High Heat Mortar Mix (3 Questions)

I have 3 questions concerning the the HiTemp mortar mix.

1.) Although the plans say not to fill in voids in the interior with HT mortar mix as it will just ultimately fall out. I unfortunately have some large gaps in the ceiling interior. I apparently did not pack it well enough when installing those rows.

2.) From other upthread posts I’ve read about water getting into the oven. Should I proactively laid a bead of the high temp calk around the the perimeter (3 sides of the hearth connection to the fire brick and also to the perimeter of the chimney before installing the final brick finish?

3.) Lastly on the front of the oven I left a 1" reveal of fiber blanket on the fire brick per the plans. I am applying a fan like finish to the front façade using brick splits. Should I be filling that gap with the hiTemp mixture before attempting to install the splits?

This is where you apply the high-temp silicone mortar patch (caulk). Do not try to apply high-temp mortar—it will fall out at the worst possible moment. Unless you’re spinning up a Fred Flintstone special, you want to avoid that.

EDIT: Dave is patching an unfired installation. So he can in fact use the high temp mortar, with a dropcloth underneath to protect the hearth and a water spray in the joints prior to pointing in the mortar. If his arch were already fired, then he’d want to apply a silicone fireplace mortar product.

Here’s the link to the details.

Yes. That certainly won’t hurt anything, and especially around the chimney base it is a good idea.

Yes. You want that sealed before you apply finishes. As we’ve previously discussed, it should not be regular mortar because it’s part of the “envelope.”

You’re making great progress, Dave!

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Thanks Matt!..don’t work so hard!