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Re: GSBN:Jeff Ruppert and clay tests

Responses below to emails from both Bruce and Tim:

Clay by itself is the binder in earthen plasters, and as we all know, works
really well when done right.  Lime, on the other hand (and portland cement)
will destroy clay's binding force by deflocculating the little water-loving
grains.  That's why highway builders, especially in wet climate/clay soil
places like the UK, like to use lime to both dry and deflocculate soils
before laying down their beloved asphalt paving.

I'm not sure, but I think you have it backwards- I think the lime causes the clay particles to glom together. My copy of the lime books are out on loan, at the moment, however. I do know that if you have a clay plaster going in the mixer and you start adding lime, you eventually hit a point where the mix suddenly gets very stiff, stiff enough that if you don't immediately add water the paddles are likely to stop turning and the breaker to pop. At this point the mix also becomes very greasy. I'm not sure, but I think this is a decent field test for the minimum amount of lime to add to a given batch of clay.

That's also why Bill
reported LOSING strength in earthen bricks when he added lime;  his binder
wasn't binding, which will always put a hitch in ol' Bill's giddyup.  But
that's only the short term (one or two month) picture . . .

 . . . The long term picture has the lime carbonation and low-grade
pozzolanic reaction between clay and lime coming into play, and I'd guess
the strength/durability would keep on growing over the years.

This has been our experience, exactly. The clay/lime mix is noticably stronger after 4 or 6 months than after 1 month. At 2 weeks or 1 month, it may well be weaker than the clay-only mix.

Recently I was afforded an
opportunity take apart some of these assemblies. (more on that later) most
notably the earth to lime connection was the week link and I would assume
extreme weather would like to pop the thin lime coat off. for this reason
(and the mold that Paul mentioned) I am interested in getting a small amount
of lime into the mix and work it to the surface then scratch so that I might
have a better chance of getting a lasting bond. I have not noticed much
degradation of the brick clays but site soils do all kinds of things that I
don't fully get yet.

Everyplace I go, I bring my building geek self along, and look for delaminating plaster. It's not hard to find, but I have never found it in a situation where the backing coats were scratched, regardless of material. This doesn't mean it can't happen; but it's clear that scratching greatly reduces the likelihood.

When we next plaster (March) I'll make a raft of samples, and send them along for testing. I can also send the 2x2x8 blocks that Tim recommends. The best may be to test at 1 month, two months, and 4 months.



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