Re: SB: Fly ash

[John Salmen <terrain@shaw.ca>]

I'm curious to hear more about this as my natural inclination is in the case
of cement renders to eliminate the lime altogether and that is how I have
worked with it. Traditionally a 'good quality' 3 coat stucco did not use
lime at all or a very small percentage (10-15% of cement max) to improve
workability. You use a cement stucco if you want the benefits of a cement
stucco which are stiffness and impermeability. You don't want to detract
from these properties as you compromise the inherent properties of the basic
material and the wall. Too much lime in a cement mix increases permeability
but also can promote map cracking (which is also a water entry source) so
you end up with a potentially overly porous wall.

Now the properties of a lime render are different; flexibility and
permeability. Is less prone to cracking in stress and can be self-healing of
cracks - a whole different material with different benefits.

I'm definitely open to other opinions and information on this.

John


----- Original Message -----
From: "John Kingsley" <johnk@magma.ca>
To: "Lori Ann & Keith" <keith.la@sk.sympatico.ca>
Cc: "Strawbale List" <strawbale@crest.org>
Sent: Wednesday, June 26, 2002 9:21 AM
Subject: Re: SB: Fly ash


> Keith,
>
> > ps.  I tried asking before, but nobody gave me a solid answer....for the
> > recipe for portland cement stucco.  Is it 4:2:1  Sand:Portland cement
> > powder:lime, along with a squeeze of dish soap.....and water (of course)
to
> > desired consistency???  How does that sound?
>
> There are 2 ratios to worry about:
> 1)  sand : cementitious material (portland cement and lime)
> and
> 2)  portland cement : lime
>
> Ratio 1) affects cracking (among other things). For 1) you
> have 4 : 3 (2+1).  This is almost 1 : 1 - very, very rich!
> You need more sand or you will end up with lots of cracks
> (not to mention it will be more expensive).  More standard
> ratios are 3 or 4 : 1.  Suppose you choose 3.  Then your
> overall ratios become 9:2:1.  If you choose 4 your ratios are 12:2:1.
>
> Ratio 2) affects vapour permeability (among other things).
> For 2) you have 2 : 1.  I would suggest decreasing
> the cement.  There has been some research
> that indicates a better ratio is 1 : 1 in terms of
> vapour permeability.
>
> On my place I used a ratio of 6:1:1.
>
> cheers,
> John
>
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