A technician suggested we seal our firebox with high-temperature mortar since allegedly the existing mortar used around the bricks in the firebox is regular mortar (according to him a crack already developed in the mortar between some of the bricks). I'm kind of skeptical since I don't really see the crack where he pointed. That's why I'm wondering, is there any way to really test if the mortar is high temperature or not (aside from continuing to burn wood in the fireplace and checking once in a while to see if cracks start to develop).
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2What kind of "technician" was this? Was he a skilled mason with many years experience or a young kid with general HVAC (or other) training doing things "by the book"? These factors would weigh heavily (in my mind) about the value of his suggestion.– FreeManCommented Jan 24, 2023 at 12:57
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1not sure what his background is, but he is a technician with a licensed and well-rated fireplace service company and seemed knowledgeable regarding the topic overall (to me at least, which is not saying much, since i have zero experience with this).– l tCommented Jan 26, 2023 at 21:15
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3 Answers
I'm a bricklayer and I've never seen a fireplace built with fire resistant mortar, that is usually used in small amounts in specific areas very close to the actual fire, like bedding fire bricks, refractory clay items, or pizza ovens and the like.
A regular, fairly hard mortar, is usually sufficient for the main walling.
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Sorry, I should have been more clear, I meant the mortar around the bricks in the fire box, not the entire fireplace. i'll update the question.– l tCommented Jan 26, 2023 at 21:12
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Ah yes, now I read it again I can see that. To be fair though, no mortar can cope 100% if there is a lot of expansion and contraction going on through heat. So the cracks aren't conclusive that the mortar is 'regular'. Like you say, keep an eye on it and scrape out and repair any cracks with a little bag of 'heat resistant' mortar.– handymanCommented Jan 28, 2023 at 1:22
From what I have seen, ht mortar is “harder”, more brittle and does not get damaged from scraping easily.
Ordinary mortar can be scratched and you get “sand” particles coming off easily.
This might just be a location thing as mixes vary…
answered Jan 24, 2023 at 7:07
from experience as a non-expert in mortar technologies...
we used this 5 years ago as mortar for bricks in the fireplace with a nominal heating power of 22 kW:
it holds very well. the thickness of the mortar is 1 cm. it developed cracks however those are on the surface only (we checked it by scraping it on multiple spots)
