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I am having a go at making a pizza oven. I know very little about DIY and as such, this project is more about the experience (not the final result!).

I purchased some fire bricks but I did not expect each brick to weigh 9 kg!!

I've laid the bricks on the floor to get an idea of dimensions of the oven and to get the shape right! I've even created an wooden arch so I can 'mount' the bricks onto the arch, cement the bricks together, remove the wooden arch and hopefully will be left with a standing arch!

enter image description here

My concern is, will a heat resistant mortar like this be strong enough to keep this structure this way, or are 9kg bricks not meant to be built up this way? Each brick is 5cm high, 23cm wide and 19cm deep

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    Arches have been constructed with much heavier bricks than those. Just have to get it right.
    – Solar Mike
    Nov 9 '20 at 18:50
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    the mortar is not what keeps the arch stable the bricks should really be wedge shaped so that they touch over a large surface ... put another row of bricks in between the bricks in the picture to give them support in the middle
    – jsotola
    Nov 9 '20 at 20:35
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    Might be worth mentioning that (from the correct suppliers) there is a wide variety in not only shapes, but also density and temperature rating of firebrick. The very heavy ones insulate poorly but may offer more wear resistance and brute strength. The lighter ones insulate very well but don't take much abuse. The very light ones are very easily cut.
    – Ecnerwal
    Nov 9 '20 at 21:42
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    As a mention, those brick you have there are meant for the floor of the oven, the "lugged back" is intended to be bedded in the mortar, the opposite side is for the surface, The edges you are working with to create the arch is not intended for a finished face since the curves creating the lugs will not give clean lines.
    – Jack
    Nov 10 '20 at 4:43
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    What brick will not do is bounce back the heat, it will absorb it and radiate it back. That I believe is the reason of the appeal of the brick oven. Even though the pizza needs to be turned while in the oven, the brick heat up and help cook the pizza more evenly than placing it by the fire. To answer your question, there are much better choices for the shape of the brick that you are choosing to use, but if you have a bounty of them, yes they will work for the floor, walls and arch. I myself would get a right angle grinder with a diamond blade, and recut the shape to better suit the purpose.
    – Jack
    Jan 12 at 5:01
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Part of the reason that an arch of this sort works is the re-distribution of force (gravity) throughout the curve. As Solar Mike suggests, you just have to get it right. This would entail ensuring to have the inner contact points of the bricks carefully aligned to each other.

The result of such an alignment is a segmented circle transferring the load from the top of the arch to the sides. It's important to secure them well (your mortar helps). My malware program blocked the web link for the mortar, but be sure to follow any curing instructions to prevent the mortar from premature failure. Such process involves slow temperature increase with holds for specific time periods until ultimate cure is reached.

Having the temporary structure you describe also permits you to pre-load the build, reducing the chance of a shift during installation causing a mis-alignment. The mortar will also provide some distribution of force, but not the majority.

If you are comfortable that the compressive strength of the bricks at the circumference of the arch is sufficient to handle the weight, you should have no problems. My experience with firebrick is that they are insanely light and equally insanely fragile. I suspect the product you found is heavier because it's quite a bit stronger than the stuff I used.

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I don't think weight - at 9 kg/brick - is an issue in this particular situation. Masonry is heavy by design. In another case I might consider weight to be an issue, just not the main thing here.

The key issue here is the shape. The way to do this is to use bricks that are designed for arches. Something like this Googled set of arch bricks for sale.

You can also cut rectangular bricks to turn them into wedges, but since this is a common problem, wedge-shaped bricks are readily available and a whole lot easier to work with.

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  • Really...weight is not the problem??? How do you account for horizontal thrust? You’re not suggesting there is no horizontal thrust are you? (All the shape of each individual block does is help create more or less mortar between each block. The Romans realized this and invented the “keystone”.) Yes, a company has created a nicely shaped set of blocks that make a pretty arched opening...but the arch has to be their size only. The biggest problem is getting heat rated mortar. Otherwise, it will all come apart when heated...regardless the shape of the blocks.
    – Lee Sam
    Nov 9 '20 at 20:15
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    @LeeSam It isn't that weight does not need to be considered, it does. And it has to be heat rated mortar (as well as heat rated bricks). But if you have simple rectangles turned into an arch as shown by OP, you end up with 1/2 the structure being mortar. If you have appropriately shaped bricks then only a little bit is mortar, which makes a big difference. There have been huge structures made without any mortar at all. Nov 9 '20 at 20:38
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    You either buy arch brick, or you cut brick into arch wedges yourself. There's also the fact that the bricks can be arranged several ways, and the way they are presently arranged is among the worst ways to to arrange rectangular bricks for an arch if not cutting them. Also pretty wasteful if cutting them. Multiple layers of bricks turned one of the shorter ways (if the overall thickness is needed) would have a much lower brick to mortar ratio. But cutting (or purchasing) wedges and not making the wedges from mortar is better.
    – Ecnerwal
    Nov 9 '20 at 20:46
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    1 - An arch (like that sample picture, can't tell 100% for sure) may have very slight wedges that are not immediately obvious; 2 - The more bricks, the smaller the angle between bricks - with a small enough angle, the mortar can span it OK. Nov 9 '20 at 21:23
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    Look more carefully at the image. I think those are actually tapered brick, but at that scale (arch to brick size) it's subtle. The basic issue is not "can you assemble bricks and mortar into an arch like that" - you can. However, bricks are much stronger than mortar, and you are putting all the geometry that makes it an arch on the mortar joints. When you've fired the oven a few times and it's gone through a winter or two, it may start dropping bricks as mortar joints crack and there's nothing really holding it together - while an arch of wedged bricks will stand up without any mortar.
    – Ecnerwal
    Nov 9 '20 at 21:29

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