PUMICE vs SCORIA

[v12spirit]

Both pumice and scoria can be used as a "natural" insulator. This fact has been puzzling me as they are both available in some territories and can be used when it is hard or expensive to get state of the art insulators..

I've been searching the web to evaluate their insulation performance compared to each other as well as to some common insulators. The comparision was made using thermal conductivity [W/m C]:

Perlite: 0.031
Vermiculite: 0.065
Pumice: 0.07 - 0.1
Scoria: 0.15 - 0.3
Glass pearls: 0.18

What makes pumice twice better than scoria is that it is rich with silica, while scoria is rich with iron that it is sometimes worse than glass pearls. Still, vermiculite is fairly better than all, while perlite is the best.

Here are some physical differences between pumice and scoria that help recognize them when found in nature:

Pumice: Light color * Light weight * Floats in water * More Silica * Vesicular texture
Scoria: Dark color * Light weight * Sinks in water * More iron * Vesicular texture

Bibliography:

Thermal conductivity of materials

Pumice Insulation: A Practical Solution for Rural Geothermal Pipelines

Difference between pumice and scoria

[david s]

Re: PUMICE vs SCORIA

The differences between the insulation materials become far less so if you mix them with cement. e.g. a 10:1 vermicrete mix contains more mass from the cement than from the vermicrete. Therefore in practice there is little difference between the insulating capacity of say a perlcrete mix and a vermicrete mix. By the way I've found that a 50/50 perlite, vermiculite mix makes a far more workable mix than either of them alone when mixed with cement. Presumably because there is a variety of grain size.This means that you can reduce the amount of cement needed to bind it.

[v12spirit]

Re: PUMICE vs SCORIA

I find scoria in tons for free in my territory. I thought for a while that it was pumice, but once I recognized it was scoria I was discouraged to use it. Do you mean mixing with cement (in different-size aggregates) makes scoria close to pumice, or even close to perlite ?? I was about to abandon using scoria altogether and replace it with glass pearls.

[david s]

Re: PUMICE vs SCORIA

Do you mean mixing with cement (in different-size aggregates) makes scoria close to pumice, or even close to perlite ??

I can't say, I've not had experience with using either scoria or pumice. But I'd be confident in saying that neither would be as good an insulator, when mixed with cement than vermicrete or perlcrete.

[wotavidone]

Re: PUMICE vs SCORIA

I used coarse scoria aggregate to make the cement slab.
If you can get both fine and coarse scoria aggregates, you can make a slab with 0.17 W/m/K and 15 MPa strength.
The thermal conductivities for perlite and vermiculite listed on engineering toolbox are for the raw materials, they won't be as good when mixed with cement.

[v12spirit]

Re: PUMICE vs SCORIA

I used coarse scoria aggregate to make the cement slab.
If you can get both fine and coarse scoria aggregates, you can make a slab with 0.17 W/m/K and 15 MPa strength.
The thermal conductivities for perlite and vermiculite listed on engineering toolbox are for the raw materials, they won't be as good when mixed with cement.

wotavidone,
Thanks for the details. I was intended to use scoria, but after knowing that it was high with iron I thought of replacing it with glass pearls. However, the thermal conductivity of the B-2* formula sounds promising.

Actually it was on your post on Anny's "Rustic primitive materials" where I first heard of scoria and pumice as insulators. Interested to know more about how you used it, ratio, thickness, size of aggregates.. Any pictures are appreciated too.

Thank you for providing the table. I hope I've understood it, and hope you correct for me:

1-I don't understand what sand and SA stand for
2-According to B-2* formula I have to mix: 250:500:410:370 water:cement:coarse-scoria:fine-scoria
3-Slump stands for slab thickness

[wotavidone]

Re: PUMICE vs SCORIA

"Normal" concrete is large aggregate, say 12mm to 15mm gravel, fine sand, and cement.
A common mix here in Australia would be 3 gravel, 2 sand, 1 cement by volume.
Sand in the table is sand.
SA is shorthand for scoria aggregate.
Fine scoria, say less than 3 mm, is what you would use to substitute for sand.
I couldn't get that here, only the larger stuff, about 20mm size.
So my slab was 3 parts 20mm scoria, 2 parts sand, I part cement by volume.
I built my dome straight on that. I put the floor inside the dome on a 60mm layer of vermicrete for extra insulation.

Scoriacrete pictures here:
http://www.fornobravo.com/forum/43/o...tml#post127337

[wotavidone]

Re: PUMICE vs SCORIA

I wouldn't let the iron content of scoria worry you. Scoria is used in direct contact with gas flames in BBQ grills here in Australia, so I reckon it won't be a problem on the outside of your bricks.
It is a pretty reasonable insulator, though perhaps not quite as good as vermiculite or perlite based concrete.
Here in Australia it is not necessarily the cheapest, by any means, either.
But, if you have lots of it and it is cheap, it is an alternative worth considering.
With all these things, you must bear in mind that the cement added detracts from the insulating ability as DavidS has pointed out.
So it is up to you. If you can get it for free, and you can get it in the right size fractions, it will be good.
If you can get scoria that is, say, 5mm in size, and use it the same way vermiculite or perlite is used, then it will work well, in my opinion.
People put a 100mm perlcrete or vermicrete layer under their ovens. get a small sample of scoria and see if you can make a similar product from the scoria.
Remember its all about trapping as much unmoving air as possible.

[v12spirit]

Re: PUMICE vs SCORIA

Helpful information. Thanks wotavidone.

[wilgaburra]

Interesting info and a great forum. Here in the Southern Highlands, I am thinking of building my hearth as 50mm perl-crete as insulator beneath mixed grain scoria-crete with 50% of the cement replaced as hydrated lime as a bit of a heat sink,semi insulator under 1" fire bricks. Any thoughts on whether this would be effective as a hearth?

Commercial materials are very difficult to get locally, so I have resorted to salvaged red bricks for my dome, Perl-crete insulating layer then a home mixed cob layer with wild sourced clay, topped off with a limey render sealing layer.
Does this sound like a recipe for disaster?

[Petter]

The insulating property is mainly dependent on the density in this temperature range. Aim for equal to slight less than perlite / vermiculite concrete.

Using splits as the floor is possible If you let it recharge between pizzas. For continuous baking, 2" is recommend.

[UtahBeehiver]

Here is a chart showing K values (thermal conductivity) of vcrete and recommended ratios depending on location, ie under floor or over dome.

[wilgaburra]

Thanks for the really useful info. I, think I'll head to the beach and collect some pumice to incorporate, as well. For me it is about doing it as cheaply and effectively as possible and trying not to learn too much the hard way.
Does anyone one know if klinker (fired clay) pavers would do under my 1" fire bricks as a heat sink?
Fire bricks are really expensive here. I figure if they crack underneath the fire bricks it wouldn't affect the cooking surface too much.
Happy Baking

[UtahBeehiver]

Do you mean thermal mass rather than a heat sink?

[wilgaburra]

I guess so, as in performing the same function as the fire bricks on the floor, above the insulating layer.