soft grey breeze blocks yup good for breaking up and packing round workpiece when using a brazing hearthI'm sure I've read somewhere: Thermolites are fine to use (don't explode) and work well
nightstorage heater bricks are free if u know anyone getting rid of oneI'm also after something of the like for the same reason. I wondered if clay house bricks would be ok as they are fired in a kilm when manufactured.
Both would be cheap enough to test with.
The other option is vermiculite bricks but they are not cheap.
I think with a gas forge you need to be looking at refractory type bricks that reflect the heat.
With other sorts of bricks they absorb the heat. You would be using the heat from the forge to get them up to temperature. You might have to use a shed load of gas and wait a while to get the forge up to a usefull temperature.
DON'T use normal portland cement concrete - it goes BANG when the water in it turns to steam.
concrete doesnt need to be damp to explode heat it dry and it willIn my experience, normal ceramics such as house bricks or ceramic chimney pots work OK for a while - two or three heats.
Then they crack.
They're also not very insulating and drain a LOT of the heat from the interior.
If you back them with insulating material, they crack sooner.
DON'T use normal portland cement concrete - it goes BANG when the water in it turns to steam.
I'm sure I've read somewhere: Thermolites are fine to use (don't explode) and work well
Well they're certainly USELESS for building walls ! I've yet to see a wall built using thermolite foamed blocks that doesn't have at least one crack in in. Also you can't fix anything to them without it coming loose.
P.S. I know of someone who built a forge using plain old dirt for an insulator. Fired it a few times without it failing.
On the concrete front, concrete has water chemically bonded to it, so it doesn't matter how dry it seems, it still has that present. Until you heat it. Same reason you shouldn't pour molten metal near concrete.