Crumbly Vermiculite/Portland Cement

[bcovey]

Hi. I poured an insulated hearth, following the instructions provided by Forno Bravo. I have noticed that around the edges, there is some crumbling of the vermiculite. How solid should this top layer of the slab be? I do not want to build my hearth on a slab that will not hold up. unfortunately, i had not noticed the fb board that is sold by forno bravo until after i had poured the insulating cement. I am wondering if I should tear it out and go with the board....
Any thoughts based on experience out there?

Thanks,
Brian Covey

[emsoven]

brian i put a course of bricks on the concrete hearth to hold the vermiculite in it will take the weight of the oven

[bcovey]

thanks, for your help.

[emsoven]

no props mate enjoy your building

[Lill Dave]

I had the same experience that you did. My solution was to retain the existing insulation and pour a 1.5 in. refractory cement surface with steel mesh reinforcement ( I used a product called bull fencing 3/16 in. diameter ) this gives you a nice smoth surface for your bricks and also with the 4 in. refactory surface you can cook bread.

By retain what I did is pour a 2 in.x5.5 in. cement wall to retain the perlite insulation, the harth slab was large enough for the wall.

[david s]

Wait as long as you can to let the vermicrete layer dry as much as possible before covering it over. It is really hard to eliminate all that water in the vermicrete layer later.

[mattyg]

My local refractory expert told me you can use rock salt to level off over the vermicrete if you need to, it is a really good and cheap insulator, he also said you can use it instead of vermicrete. good luck.

[david s]

I would think that salt will dissolve if it gets wet and also will contribute to corrosion problems. I think you are better off using 50/50 sand, fireclay if you just want to get a good level surface to put the floor on. Add some lime to it if you want it stronger and waterproof.

[Tscarborough]

I left the form on my perlcrete slab until I was finished with the oven to prevent damage to the edges.

[benguilford]

I found that due to the porosity of standard vermiculite and perlite that when it is mixed with Portland Cement to form an 'insulating concrete' that an excessive amount of water is needed in the mix. Given that in standard concrete, the more water you add the weaker the resulting concrete becomes, it seems logical that these vermiculite mixes are going to be inherently weak due to the amount of water that is needed to 'wet out' the mix (as the water is soaked into the aggregate, unlike normal concrete).

To overcome this I found a supplier who makes 'Sealed Perlite', Perlite treated such that it has no pores and does not absorb water. I made my dome insulation layer using the 5:1 ratio with this Perlite and used chicken wire for the reinforcement, and even at only 50mm thick it feels as solid as any concrete I've ever seen. My neighbor used normal vermiculite with the same ratio and ended up with a crumbly mess that he broke off and threw away!

Has anyone else ever used the sealed perlite before?