide to kill the mold, and it drank it down and grew stronger. 
I tried a solution of bleach and water; this knocked it back until the
bleach went off into the ozone layer.  Then I tried a limewash,
and the mold receded. </font>
<font color="#000000"> </font>
<font color="#000000">Mold doesn't grow on the clay, it grows on
straw in the plaster mix, and on the straw behind the plaster. 
We are careful now about keeping our milled straw dry; but the key
element has been the inclusion of lime in the plaster.   We
haven't had a signifcant mold issue in the three years we've been
using lime stabilized plasters.</font>
<font color="#000000"> </font>
<font color="#000000"><b>Weather resistance</b></font>
<font color="#000000">In our climate, it's not practical to use
clay plaster as a final coat; but because of our short construction
season the clay base coat almost always goes through one winter before
receiving a lime finish coat and limewash.   So in the
spring, I get to inspect the damage.  There is no question that
the lime-stabilized mixes are performing better than the clay-only
mixes.  On one house, we used both, and contrast was amazing. 
One whole corner of the clay-only mix had eroded away, while on the
lime-stabilized mix there was no discernible erosion of the horizontal
ridges made by the scratching tool.  Generally, the only damage
we see on the lime-stabilized walls is right at the top layer of the
winter snow pile, where the snow does not melt the usual inch or so
away from the face of the wall, and the wall is subjected to alot of
freeze-thaw.  This is easily repaired before the final
coat. </font>
<font color="#000000">(Incidentally, I love this system of
leaving the finish coat for the next year.  It allows time for
the building to shrink and settle, and our finish coats generally show
no stress-related cracks.  It also leaves plenty of time for the
bales to dry off the trendous amount of moisture introduced by the
base coat.)</font>
<font color="#000000"> </font>
<font color="#000000"><b>Reduced shrinkage</b></font>
<font color="#000000">We apply our base coat at about an inch
thick, with a sprayer and therefore quite wet.  The clay-only
plasters would consistently show a pattern of short (2-4")
vertical cracks, covering the entire building.  This was not a
long-term problem, as they didn't seem to telescope through to
succeeding coats; but I like the fact that in the lime-stabilized
samples these cracks do not appear, except in areas that were filled
quite deeply.  At the very least, there is less chance of water
entering the building during the first season, before the finish coat
is applied.  I also believe the reduction in cracking makes for a
more solid substate for the finish coat.</font>
<font color="#000000"> </font>
<font color="#000000"><b>Increased compressive
<font color="#000000">Here is where a lab test would be very
useful.  There is no question that our lime-stabilized plasters
are harder and more crush-reistant than their clay only forebears;
I've pounded on enough walls and crushed enough drops with my foot or
a hammer to say this with certainty.  What I'm not sure of his
how<i> much</i> harder....</font>
<font color="#000000"> </font>
<font color="#000000"> </font>
A further benefit of the lime is that it makes spraying much
easier.  The lime makes the plaster much more slippery, so that
there is less friction in the spray tube.  With the rather
underpowered Quickspray Carousel, this is a very significant
improvement.  (By the way, if anybody has a recommendation for a
better sprayer, I'd love to hear about it.)
<font color="#000000">The ideal percentage of lime for
stabilization varies according to the richness of the clay soil used,
and also according to the specific clay particles in the soil. 
The properties of clays vary widely.  We usually work with a clay
from a brick factory which is somewhere in the area of 60-70% clay
content.  We have found that a 1-1 ratio of dry, milled clay to
lime, added to 4 parts sand and 2 parts milled straw, makes an
excellent base coat plaster.  This is a rairly rich mix, with
excellent properties of adhesion and workability.</font>  After
seeing Jeff's test, we may try increasing the sand to six parts,
omitting the straw, and using synthetic fibers.  (We have had
very good luck with synthetic fibers in lime plasters.)
I don't know what happens if the mix is too rich on lime; I do
know that if there is not enough lime, the plaster can actually end up
feeling spongy and crumbly, much weaker than the original.  I've
found that this 1:1 mixes usually works well.  It may be overkill
for some soils, but I've never found a situation where it is too lean
on the lime. 
<b>Caution and Chemistry</b>
My only concern about this mix involves the degree to which vapor
permeability may be reduced.  In<i> Building with Lime</i>,
Michael Wingate and Stafford Holmes talk about a low-grade cement
being formed by the reaction between the lime and some of the minerals
in the clay.  This is only one of a set of reactions that
supposedly combine to form the harder plaster.  The
others-flocculation (sticking-together) of the clay particles,
pozzolanic reactions between the lime and minerals in the clay,
standard carbonation in the lime- don't seem as if they would cause
significant reductions in vapor permeability.  Though the nature
of these reactions would certainly vary according to the different
clays and limes used, it would be useful to have some general sense of
how this plaster would perfom, compared with the clay-only version.

The walls seem to dry well- we always get them down to 14% before
proceeding with interior or exterior finish plaster- but I don't know
whether they would dry faster with a clay-only plaster.
So, I hope you find this useful.  If anyone has any ideas or
needs more specific thoughts or mixes, please let me know.  And
maybe someone who is more laboratory-oriented than myself wants to
crush a few samples?  I'd send you a virtual kiss for that, which
you are absolutely free to refuse!
Paul Lacinski 
Amy Klippenstein 
GreenSpace Collaborative 
Sidehill Farm 
PO Box 107 
463 Main St. 
Ashfield, MA 01330 USA