I have a custom-made outdoor fireplace where the sides and top are lined with firebrick. One of the bricks on the "ceiling" has come out, and I'd like to just reattach it somehow. The ceiling if flat at about a 20 degree angle to horizontal. I tried using heavy-duty construction adhesive on the top and mortar on the sides, but it didn't last a day.
As a fallback, I might just use a long piece of angle-bracket to hold the brick in place. The bricks are not visible or easily reachable from outside so I'm not worried about it getting touched, but I'd prefer to just reattach the brick if possible.
What's the best adhesive to use on brick that's exposed to winter cold air and extreme heat?