Has anyone used clay pebbles for RMH mass?

Hi. I was given a lot of clay pebbles that were used as insulation. I'm wondering if I can use them for a RMH?

Maggie

Hi Maggie;
I have not heard of anyone using them as a mass.
A short answer is yes, you could use them, although there would be insulating air gaps similar to a stone pebble mass.
If you want a solid mass system,  I recommend using large stones and packing your clay pebbles tightly around them, leaving no air pockets.
Most new RMHs are a bell stratification system rather than a solid piped mass.

If you can post a picture that would help.
Baked clay has its uses but mainly for making insulating cement or lightweight insulating bricks.

Hi.
Thanks for replying. Here's a photo. Now I'm wondering if these used with sand would resolve the gaps between the pebbles.

Maggie

Hi Maggie,
with the picture it becomes much clearer. Those pebbles are expanded clay, they have air bubbles inside of themselves. So apart from the air pockets they will have in between them, they also have air pockets inside. As you said, they're used for insulation and are rather light weight. For me that rues them out as a thermal mass.
The builder that I'm learning from uses them with a clay slip as the insulating foundation and if the bench/mass needs to be insulated from an outside wall.
What did you intend on using them for?

They were given to me. I initially thought to explore using them for an RMH. If they can be repurposed into bricks, that would be cool. I'll never have enough potted plants to use this amount up.

Yep that is Leca Horticultural, Clay Pebbles, Lava rock.
They are used for lightweight building blocks as well  as for horticultural use, bonsai soil, cat tiller and other uses.
You can use them as Benjamin suggests, mixed with Portland cement 6-1 ratio, they can be used as a structural,  insulating base mix. Or mixed with fondu cement for higher temperature areas but, not very suitable for use in the flame path.

Fox,
 with your back ground, you may an answer-

I would only attempt with a really good vibrating table, and a super big and very controllable kiln. I keep running into a fellow  (online) that insist a mixture of fire clay at 15% and 85% pumice (fine ground-much smaller than the clay pellets shown)  will produce the best of both worlds.  A TUFF and insulative brick or casting.

Although he doesn't say, I am presuming the clay is mixed like we would do in ceramic's  (a pourable thick soup)  And then add in the pumice to the point where it is simply coated and considered very dry.  

I have always presumed it would NOT be tuff, but being I have both kiln and table available, I have been pondering. My biggest obstacle is our pumice is almost always lave stone of 1-2" size so not suitable for casting.

Thoughts?

Scott, sorry I dont have much knowledge using pumice and fire clay.
Very few materials can survive naked flame for repeated heat cycles  but you can buy insulating mix, I bought a bag a few years ago, it did not state the exact ingredients on the bag but, like most insulating fire brick, it dried pure white.  
To be honest I find vermiculite board is just so easy to use and it can last as well as any other insulating product, I tend to stick with that.
It is a shame vermiculite is not readily available in the states, I believe it has something to do with a belief it contains asbestos but, it fact it has no know heath hazards and is even used as a mineral supplement in cattle feed.
In recent years throughout Europe, box stoves have had to comply with strict emission laws and most box stoves are now factory lined with vermiculite board.
I think this use has lead to a better quality board becoming readily available.
Anyway, I have used a lot of Leca clay and especially Moler clay cat litter, mixed with portland cement, as a solid insulating foundation for under the fire brick cooking base of pizza ovens.
Although portland cement is said to crumble at 275-300c in in fact last very well under the fire brick where topside brick surface temps can reach 500c and underside where it touches the Moler Portland mix, 400c .
I have read huge amounts about various DIY insulating mixes, some folk use salt or polystyrene balls that melt out making air spaces and lots of other ideas but I dont think many work in the flame path of a rocket stove.