[Cob] STABLE WALL- lime weakens?

[rodger at inbox.com]

Hi Charmaine,

A cob house I helped build and plaster had beautiful clay soil on site, about 30% clay, 20% silt and 50 % sand by volume... and it required about a 1:1 addition of sand to get it to be perfect cob or plaster.  Once burnished and dry it didn't dust at all.

The owners were worried about it being durable enough on the exterior so we experimented with various plaster mixes that included well slaked Type S Hydrated Lime (not the best lime in my experience, but good enough).  I think we did a whack of mixes up to about 50% lime.  All hardened well as you say... but it was my experience that all were more brittle than the clay plaster OR a pure Lime render.  Harder yes, but more easily broken, chipped and cracked, which I extrapolated to mean... weaker.

It seems to me that the lime binds chemically and the clay binds physically (+ and - forces in the clay platelets)... so if there's a clay particle between lime particles, it will interfere with the lime process, and if there's a lime particle between clay platelets, it will interfere with clay binding.  Best to leave them to their own ways.  So I never mix lime and clay with the intention of making it stronger.  

But if you are determined to use less than ideal soil without bringing in extra clay, then it makes perfect sense to me to add lime or portland.  If soil isn't ideal I usually just get a load of high clay soil to mix in... cheap stuff as you know;)

thoughts?

Rodger


> -----Original Message-----
> From: dirtcheapbuilderbooks at gmail.com
> Sent: Thu, 9 Jul 2009 15:14:09 -0700
> To: coblist at deatech.com
> Subject: [Cob] STABLE WALL- lime weakens?
> 
> RODGER said:   Lime could actually weaken the wall, interfering with
> the clay's binding.
> 
> HI Rodger can you give an example or anecdote of this ?  I do add
> 2%-10% lime to all my clay projects specifically to bind the clay
> forever.
> 
> it is true a really good clay is perfect as is.
> 
> but if  HER clay is not, then a 2-3% addition of lime  at least will
> bind very well all the parts of the clay and sand,  and also get hard
> as a rock.
> 
>  I am most interested in  where you learned/observed/heard  this, so I
> can  educate myself-- any details are appreciated.  3rd or 4th hand
> info is hard to validate.
> 
> I have very high silty& sandy clay soil here, so the lime binds it all
> very well. we call it Blue goo- it is sometimes all silt, and a
> gorgeous rich blue color when fresh  harvested, then turns greyish in
> the air.  but the blue color is fabulous-- wish it could stay that
> way!
> 
> 
> --
> Ms. Charmaine  Taylor/ Taylor Publishing
> Toll Free Order: 1-888-441-1632
> www.dirtcheapbuilder.com   www. papercrete.com
> PO Box 375, Cutten CA 95534
> 
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