Cob: stucco question?

[jamie.ayres at m2.com]

Bearing all of this in mind, and that correctly constructed dry stone 
walling may be strong enough to hold up a cob house, would it be safe to 
say that you could simply use a softer (less pozzolan) lime mortar simply 
for weather proofing?

ie, instead of relying on an unknown factor for such an important job as 
holding the house up you could rely on the relatively safe dry stone 
construction and just point the wall with softer lime mortar that will 
prevent water ingress?

Jay


Jamie Ayres / M2 Communications Ltd / http://www.m2.com
Any views expressed in this e-mail are not necessarily the same as those 
of my employer.






"Mike Wye" <mike at mikewye.co.uk>
Sent by: owner-coblist at deatech.com
08/10/01 08:50 PM
Please respond to "Mike Wye"

 
        To:     "Charmaine R Taylor" <tms at northcoast.com>
        cc:     <coblist at deatech.com>
        Subject:        Re: Cob: stucco question?


Charmaine wrote,
> Mike, as a matter of course would you recommend hydraulic hydrated lime
> for all base stone work (foundations, etc?)  or is is priudent for owner
> builders of stone bases mix their own clay-ash pozzolans into reg lime?

Yes I would recommend an hydraulic lime mortar mix, by either route that I
outlined, if the stone foundations/plinth are going to be permanently 
damp.
How much pozzolan you add depends on how reactive the burnt clay portion 
is
and how much compressive strength you need in the bedding lime putty 
mortar.
The questions as to how a structure will resist earthquakes are academic 
if
there isn't sufficient compressive strength in the
foundations/footings/plinth to actually take the weight of cob above. Lime
putty mortars have relatively low compressive and tensile strengths to 
start
with. This is an advantage in many uses but not where the wall is
loadbearing and the pure lime putty mortar's compressive strength would be
insufficient. When adding a pozzolan its a question of how reactive is the
pozzolan and how much you choose to add.
At one end of the scale you can add a little pozzolan and barely change 
the
characteristics of the lime putty mortar or you can add so much pozzolan
that it reacts with every bit of available lime and makes a mortar as hard
as any concrete. Where you want to be on that scale depends also on the
location of your structure, the weight and thickness of the walls, climate
its exposed to, position of water table etc etc. Historically there were
wide differences in the lime/earth bedding mortar mixes used and their
characteristics.
We're aware of cases in England where people have used pure lime putty
mortars for building and rendering externally without being aware of the
history and existence of pozzolans and have "rediscovered" belatedly why
they form an important part of lime building practice when they find that
their pure lime mortar is still soft and uncarbonated months after
application and then fails in the first frosts of winter.
regards,
Mike
Mike Wye & Associates,
Traditional & Ecological Building Products
www.mikewye.co.uk
01409-281644


> Mike said:When working with a lime mortar in foundations and stone
> plinths
> they traditionally used an hydraulic lime mortar either by
> 1. adding a pozzolan to a lime putty/sand mix to set off some part
> of the mix prior to carbonation.
>     ( a pozzolan is a burnt clay such as volcanic ash, brick dust,
> crushed
> clay tiles etc which reacts with the lime to     form cementitious
> compounds)
> 2. using a natural hydraulic lime mixed with sand.
> Permanently damp stone structures don't allow sufficient air and
> hence
> carbon dioxide into the mortar matrix to carbonate the lime to the
> extent
> needed.
>




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