An Annular Patch, of Structolite and Flexible Grout

From the attic, I had seen a lot of light through a collar-hidden annulus of the wood stove flue ceiling penetration. Maximum gaps are about 5/8", and the ceiling drywall is fractured above face paper. I can't close the gap with flexible grout alone, over more than half the circumference. With this prep, the plaster securely bonds to the ceiling-set ring, and to drywall edges. Plasterweld would have almost no value here.