trader_4 wrote:
On Monday, March 28, 2016 at 9:23:34 PM UTC-4, wrote:

Maybe pour some of that leveling compound in there.
It has a low enough viscosity that it should go into the gap and it
has pretty good compressive strength.

Or epoxy. Epoxy is thin enough that it will run in and fill a gap
like that.

Epoxy can be of various consistencies. "Thin" epoxy would run in quite a
way, "thick" epoxy not much.