Preventing Freezing Temperatures from Ruining Wet Mortar

We finally started building our Mattone Barile pizza oven last week. We got 4 rows of fire brick on both sides then had company several days. We started again today and laid the rest of the barrel brick. It was a sunny day in the 50’s. I checked the weather later and seen it is suppose to get to 28 tonight. We wrapped the oven with insulation and put a tarp over it. I put a couple gallon containers of hot water inside the oven, however the foam form will block a lot of that heat. I tried to look up how mortar does with freezing temperature and the sites I found said it wouldn’t set right. They said it would crumb and not have structural strength. The sites were talking about bricking houses or trying to sell me a masonry heating blanket. However, if there is going to be a problem with the mortar, I would rather fix it now. Do I need to redo the 9 rows we did today? Is there any way to see if the mortar is ok at this time?

Hi Melva, and welcome to the BrickWood forum.

I think you’re going to be okay. Ordinary (Type “S”) mortar would not do well if the temperature stayed below freezing for a while. You’re using a different mix, and while it’s not a good idea to stay out in the cold and set in brick while the temperature is freezing, much of the setting it is going to do happened within the first hour. Unlike Type S, the high heat mortar actually depends on firing for its final set, so that’s not going to happen until your curing fires.

Knowing that it’s going to be cold for the next few days, and of course with late fall temperatures finally settling in, I’d call it a season. But you should be fine with what you already laid in.

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One surefire way to keep your oven warm and cozy in the winter weather is by placing a small space heater inside the oven and placing a tarp over the oven.

This drying / heating method is great for slowly drying a wet oven (kids + water hose = wet oven). And it can also prevent the moisture in your mortar from freezing.

Best of all - it’s not too expensive (about $20 for a simple heater at True Value or ACE). Just make sure you put the heater on the LOW setting - not High. You’re not trying to cure the oven… just trying to keep her warm.

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How long do you need to keep the heater in the oven? We ended up getting a week of mild weather, high 74 to low 37; so we finished the oven. There will be 5 days above freezing after it is completed. Then when it is time to take the foam out to let the oven finish drying, it looks like we will have a few nights that drops into the upper twenties. We can definitely use a heater during that time. At that point will we be able to fire cure the oven if there are nights that drop into the 20’s?

If you finished the barrel, and you are in the process (or have completed) adding insulation and stucco, you don’t have to wait to take out your form. You can do that while you are finishing the insulation work, which will give you a jump start on drying. (And for that matter, you can use @BrickWood 's tip about the small space heater during those nights, even if the temperatures are above freezing.)

The form holds up the bricks as you form them into an arch. The mortar gains the initial strength it needs to fix them in place within a day (though I wouldn’t go jumping on top of it), and at that point the form has done its job. Slip it out so that air curing can continue while you are finishing the insulation and stucco.

Once your stucco has cured and you have sealed it (I presume you’re painting it for this season), you can go ahead and do curing fires. It doesn’t matter if the nighttime temperatures are going down into the 20s at that point. Your firebrick (1) will be well-insulated from the outside; and (2) don’t really care because you are slowly fire-curing the mortar. Just do what is recommended for every oven during the 6 day initial curing process: low and slow.

The idea here is both to drive out additional moisture from the oven cavity, and to fire-cure the high heat mortar. It requires this to become rock-hard and fix the firebrick in place.

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Thanks. We took the form out today and are putting in the electric heater tonight.