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Re: GSBN: Lime and Cement Plaster



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.

Derek Roff

--On Friday, December 7, 2001 6:07 PM +1100 Frank & Ingrid strawbales@... wrote:

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
render.

Derek Roff
Language Learning Center, Ortega Hall Rm 129, University of New Mexico
Albuquerque, NM 87131  505/277-7368 fax 505/277-3885
Internet: derek@...