Properties of perlite and waterglass plasters?

Hey, so I’m considering a cob project in a cold climate.
( https://permies.com/t/170675/Crazy-Cold-Climate-Cob-Vault#1340579 )

Obviously it needs insulation—real insulation, not just a lick and a promise—and obviously this is a problem other people have tried to solve before me. I want to avoid building out a second wall and infilling the cavity, because that would be a lot of extra work, and also because the cob will have a whole lot of curvature and sculptural work, which any insulation method would need to follow. I want to avoid sprayfoam, because it’s nasty, and also, it doesn’t breathe. No matter how many assurances I receive to the contrary, I don’t really believe strawbale is appropriate in our soggy climate, and also, those big blocky shapes will not marry well to the shape I need them to cover. A couple inches of slipstraw are not going to be warm enough.

So I am considering a super-thick perlite plaster, say 6”-8” (15-20 cm). Perlite coated with clay slip probably doesn’t hold together well enough to do that without being packed into an exterior form, and if I had to do that, I’d pretty much be back to the “second wall + cavity infill” thing. I hear interesting things about perlite or perlite-slip stabilized with waterglass (also spelled water glass), but I don’t know anyone who has tried that, and usually when I read about someone who does try it, it is for an application that will then be fired, which this would not be.

Anyone out there know anything about this? First, does waterglass stabilize a thick perlite plaster enough for it to adhere to itself and the wall while curing? Can it form or follow shapes? Does it set up hard enough to take some weathering and/or a final exterior plaster? As I understand it, perlite is rather strongly absorptive of moisture, and so although it would be protected from direct rainfall, I’d have some concern about it taking up ambient water vapor (of which we have plenty; this really is a soggy climate!) and either becoming heavy, softening a clay slip binder, or worse yet, softening the cob wall beneath. Conversely, waterglass might set up so waterproof as to inhibit the breathability of the walls. I have no idea how those two contradictory properties might interact. Do you?

Appreciate your help!

Waterglass must be fired to work.  That's all there is to it.  When first reading your post my very first reaction was "waterglass" on the outside of your cob, then fire it with one of those big weed killing torches.  I'm not sure of the mechanics, but I have seen the application done on a product called aircrete to make the aircrete fire resistant.  You might give it a try.  Don't overthink this stuff or else you will wind up paralyzed and do nothing.  Do small things first as an experiment to see if they work.  Perlite + slip I don't think is very strong so you need something like waterglass painted on then fired to keep the perlite/slip from falling apart.

Interesting idea. My experience with waterglass is that it forms a hard skin and I don't think it's breathable. What about a lime plaster with perlite mixed in? That would be easy to work with, and have some insulating qualities as well as a protective function.

Did a bit more digging on the internet. Looks like road stabilizing engineers have done a good bit of work with this, not with perlite but with clay soils, so some of what they discovered still applies. Found this in-depth white paper:

“Lime and Sodium Silicate Stabilization of Montmorillonite Clay Soil.”
Iowa State University, Clarence G. Ruff (Army Corps of Engineers) and Donald T. Davidson (Prof Civil Engineering).
Think this was published in 1963? Date not so well marked. Anyway, publicly available online.

To summarize what they found:
1) Yup, unfired waterglass by itself doesn't do jack.
2) But waterglass DOES react with hydrated lime to form a stable calcium silicate matrix. They were able to stabilize an expansive clay soil with a mix of 6% calcitic hydrated lime (dry weight) and 4% waterglass (preferred form sodium sesqui-silicate pentahydrate) and achieve a SATURATED compressive strength of 450 psi after a 28 day cure.* Which is to say they subjected their test blocks to full immersion and then tried to squish them. So that's pretty good, really.
* The lime+clay alone was 120 psi, saturated. The waterglass certainly made a difference, but only in these relatively high amounts.
3) Unfortunately, waterglass in the quantities needed for a whole building gets expensive fast. Looks like ordering it by the 1-ton pallet, it's $1700 + shipping, so roughly $1/lb (USA pricing, in case the imperial measurements hadn't already made that obvious). This plus lime plus clay slip puts you in a similar price range as Portland cement, for a poorer end strength. And it's not like waterglass or hydrated lime are THAT much less embodied energy. Probably not all that much more breathable either.

So it looks like it wouldn't be worth it as a stabilizer for the cob mix as a whole. It *might* still be worth it as a stabilizer for an 8" perlite-slip plaster, but it's certainly not a shoo-in.

The other thing the road engineer people have been getting excited about is "enzymes". This looks *very* interesting, possibly even as a stabilizer for the structural cob mix, but I haven't found much data or experience shared about it in a building context? I'd love to hear more if anyone knows anything. I see this thread: https://permies.com/t/161414/DIY-Permazyme-recipe-alternative but it kinda peters out with not much engagement. I've also run across papers suggesting the bond is not calcium carbonate but a cationic bonding of the clay, in which case it wouldn't work in sandy/gravelly soils at all. I'd guess that's a different enzyme mix than Lia researched. I'd guess there's a whole world of this we don't know much about, but I tried to contact a couple of these enzyme companies and so far, crickets. Haven't tried Perma-Zyme / Substrata yet because it ticked me off that they wouldn't share their whitepapers until you give them your personal info, but they might be the ones I'll have to ask. Unless any of you know more about it???

I have tried cow poo "litema" clay plasters, by the way. They DO hold up to driving rain, for at least 10 years, possibly precisely because of the enzymatic reaction with the clay. I don't have any info about their long-term effect structurally in a cob mix and I don't know if they'd help stick perlite to itself long-term. But for a finish plaster they work.

Also I see there's a pond-building thread on here which suggests that casein-rich milk-fed yellow calf poop works particularly well. Having tried to scrape that stuff off my boot, I don't doubt it! But you'd need a pretty darn big dairy to get enough of the stuff, and you'd have to convince/pay them to separate it out for you. Personally I try NOT to do business with 5000-cow mega dairies, but it would probably be magic. Stinky, stinky, *much* worse than adult cow poo, magic.