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.
Hi mourits, Thanks for taking the time to research/write this excellent instructable, Ive followed these instructions and your tips at the end and built a chiminea of my own. It looks great and works a treat! Ive had some roaring fires in it and spent many nights by it. Its standing up the fire well and looks like I’m going to get many years of use out of it. My hands and arms looked a right state by the end of the project but it was well worth the pain! Everybody who visits asks where they can get one. Thanks again. Awesome Instructable!
That is a totally awesome product! Much neater than then mine actually ;-) I had a very hard time getting the wiremesh to behave the wanted to. My chimenea is being looked after by a friend in Wellington while we are in Amsterdam, but when we return I will start the contruction of a wood fired pizza oven, using the same technique.<br> <br> Do you have any hints or additions to add to the instructable to improve on what I found out?<br> <br> cheers,<br> <br> Matt<br>
<p>Kei te pai e hoa! Mean chiminea bro.</p>
<p>thanks bro! Awaiting pictures of yours ;) </p>
<p>KiWi !!</p>
No 'terra cotta' for this kid...any comment(s) on mixing in refractory clay in a proper ratio to the normal concrete mix ?<br>
I have not tried myself yet,although i am about to for a pizza oven project.<br><br>you might want to read up on these links:<br>plus the forum at fornobravo is always a good place to ask questions.<br><br>http://www.fornobravo.com/pompeii_oven/high_heat_mortar.html<br>http://www.traditionaloven.com/articles/101/what-is-fire-clay-and-where-to-get-it<br>http://www.traditionaloven.com/tutorials/concrete.html
How has it held up to repeated use?
O, by &quot;brittle&quot; I mean that small bits come off if you poke and rub on it, not that it is in danger of shattering... <br><br>But I think that's because my mix was very rough the begin with (let's of large grit in the sand)
Absolutely. Although it is quite brittle the high amount of chicken wire makes it really really strong. Of course we haven't used it daily. (or weekly for that matter)
Just a reminder, you need to use multiple(3+) layers if you are using chicken wire. My first attempt was an epic failure due to hastily using 2 layers. I think that a heavier gauge wire that others have used would be more effective. <br><br>I will post a full project including failed design once mine is complete. I will say that a larger design like mine places a ton of weight on the wire structure, 160lbs of cement on two layers doesn't make much sense now that I think about it.<br><br>AL
One thing you didn't mention was the 10:1 (opening size compared to chimney size) ratio needed for a proper fireplace draw.<br><br>I used a refractory cement mixture for mine to avoid it breaking down. My ratio was 3 sand, 2 plastic cement (I was told this was stickier), 1 fire clay. I forgot to buy lime and when I called back to the shop was told it was not necessary. What is the purpose of lime anyway?<br><br>Completed 3 days ago. Can't wait to give it a try!
Good point about the ration between the chimney and the opening. I was only slightly aware of this and on this project have not taken it into account at all.<br><br>My experience is that the chimney could be wider or, that their should be more roof-spanning-space between the top of the opening and the bottom of the chimney.<br><br>I'm working on a wood fired pizza oven at the moment and that design is very clear that the height of the opening (or bottom of the chimney) should be set at 63% of the height of the dome. So there is quite a bit of theory about this.<br><br>Not sure about the lime. In clay ovens it is used to make a waterproof outer layer. Not sure what it does in cement.<br><br>Good luck!
I only had enough chicken wire for 3 layers except around openings where I made sure to have 5 layers. My base was rectangular (stand from an abandoned coin operated newspaper dispenser.) From the corners I created a frame of 1/4 inch metal rods coming up and a little past a 1/4 inch metal rod ring that defined the basic size of my chimney. My chicken wire pattern was random as I just cut and then wired in pieces as the structure seemed to need it for strength. There ended up being about 1/2 inch of space between the inside and outside layers of chicken wire. Filling that space and putting a layer over both sides made my cement about 1 1/4 inch (3 cm) thick. Do most ferrocement structures have thinner walls?<br><br>I'm wondering if the walls will become hot and radiate the heat or if the fire clay (refractory cement mixture) will prevent this effect.<br><br>As it is, with my chiminea standing 4 feet 7 inches (140 cm) and cement even making up the chimney, it must weigh somewhere around 165 lbs (75 kg). How much did yours end up weighing?<br><br>Veel geluk met uw pizza oven!
Cheers Matt, The main thing was your advice on keeping it simple, the frame started as two rolls of wire mesh which I then bound together with garden wire. One thing that may have made it easier was the fact I used a smaller size mesh, the holes where only 16mm so the cement stayed in the frame quite well. I was worried the walls of the belly might suffer from the heat of the fire so I cheated a little and bought a small tub of ready mixed fire cement. I used it to tile the inside with broken paving slabs which I had laying around. This seemed the easiest way to bulk up the thickness of the walls. The base is filled with sand and more broken slabs. I painted the outside with exterior wood varnish to finish it off and it&rsquo;s stood up fairly well, the heat has worn it off in a couple of places but it&rsquo;s a only a 2 minute job to touch it up. Other than the above I followed your instructions and it turned out really well. The pizza oven sound really cool, please make sure you upload some photos Thanks again Will
Wicked cool....looks like a great project for my daughter and I to tackle this summer....thanks for making it LOOK easy....
lovely actually, my brother and I with friends and insane family made a castle facade on a building&nbsp;using this technique. It was to be a bar and was quite intresting, well except for theneighbors who hated it. with a name like Port'o'call and a carribean themed insides who couldn't &nbsp;like it .&nbsp; arrgghh<br /> <br /> If you make another use &quot;white Cement&quot; it is used around pools and must have alumina in it it resists fire better. If you have a guiotine for paper you can chop fibergalss insulation to 1/4-1/2 inch threads and mix in the mix to add strength. <br /> <br /> if you fill the bottom with sand and line the bottom with splits ( thin ner bricks then standard bricks, you can fire it for hours then after the booze works its magic AND&nbsp;the 3 hour fire finishes up, you can bake a few small pizza's or pita and nan. Then drink more beer, and restoke the fire. If itis wide enough and the pie small (for 1-2 people only) you get some coals going brush to sides and back and slide the pie right in front of it. An edgeless &nbsp;cookie sheet will work as a peel. put the pie right on the bricks. For low use ovens (or this) plain vanilla red clay bricks work fine.<br /> <br /> I&nbsp;enjoyed this a lot and may yet build a brick oven soon. Forno Bravo is great and is not cheap, err rather inexpensive&nbsp; or affordable.<br /> <br /> <br /> a health warning,do not use galvanized metal with direct , near direct contact with fire. It can release zinc fumes that can destroy lung tissue.<br /> <br /> using this technique in old issues of Mother Earth News a person built a small out building like your Chimenea<br /> <br /> thanks again for the intructable , this was fun and a trip down memory lane for me.
Thanks for the awesome reply. I would love to share a beer with the spark master that created a castle facade that was so bodacious that it pissed of the neighbours ;-)<br />
I&nbsp;will see if big brother has a pic of it after completeing . It was very hokey (childish silly) and the door was made from planks they use to hold open the streets (4-5 inches thick and 8-12 inches wide), bullet proof unless your toten armour piercing&nbsp; or 50 cal. Some of the local &quot;stand up fellas&quot;&nbsp;tried shooting it, didn't work.&nbsp; I&nbsp;did not create it, my brother did we just helped. we made wire frames cut up styro back boards (reduced weight and materials/cost used a ram set (22 cal) to shoot the silly frames onto the brick walls then stucco's them same technique as this thingie. Then it was painted and &quot;marble ized&quot; to make it appear granite like. <br /> <br /> Inside was cool too, trump l'oi pics with mirrors set next them in the walls. On a good night heavy party types would get wacked out and walk into the mirrors or doors (percieved) only to bounce off, Easy way to assume a drinker was cut off for the evening.<br /> <br /> see ya
<em>congrats for a wonderfull &amp;highly detailed awesome instructable very informative especialy about ferocement as i heard about it used in boats some years ago hope you have lots of fun&amp;hot times with it &nbsp;</em>
I did.&nbsp; Unfortunately I had to leave it behind as we are currently back in Amsterdam (where two friends half already commissioned a chimenea for their back yard)<br /> <br /> So, perhaps during the summer months a version 2.0.<br />
Very good work!<br /> <br /> I'm impressed for the use of wigs... <br /> <br /> Years ago I maked a &quot;bell roof chimney&quot; for my BBQ, and I used expanded metal (&quot;metal desplegado&quot; in Spanish) covered with Plasticor, a precast masonry mix.<br /> <br />
Thanks!<br /> <br /> What is &quot;expanded metal&quot; and why would you use it in this case?&nbsp; I tried googleing it but could not find a quick answer.<br />
Expanded metal is simply a heavy sheet of metal that has a series of small cuts made in it and is stretched to create small diamond shaped spaces in the metal.&nbsp; It is normally used in welded contructions like gates, burglar proof screens and in the heaver gages, I've even seen steps made from the stuff.<br /> Go to http://www.mechanicalmetals.com/expandedmetal.html to see what the stuff looks like and the sizes out there.<br /> <br /> For something like this fire place, you have the choice of either welding it into shape, or wiring it together before you cover it with your selected cement.&nbsp; Be aware that this stuff being steel it will <strong>RUST</strong> as heat aids oxidation, even in the concrete.&nbsp; I suggest once you have the form shaped, spray it with a rust proof BBQ high temp paint to seal the steel, and then cement over this (It will adhere just fine.)<br />
Expanded metal is <strong>sometimes </strong>a heavy sheet, other times is a <strong>very thin</strong> sheet... <br /> It can be made of distinct metals: iron, steel, aluminum, brass, etc.<br />
Correct,but I covered only the heavier steel metal steel type as this is what would be used in such a fireplace.&nbsp; I'm a mechanical designer by profession, and I have used this material in many systems and parts I have designed over the years.&nbsp; The heavier types are more suitable for the cement armature (wire frame work used to give the cement shaping and integrity) for their strength and resistance to heat, especially over time.<br /> <br /> This material can be expensive, but with a bit of forethought, you can use only what you need and save money.&nbsp; By the way, with good planning and some rough measurements, most shops can cut this stuff to your sizes easily and cheaply and you save yourself a lot of work.<br />
http://images.google.com.ar/images?q=%22metal+expandido%22&amp;oe=utf-8&amp;rls=org.mozilla:es-AR:official&amp;client=firefox-a&amp;um=1&amp;ie=UTF-8&amp;ei=yZtnS-6eE82vtgfVypHRBg&amp;sa=X&amp;oi=image_result_group&amp;ct=title&amp;resnum=4&amp;ved=0CCUQsAQwAw<br />
That looks very interesting. I've added this comment to the final thoughts:<br /> <blockquote> <div>&quot;<a href="https://www.instructables.com/member/rimar2000/" rel="nofollow">rimar2000 </a>suggests using <a href="http://en.wikipedia.org/wiki/Expanded_metal" rel="nofollow">&quot;expanded metal&quot;</a> sheets instead of chicken wire. It is apparently cheaper and from the looks of it it is ticker and the gaps are smaller so you might need less layers.&quot;</div> </blockquote>Thanks for the tip.<br />
There are many classes of&nbsp; &quot;metal desplegado&quot;: thickness, size of the holes, metal (aluminum, iron, etc). The most common is that is used for plasterers to make ceilings. That is that I used.<br />
I used it because it is cheaper than chicken wire.<br />
that is totally WILD!&nbsp; If I didn't already have a killer burn barrel Id definately be making one like yours !
Awesome instructable. Love the detail and tips.<br />
I am impressed, that's a great build.<br /> <br /> L<br />

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More by thijsatteiltje:The Silicon Graphics Refrigerator Project Outdoor fire place (Chimenea) from ferrocement 
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