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Re: GSBN: Lime and Cement Plaster
- To: GSBN GSBN@...
- Subject: Re: GSBN: Lime and Cement Plaster
- From: Derek Roff derek@...
- Date: Sat, 08 Dec 2001 08:24:17 -0700
- Reply-to: "GSBN" GSBN@...
- Sender: "GSBN" GSBN@...
I disagree with Frank. A major reason to use lime is to increase the
permeability of the mix, and the permeance of the stucco layer. His
assertion that sand is responsible for permeability is contrary to
all the reading that I have done on the subject. John Straube's
permeability testing, which was done with strawbale building in mind,
shows that portland cement/sand mixtures are quite impermeable to
moisture. Adding a moderate amount of lime to the mix improves
permeability dramatically. Lime/sand plasters with no portland
cement are still more permeable.
A summary of the report is available on-line, and the details were in
a recent issue of The Last Straw (Spring 2001, #33, I believe).
Unfortunately, my copy is loaned out. If anyone has a copy handy,
post the numbers.
--On Friday, December 7, 2001 6:07 PM +1100 Frank & Ingrid
The important question really is, why do we need lime in a cement
render at all? There is only two reasons why we need to add lime.
1) without lime, the mix will not stay homogenous, it will separate
into its components [snip]
2) lime makes the mix creamier and easier to apply.
[snip] Using the smaller amount of lime, as
in the suggested mix of 1 bag cement, 1/4 bag lime and 25 shovels
of sand, will not diminish vapour permeability, because the sand is
responsible for that. Adding more lime may even result in less
permeability, because the lime particles are much finer than the
cement and sand particles, and clog the oxygen pockets of the
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