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Re: GSBN:test results


Well yes, I find this just as perplexing as everyone else, since it contradicts my experience just as directly as everyone else's. Here's what I did for a test: I brought a piece of an old chair around to the back wall of our house, which is partially covered by a lime-stabilized clay plaster, about 1" thick, and partially covered by this same plaster with a 3/8" lime/sand finish coat. (The system that we always use.) I then attempted to gouge these two plasters with the corner of the chair, mimicking, as best I could, a portly and slightly drunk matri or patriarch jumping up from the table to answer the door or phone or whatever. And what do you know, the lime-coated wall is much more resistant to this type of abuse than the clay-only wall.

So, having re-examined the field conditions, I am led to wonder what might possibly have been different about the lab conditions. The only idea I have is that 2" is much thicker than I would ever go with lime, without a pozzolanic additive. (We have poured 2-3" thick lime concrete with ground bricks or pottery as a pozzolan and come up with compressive strengths that, while never tested by the psi, are definately much better than could ever be achieved with earth. (Children jumping around on windowsills made of this material seem to have no effect.) I know what when you pick up a chunk of dropped lime plaster that has been sitting around for a few weeks at the base of a wall, the surface (3/8" or so) will be well set, while the material below will be quite weak. I've gouged these limeplops with a screwdriver before, just to see. The surface is about as hard as a finshed plaster; the underside is not much better than the gypsum in wallboard.

So, maybe we have a situation where the lime didn't cure much, except on the surface, and the cement dried out too quickly to cure properly. If there are any samples left intact, it would be interesting to go at them with an awl or screwdriver, to test this theory. Also- is there any chance it was a bad batch of lime? Was the bag of lime left around the lab for awhile before it was used? Was it leftover from a site somewhere? Is there any chance it got damp?

Maybe someone could look at a bit of this material under the special microscope (whatever it's called) to check on the cement crystal growth?

In any case, thanks again for all of this work. Whether or not I like the outcomes, I'm very happy that the sb world is moving beyond testing plasters with the back of a chair.


Paul Lacinski
Amy Klippenstein
GreenSpace Collaborative
Sidehill Farm
PO Box 107
463 Main St.
Ashfield, MA 01330 USA