Cast floor insulation in one or many pieces?

[Petter]

I'm planing on how to make the floor insulation. I will make a mix of perlite and Sodium Silicate. However, I can either cast indivudual
bricks in any shape or make a large cast (circular, 140 cm / 55" diameter and 15 cm / 6 inch thick).

Thing is that the castings need to cure in some 150 C to be water resistent and gain strength. I can make a temporary kiln (have lots of IFB or even in the sauna, above 100 C will likeky do). Smaller bricks mean more work but can be cured on the bbq or in the oven.

My biggest consern is what is easiest to get level and to build on. One large cast will maybe twist slighty and crack in few pieces once loaded with the dome. It cant go anywhere though. Indivudual bricks involve cutting the edge bricks to fit the circle and its difficult to get them perfectly rectangular and minimize gaps.

What would you do? Input is welcome.

[UtahBeehiver]

It will be interesting to see this process, Sodium Silicate (water glass) has been mention in the past but never documented on success or not so keep the blog in the loop.It may have a lower K value than v or pcrete and compressive strength only draw back I see for the regular builder is the 150C heat cure. As far as cutting the bricks to fit inside the dome (more precision cutting), you can always set the dome on the bricks where the cuts can be a little rougher or proud since insulation will cover the floor and dome interface.

[Petter]

Thanks, The research I've found implies that the density, i.e. conduction trumphs everything (radiation, convection) below some 500-750 C. The key is thus to find a light weight binder, does not matter which - as long as it adds a minimum of weight. Sodium Silicate is true refractory, has good strength and can be applied very thin on every surface to make them stickey.

I have tested to make test cupoons in washed milk packages and cured the mini brick in the oven. It works very well and is simple. I m confident in the material, just need to get the ratio right for a density of 0.18 +-0.2 g/cm3.

The pressure from the dome is silly low, I even doubt a binder is needed at all. Just compacted perlite would do fine if it was constrained on the walls and surface. The binder is more there to prevent particle movement than to add strength.

i could have bought CaSi, but this is more fun in a scientific point of view...

[david s]

I’ll be interested in your experiments. For the underfloor insulation you need to get the strength up to around 100 psi

The problem with using cement as a binder is that it reduces insulation capacity. I tried using cellulose powder which swells up when water is added and it makes a paste. It works quite well and when the water evaporates results in a low density but quite firm product. The problem is it’s cost. It ends up being significantly more expensive than cement. I suspect sodium silicate will be dearer too.

[Petter]

I'm too looking forward to the evaluation. My initial guess is that 0.7 +-0.3 Sodium Silicate (35% solid) of the perlite weight should do.

Pure cellulose is as you say quite expensive. You only need something to make porosity. Could be sawdust too for example though it needs to be fired to get the organic component out. Actual, anything organic less than 5 mm with a good size distribution would do just as fine.

I make my own Sodium Silicate from lye and cat sand (silica lumps). It costs maybe $1-2 / kg. If purchased as finished product, it is about $10 / kg in Sweden, and that, as you say would make the ceramic blanket solution cheaper.

[Petter]

Do you have any reference to the 100 psi requirement? I get only a few psi when calculating dome pressure. My guess is that "spongeyness" is of greater importance. You don't want the casting to flex.

[david s]

Hi Petter,

Because of the problem of cement adding density and reducing insulation value I began looking for a lightweight/cheap binder which led me to cellulose fibre. The cellulose fibre will act ok as a glue to hold the vermiculite together, but does nothing to add strength, So whilst it might be ok for over the dome would be inadequate for underfloor insulation. I gave up on it because it wasn't cost effective, but then I got led to replacing the vermiculite with an alternative.and started investigating aerated concrete. You may be interested in this following thread where the foamcrete can make a surprisingly good product that is cheap and less water absorbent than standard vermicrete whilst also having the ability to adjust for different strength requirements.
https://community.fornobravo.com/for...rete#post16031
Dave

[david s]

Regarding underfloor strength, take a look at the attached table. an 8:1 vermicrete is so weak it will break apart in your hands. Fine for over the top of the oven but nowhere strong enough for under the floor IMO

.I agree that a higher density is usually accompanied with a corresponding reduction of insulation as the table shows, however in the case of insulating ceramic fibre blanket the higher density blanket has a better insulation value at all temperatures than one with a lower density.

[Petter]

Hi Petter,

Because of the problem of cement adding density and reducing insulation value I began looking for a lightweight/cheap binder which led me to cellulose fibre. The cellulose fibre will act ok as a glue to hold the vermiculite together, but does nothing to add strength, So whilst it might be ok for over the dome would be inadequate for underfloor insulation. I gave up on it because it wasn't cost effective, but then I got led to replacing the vermiculite with an alternative.and started investigating aerated concrete. You may be interested in this following thread where the foamcrete can make a surprisingly good product that is cheap and less water absorbent than standard vermicrete whilst also having the ability to adjust for different strength requirements.
https://community.fornobravo.com/for...rete#post16031
Dave

Thanks Dave, I gave your link a read. Interesting. Did you ever try with CAC or did you measure the density of your casts? I found a few data points in the literature on FoamCrete and they fit the trend curve of Density and Thermal Conductivity very well. It indicates that the foam was well dispersed with no to few larger agglomerates of cement.

V-crete in Red and FoamCrete in Green.


Unfortunately, I have already bought the perlite. However - I find this very good for the common knowledge and therefore worthy a discussion.

[Petter]

Regarding underfloor strength, take a look at the attached table. an 8:1 vermicrete is so weak it will break apart in your hands. Fine for over the top of the oven but nowhere strong enough for under the floor IMO

.I agree that a higher density is usually accompanied with a corresponding reduction of insulation as the table shows, however in the case of insulating ceramic fibre blanket the higher density blanket has a better insulation value at all temperatures than one with a lower density.

Hmmm, I see your point. What I don't understand is how the strength correlate to the touch. I have for example wetted perlite with only water and compacted it. According to literature, it should have a compression strength of about 50 PSI if I remember correctly. When I push my finger into it, it feels strong and responsive. I have also mixed Perlite and Sodium Silicate at about 3:1 in solid weight ratio and it also feels pretty strong though fragile.

Soon, I will do some testing with an indent like Vickers or Brinell on Perlite - SS castings. I have found three ways to measure compression strength of porous ceramics.

1) Like regular concrete. Uniaxial pressing until failure of a cylinder with a height : diameter ratio of about 3:1. Fix area, measure force to calculate pressure.
2) Indent with a spherical or conical object with fix load. Measure indent size and calculate pressure.
3) Pressure required for 5 % compression. Fix area, measure force to achieve 5 % compression.

For the DIY:er, I think #2 is easiest. Load a boule ball with a known weight and measure the indent.

I get the dome pressure to about 2 PSI for basically any type of dome. Look here:

https://en.wikipedia.org/wiki/Factor_of_safety

A factor of 2 is commonly used in buildings. Therefore - it must be something weird going on with the type of measurement or values recorded if 100 PSI is necessary for sufficient strength.

/Petter

[david s]

As builders have found a 5:1 by volume not weight, vermiculite or perlite to cement ratio is required for underfloor insulation and 10:1 for over the dome, I suggest you cast the two ratios as samples so you can compare their strength with your sodium silicate vermiculite brew.