Not to be mistaken for thermite which is not a suitable building material!!
I was curious to know about these blocks as they are a easy to get hold of in the UK, they are made from recycled material, are a lot lighter than refractory bricks and can be carved using a knife or hand saw.
What is thermalite?
“Up to 80% of the raw material used in the manufacture of Thermalite is pulverised fuel ash (PFA), a stable `by-product’ of coal-burning power stations. This recycled material is mixed with sand, cement, lime, aluminium powder, together with processed waste and water, to produce a range of blocks noted for their high thermal insulation.”
i.e. These blocks are not just aerated gypsum.
The experts view
What the expert said when I asked him about using these for furnace applications:
“Thermalite blocks in domestic situations are usually serviceable after a fire of approx 900-1000 Celsius, however, exposure is only usually for about 30 minutes and the blocks will have a protection layer of plaster or plasterboard. The protection of your insulating layer is all important, however, asthe blocks dryout they will shrink and crack if they are held togetherby conventional mortar, it would be better to oven dry the blocks priorto incorporation into a structure (to limit their shrinkage potential)and to investigate if a special mortar is required to glue themtogether. We cannot give a definative answer as to wether the blocks will crack due to thermal shock as we have no experience of this type of block used in this application.”
Charles from Home Foundry and Casting group suggested that I do a “Myth Busters” on this material.
Blow torch test
A fine flamed butane gas blow torch was held on the block.
The result was that the block glowed red in the area of the flame whilst the flame was applied, the surface of the block was cool a distance of approx 30mm away. When the flame was removed the block rapidly cooled and could be touched with a damp finger seconds after the removal. The estimated temperature for these kind of torches is upto 2400°F / 1300°C.
Blow torch scratch test
In order to determine if the block was softer when hot, a metal rod was used to scratch it. The block is quite soft when cold and the rod produced a medium sized scratch with very little effort. When heated, the block was slightly softer but this was only on the surface for about 2-3mm.
These blocks are quite soft and very brittle, even small bumps and scrapes will cause pieces to be broken off. The blocks can be cut with a hand saw and shaped with tools such as a surform. Don’t try to snap a partially cut block as it will break along a completely different line. The blocks are very light and the example shown below can be easily lifted with one hand.
An model makers hearth for small brazing and heat treatment jobs, made from a thermalite block and patched back together with fire cement.
I see no reason why this material can’t be used for a hearth as above, with no additional treatment, time will tell if it is too soft or if it generates too much dust. It should also be ok as a surround for an aluminium melting furnace. Given that it’s a bit soft it needs to be protected with a layer of a harder refractory material on the inside and something such as brick or metal on the outside. For higher temperatures needed for steel and bronze further experimentation is advised to see if it meets your needs.
These results are not endorsed by Hanson the make of these blocks.
My experience of Thermalite leads me to much the same conclusions re Kiln use. I am currently building a 2kw glass annealing kiln and intend to use Thermalite for the outer 1″ of the 3″ insulation, the other 2″ being (expensive)Ceramic Fiberboard.
Though the normal working temp will be 550C we are hoping to go as high as 1100C for some purposes.
I bought thermalite blocks to build a ceramic kiln in my garden, insulating the outside and providing a cool wall. With no kiln shelf I tried some of the blocks inside the kiln to support my pots. The pots were bisque fired between 891 and 1025 celsius coming out pink and hardened as expected. The thermalite had changed also: they were distorted, pink in places where the heat was fiercest, and with interesting cracks running through the body. They had become fragile and easily broken in my fingers.
Since this experiment I have invested in some proper firebricks which will withstand 1600 celsius so suitable for bisque and glazing.
Hope that info helps!
That’s really helpful, thanks for describing your results.
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I would like to build a wood fired pizza oven in my garden using leftover Thermalit blocks. Is this feasible??
Hi John, I’d be wary of this idea for a couple of reasons. The first is that thermlit blocks often generate a lot of dust which would not be nice if it got all over your pizzas. The second is that the bricks in the oven store and radiate the heat giving a constant temperature. That would not happen with these kind of blocks. But you could use the blocks for the supports and base of the oven. Good luck with your project.