Instructables

Outdoor fire place (Chimenea) from ferrocement

Step 2: Choose the cement mix

The cement mix
First of, I have used ordinary ready-made mortar mix – the kind used for regular brickwork, that consists of just Portland cement and sand. I recommend using the ready made mix, for a small batch it is not worth the hassle to mix your own from cement and sand. The ratio of the ingredients is very important in making concrete so you don't want to mess that up. Ready made mix also contains some additives that make it easier to work with.

Tip: Do not get ready-made 'concrete' mix! (as I did by accident). Concrete mix contains gravel (duh) and that makes it impossible to smear the cement between the chicken wire mesh or to end up with a smooth finish. Because I wanted to get started on Christmas day and therefor could not get any other mix I ended up sieving the gravel out of  my dry cement with an artifact I had left over from making a WiFi antenna. ;-)

The colour
There is a wide range of colours available for concrete, based on iron oxide, and it's very cheap.
Originally I wanted to colour the chimenea in a terra cotta orange-reddish-brown like its Mexican brethren but Ablaze only had black available and as I was impatient I went with that. I would go for terra cotta the next time though. Portland cement is gray and will influence the final colour. Use white cement and sand if you want the colour to come out better.

Concrete and heat
I did not use any 'fire resistant' cement mix mainly because cvdukes over at the Forno Bravo Forum for Wood-fired-pizza-oven geeks told me it would be ok. The guy that works at Ablaze, a huge firewood and fireplace supplier nearby also thought that for this particular project I would probably be fine with regular cement mix.

However: Fire can destroy concrete. When concrete hardens ('cures') it does not actually 'dry'. The water in the mixture does not evaporate but reacts with the solids in Portland cement and remains locked inside the concrete and is a vital element to give strength to the structure. This is why you need to keep concrete wet while it 'dries' and cover it with plastic to prevent evaporation.

A hot fire removes the water and can destroy the integrity of the concrete structure.

Air pockets and cheap wigs
Besides that there may be air pockets left trapped inside the cured concrete. When heated, the air will expand and can cause many 'micro explosions' that weaken the concrete from the inside.
I read somewhere that to fire-proof concrete buildings they add plastic fibers to the mix that will melt in case of a fire and create little channels to let the expanding air escape. I decided to go with this idea and bought two black wigs at the $2 Shop which I will cut in small pieces and add to the mix.

An alternative mix

For future reference sake i will give you the recipe for an alternative and more 'fire resistant' concrete mix although cvdukes warned that it might be to brittle to hold up a structure on it's own. I think that since the chicken wire adds so much strength to the structure I would be interested to try it next time. On the other hand, if my chimenea with regular cement mix holds up after repeated thermal abuse there seems to be little reason to spend the extra cash and effort.

Fire-resistant cement, called 'refractory mortar' (NOTE: not used in this project!) is sold as "RefMix" or to make it yourself use:
  • 1 part Portland cement
  • 3 parts sand
  • 1 part lime
  • 1 part fireclay
For more information on this read the High Heat Mortar Primer at Forno Bravo.
 
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