Lime plaster mixes

Hi,  another lime plaster question!  I heard you should make your mix 2 weeks prior to application.  Does this apply if using mature lime putty or can i just mix and go?  Thanks.

I think the aging only applies to the lime putty, and you can mix in the sand and use it immediately. I put dry hydrated lime in a plastic bucket to soak for a few months, then mixed it with sand and applied the same day, and it seems in very good condition a month later.

Thanks Glenn.

Hydrated lime is lime that started as "hot lime" that has been slaked, then dried. It is not a setting material when water is added because the chemical reaction has already occurred from the kiln, then the slaking and drying process. This lime is good for binding other materials together and making them more plastic but not really a strong material by itself. Hot lime is slaked by sifting lime powder into water carefully (with eye protection!) and letting the chemical reaction commence until is is fully worked and excess water still stands on the surface.The lime is allowed to settle into a putty and completely hydrate before it is used. Hot lime is a setting material, it will take 50 years to reach it full strength but is sufficiently strong for construction even  after the initial set. Hydrated lime putty can be mixed 50/50 with gauging plaster for smooth plaster work, gauging plaster is a gypsum based plaster. I have worked as a plasterer for 25 years and as a hod-carrier for another 10. Most of that work was commercial work so most all materials were processed and bagged. I am now retired and really enjoy reading, on this site, about old/new methods of earthen plasters and would like to work with them. I don't see where gypsum is discussed or used much by permies which makes me wonder if it might not seal off walls from water vapor on the inside just as cement stucco seals from the outside, or maybe there is another reason.

Arlie Grunseth wrote:... I don't see where gypsum is discussed or used much by permies which makes me wonder if it might not seal off walls from water vapor on the inside just as cement stucco seals from the outside, or maybe there is another reason.

Mostly it's because people dont understand gypsum.

It's a graet material if you know how to use it.  It's about the same pH as human skin, very low embodied energy as it calcines at low temp.  Not so great in wet areas.  It does breathe.  When mixed 1:1 with Lime can achieve something like 256psi compressive strength versus gypsum or lime alone yeilding about 75psi.  I could go on and on

Hi everyone.

It seems "hot lime" and hydrated are completely different beasts regarding plaster behavior.
For a lath and plaster (wood lath and lime plaster) ceiling, what should be used ?
Hot slaked aged for some months or hydrated lime aged for weeks ?

I'm asking because hot lime is not easy to obtain and slaking large quantities can be a problem.
On the other hand, hydrated lime in bags is only 80% purity ...

In any case, i know how much plaster volume i need.
But how much powder lime ?
If hydrated, i calculate 1/3 hydrated powder lime / sand and age that powder underwater for weeks.
If i use lime paste, the ratio should be about 1/2.5.

But if i can use hot lime, how much do i need to slake ?
I could not find any relevant info on this.

Regarding gypsum, i want to know more about how it can improve lime plaster and maybe how can it be used by itself.
Application is for a one level home that has a cold roof.
The insulation will be on the attic floor (mineral / rock wool).
The ceiling of the rooms should be pretty airtight and not very permeable so vapor can't go in large quantities and condense inside the insulation.
The walls will be lime plaster (over strawbale) and that has to have a good connection with the ceiling to avoid air leaks.

Lime plaster is preferable since it can handle humidity variations well and also some water if the roof somehow fails and it has a leak.

Alternative would be drywall but it's going to be terrible to keep air tight and will fall apart in case of water.

If I were to plaster a wood lathe ceiling I would choose gypsum scratch and brown coats and a lime and gauging plaster finish. Gypsum plaster can be mixed with sand or perlite and is easy to work with. It will set slowly unless you 'gauge' it up with agircultural gypsum, a hand full per mixed bucket is good for 45 minute set, more or less.

Thanks Arlie.
I really appreciate your experience.

You said gypsum scratch and brown coats.
What kind of gypsum ?
I have access to bagged gypsum (not agricutural).
It comes in roughly 2 flavours: builder and tinkerer / modeler
Builder's gypsum i assume is raw material, without additives, setting time between 7-15min.
The others have setting time of 30' to 1hr.
I prefer the raw gypsum, even if it's harder to work with (fast setting time) as i'm more certain there are no nasty additives.

Regarding lime, what kind ?
Bagged, hydrated ?
If so, it's available everywhere in 80% purity.
I would prefer 90% purity but that's harder to come by.

Regarding the 3 layers, why do you have this specific preference?
I'm genuinely curious.
The scratch and brown is just pure gypsum or mixed with sand ?
And why not a gypsum / lime mix (maybe 50/50) ?

And the finish, just lime / gypsum without sand ?

Also, on every bagged gypsum product (builder's or gauging plaster) it's specified to not use on wooden substrate.
Do you know why it's so ?
Is there some chemical / mechanical (moisture absorbtion) going on ?
Or it's just that gypsum can't grip a solid wooden sheet (like plywood or OSB) ?

Thanks a lot for taking your time to answer.
This is really important for me.
The walls have to be more permeable than the ceiling.
For the strawbale walls i will definitely use lime plaster because of the vapor permeance and since it's anti-fungal.
The ceiling needs to be much less permeable since warm air that contains humidity will rise to the top.
If it manages to get thru the plaster and into the rockwool insulation, it will condense and cause trouble.
That condensate will reduce insulation performance and at some point it will drip down to the plaster.
And that's not good.

In your experience, what's the behavior of this system if something happens to the roof and water drips on the insulation and finds it's way to the plaster ?

PS
As anecdote, i remember some years ago i visited a local village and had a talk to the resident priest.
He told me that when they built the church in 1910 or something like that, they had wooden formwork supported by some steel framing for the cupola.
They poured lime (i guess it was actually lime mortar) in that formwork.
The interesting thing is that they used some gypsum in the mixture, as a fast setting agent, so they could remove the formwork in about a week and, because gypsum's fast set, it had enough structural strength for the whole thing to self support.
If it would have been lime alone, that formwork would have been there for months at a minimum.