I had a look at Leca, but went with Vermiculite as it was easier to get hold of. Leca seemed to be okay to use, so long as it would be the sort made up of many sized particles to lock together. I thought Leca aould be better through not being so open to soaking up the water needed to create the cement slurry. It would soak it up, but not so dramatically. Thereby not needing so long to drive out the water later in the curing process. Its not tried, so i'd say you be the guinea pig. If you don't mind that, and can take a failure, then go for it and report your progress for the betterment of all. Leca would work well in an oven within an enclosure as loose fill insualtion. The weight issue is only an issue to the base structure, as any Pompeii or Vault oven gets stronger with more load, so long as the bottom cannot move outwards.
Hi, i'm Pietro. I've only recently joined this forum. I didn't get time to write my intoduction yet, but i'm just in the proccess of insulatin my concrete base and was looking to see wich is the best material here in the uk. About your discussion about leca i can tell u that i'm in an italian forum and there pretty much everibody uses leca under the layer of bricks. I don't know if i'm allowed to post a link