Step 6. Reassemble Stones With Concrete
I then reassembled the whole thing while setting the stones in concrete, using standard concrete mix with small gravel in the mix. The mix with gravel in it (concrete mix) is much stronger than the mix without (mortar mix), but it’s not as easy to get into every nook and cranny.
In order to make the concrete workable, I mixed it to a drier consistency than normal, almost like mud. When I could grab a handful of concrete and make a mudball out of it that would hold its shape, I stopped adding water.
I jammed concrete all along the interior of the circle, fully supporting any open edges under the stones and filling all the spaces between them.
Then just start stacking. Use the natural shapes of the stones to arrange them so that they “lock” together with each other. Your structure should easily and firmly stay in place with no concrete. Then pack concrete into the gaps and joints to fully lock the assembly.