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GSBN: Lime and Cement Plaster

<x-charset iso-8859-1>Greetings Kelly and all,

not being an advocate of cement render at all, I still have to agree with
Kelly's plasterer contractor. He is basically using the traditional cement
render mixture. I have rendered with that recipe in Europe and Australia

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 (sand on
the bottom/cement in the middle/and water on top) and needs constant mixing
to avoid this separation.  2) lime makes the mix creamier and easier to
apply. A small amount of lime, such as suggested by Kelly's plasterer, will
actually perform these tasks already. 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.

If you are worried about a hot mix there are actually some clay products you
can use (instead of lime) to keep the mix from separating into its
components. In Australia these are called Plaster Master, Builders Clay or
Renderers Clay and are usually available in 25 kilo bags. This clay will
help keep your mix homogenous and you don't have to worry about lime burning
anything. It costs about the same than hydrated lime (or even less) and is
easy to work with. You can use it as the same mix (as above), that will work
fine and will not affect permeability unfavourable.

We have actually not rendered with cement for several years, thankfully clay
and lime renders are very well received in Australia. I used to hate working
with cement render, it feels dead and is not very pleasant on the skin no
matter how much lime is in it.

I hope I have not confused anyone and good luck with the project.

Regards, Frank Thomas

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