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I mistakenly did not add any stones to my foundation mix and now have a mortar foundation. It is 1.6m x 1.6m x 0.2m with reinforced mesh. I am creating a brick pizza oven with hollow concrete blocks built on top of the foundations as the frame and a soon to be concrete top 100mm thick, before laying the brick work for the pizza oven, roughly using 150 firebricks. My main question is will the mortar foundation be strong enough or have I messed up so badly to the point I need to start again??? Any help would be greatly appreciated.

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    Did you use mortar mix, or did you use "sand and cement powder", mixed onsite? If so, in what proportion? Mortar is not just "concrete without big stones" so it makes a difference...
    – Ecnerwal
    Aug 10 '21 at 0:30
  • I did a 5 to 1 mix of sharp sand to general cement. The base has hollow concrete blocks sat on it around the edge, layed 5cm in from the edge . It is 4 blocks high. I was then going to have a 100mm concrete top but use perlite as an insulation base before laying the firebrick so heat transfer isn't a problem. I'm just concerned with all this weight will the foundation fail and cause huge problems meaning I'm wasting money as the whole structure will break. Aug 10 '21 at 5:13
  • So, that's more grout than mortar, but a bit on the low-cement side for standard grout (sand has more surface area than stone, so it's usually mixed more like 3-1.) However, your actual stresses are likely to be quite undemanding - let me see if I can find a prior pizza oven slab question where the load was calculated for an example. Here we go: diy.stackexchange.com/a/218863/18078
    – Ecnerwal
    Aug 10 '21 at 11:07
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As per https://diy.stackexchange.com/a/218863/18078 your slab loading is likely to be pretty mild, even with a slightly smaller slab and more bricks.

Your grout mixture is a bit on the low cement side, but if you didn't mix it very wet as grout typically is for pumping, the lower water content will add some relative strength back. Sand has more surface area than stone, so grout is usually mixed more like 3-1 rather than your 5-1 ratio. Per the above answer, total loading was on the order of 1/1000 of typical concrete strength, (kiloPascals for load, megaPascals for concrete) so even with some loss of strength from a low cement mix, almost certainly more than plenty. Your grout strength might be "only 50-100 times more than needed" rather than 1000...

This linked article was looking at "mortar as grout" - other than the lime addition, the sand/cement ratio is similar to your mix, and compressive strengths were 1400 PSI to more than 2000 PSI (where concrete is typically 3000 PSI and up) suggesting that your mix is not likely to be worrisomely weak for its job. 300-500 times stronger than it needs to be, perhaps.

More than likely the thickness of your slab and providing mesh will result in it being perfectly adequate, but if you'd like to hedge your bets, replace your bottom layer of "hollow concrete blocks" with the open-ended variant of hollow concrete blocks used to make a "bond-beam" and place steel reinforcing bar and concrete in them before building up from there with regular hollow concrete blocks. That should add considerable strength to the structure where it counts, and should cost a good deal less than replacing the whole slab.

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At 750psi, mortar has 1/4 the strength of concrete.

Also, your base at 0.2m (almost 8in) is quite thick which offsets the lower strength. Perhaps this could be the equivalent of a 2in concrete slab.

The mesh helps should cracks develop.

But fire bricks are very light, and I don't think you are going to stand on it.

Whether it will hold depends mostly on what's underneath... what is it? Compacted gravel? Organic sludge?

The pad you poured is not large, so torque is low and this helps if the ground gives.

Since the bedding will determine if it's strong enough, adding a layer over top to make up for the mortar is likely not going to help.

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  • No, no, no. I’ve got to downvote this answer because it makes a couple of incorrect statements: 1) Don’t add rebar into the mortar foundation, it will only transfer the heat from the oven deeper into your base and create even more problems, 2) Don’t add a concrete slab on top of your base. Concrete will transfer heat into your base and cause the base to warp, sag, crack, etc.
    – Lee Sam
    Aug 10 '21 at 1:00
  • @LeeSam I appreciate all feedback, but I never suggested he add rebar: only that any additional layering would only strengthen the base IF it were properly bonded. A second layer, e.g. of concrete on top, to make up for the mortar won't do anything for strength so don't bother. Interesting point on the heat transfer though. Anyway, I'll edit to be more clear.
    – P2000
    Aug 10 '21 at 1:14
  • Adding weight of a concrete slab that can deform due to heat seems like the wrong thing and will only accelerate the issue. Plus adding steel mesh that will heat up and cause cracking of the slab.
    – Lee Sam
    Aug 10 '21 at 1:40
  • I did a 200mm base below the foundation using type 1 mot. I used a whacker plate to compact it before pouring the foundation on top. Aug 10 '21 at 5:20
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    @LeeSam OP isn't planning to add a concrete slab right on top of the foundation. The slab goes on top of the walls made of concrete blocks (which in turn sit on the foundation). The slab has to be there to serve as the base for the actual refractory brickwork. So the actual oven will be separated from the foundation by the concrete block walls and a lot of air, which provides plenty of insulation so that heat transfer into the foundation is not a concern at all.
    – TooTea
    Aug 10 '21 at 7:08
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You have a bigger problem than making sure your foundation is strong enough.

You should have used heat resistant mortar or “refractory mortar” often used with fire brick or flue liners.

When you heat that oven up it will make your mortar “flow” and cause deformation of your foundation. You can add a layer to isolate the foundation from the oven or remove some of your mortar sufficiently to add fire brick / refractory mortar.

To answer your question, yes, the mortar will be strong enough to support a pizza oven, (depending on the type of mortar you used) but it won’t last (cracking, sagging, etc.) as soon as you fire up the oven. This article explains it better.

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    Typical home-scale pizza oven the fire is nowhere near the foundation. What is described: Foundation, blocks on top to a comfortable level, 100mm concrete slab on those, then firebrick for the fire. You are addressing a situation that's not at all the situation being described.
    – Ecnerwal
    Aug 10 '21 at 1:52

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