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Before starting, lay down a drop cloth to protect the floor; the cloth also makes clean-up easy.
Mix the mortar. The mix should be thick but still workable, with a consistency similar to peanut butter. Allow the mixture time to rest and soak up all the water. Refer to the resources below for more info about mixing mortar.
While the mortar is resting, prep the hole by scraping the area clean and removing any loose debris.
Fill the hole with the mortar, using the trowel to compact the mixture tightly. Allow the mortar to cure for at least 24 hours before testing it.
Clear out the broken mortar and loose debris.
Spread the mortar evenly around the area, filling in the gaps. The ash dump should rest flush with the floor when it’s in the closed position (Image 1).
When the ash dump is in place, tap it with the butt end of the trowel and scrape off excess mortar; fill in any joints, making sure all surfaces clean (Image 2).
Note: Depending on the mortar, it may help to dampen the old mortar to help it get a better grip with the new.