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Old 12-07-2009, 07:41 AM
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Default Cold temps, unfinished oven, not cured yet

I know that freshly set mortar wants a good 48 hours above 40 degrees, and that is easily achieved with a halogen work light and a cover (a tarp). However, I haven't been running a halogen all the time, only after I recently mortar new bricks in. Is my entire oven at risk because it hasn't actually been cured by real heat yet, a fire? I mean, it went down to 'teens last night. What does that mean for uncured mortar that may be several days to weeks old but isn't truly cured yet?
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Old 12-07-2009, 08:47 AM
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Default Re: Cold temps, unfinished oven, not cured yet

are you using a cementitious mortar like Heatstop or homebrew, or an air drying/heat curing premixed mortar like Sairset?
I don't know about the premixed mortars for sure, but I'd guess that as long as they're dry enough not to have water expanding in them, they're probably fine in the cold (you could look this up in the mfgr. data for whatever product you have). If you're using a cement based mortar, "curing by real heat" doesn't make any difference, AFIK. What matters there is the strength of the concrete bond, and in that case I wouldn't want any freezing for at least a week, but longer is always better, since the hydration reaction will continue for as long as there is water available to drive it, and if those water molecules are frozen they a. can't react, and b. might/will weaken the developing concrete matrix by expanding when frozen.
At this point I really wouldn't worry about whatever you've already completed, but it does take a loooong time for all that mud (either type) in the joints to dry out so it might not be a bad idea to keep the whole deal above freezing from now on.
I guess I'd see what others think though, too. I'm just giving you the "what I would do."
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Old 12-07-2009, 08:52 AM
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Default Re: Cold temps, unfinished oven, not cured yet

I'm using homebrew 3:2:1:1. Ugh, the oven has definitely gone way below freezing for several hours...multiple times. Like I said, I keep fresh mortar warm for a day or two, but after that I try to salvage some of my electricity bill.

I'll bear it in mind.

Thanks.
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Old 12-07-2009, 04:31 PM
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Default Re: Cold temps, unfinished oven, not cured yet

In general, so long as you get 24 hours of hydration for cementious mortars you are good. As a rule, most of the required hydration is done by then, and it takes liquid water for crystallization, i.e.expansion.

The general specification for normal masonry construction is 40 degrees and rising with no temps below 40 within 24 hours. Masonry can be done under lower temperatures, but it requires special means and methods.

Refractory cement is a different animal and I can't speak to it.
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Old 12-07-2009, 04:35 PM
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Default Re: Cold temps, unfinished oven, not cured yet

Okay, but I should make sure things stay above forty for a good day or so after every mortar job. That's really easy with halogen worklight inside the oven...I'm not dead certain what temps I'm getting on the outer surface of the bricks, whether the halogen on the inside and the tarp is keeping the outer surface sufficiently warm. I have an IR thermometer though. I'll keep a close eye on things.

Thanks.
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Old 12-07-2009, 04:44 PM
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Default Re: Cold temps, unfinished oven, not cured yet

And as an FYI, cementious mortar and concrete are exothermic, meaning that the process itself generates heat.
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