On Dec 9, 1:09 pm, beecrofter wrote:

I think you are probably a little out of your depth on this project.
If this gap is between two items of masonry then you need to use
mortar.
If this gap is subject to the fire itself you will need to use
fireclay based mortar.
If this gap is between the hearth and the surrounding floor you may
have other options to provide some flexability as the two different
materials move at different rates.

The gap is between the brick hearth and the surrounding wood floor and
is not subject to the fire.