Conventional concrete is one example of the insufficiently refractory stone types. The stone itself can withstand heat well, but its ability to soak up water is problematic. Concreted outdoor fireplaces are exposed to wind and weather and, of course, rain. The moisture seeps into the concrete - only to evaporate or even to fizzle out as soon as the fireplace is lit again at the next opportunity. The result is that the concrete bursts and concrete blocks (as well as Ytong blocks, however, because of the air they contain) can even explode in the worst case. For the same reason, the foundation of a fireplace should not be concreted either.
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Concrete fireplace ideas
If you still want to use concrete for your fireplace, the stone must be protected from rain and moisture. This can be done, for example, by a canopy, but also by a cover with a water-impermeable material. If these requirements are met, you may not necessarily be able to line the fireplace yourself with concrete blocks, but you can use them for the border. For example, disused manhole rings of various sizes are very suitable for this. You are absolutely on the safe side if you use fireclay bricks for your fireplace, because these are completely fire-proof. Fireclay bricks are specially made for lining chimneys and blast furnaces and must therefore be able to withstand high, direct heat.
There is also the option of using refractory instead of conventional concrete. So-called refractory concrete can withstand temperatures from 1,100 ° C to 2,000 ° C and is therefore perfect for concreting a fireplace. By the way, if you are planning a brick fireplace, the use of fireproof mortar is recommended - otherwise the wall could not withstand the great stress caused by the heat.
Instead of concrete, baked stones can be used excellently for the construction of a fireplace, for example clinker or bricks. These stones are also inexpensive to purchase and naturally fire-resistant.