can it be a surface that single wall pipe passes though with, say, 3 inches of clearance around the pipe? as I understand it, the stuff is cement and sand (so far so good) with some fibers and additives (hmmmmm).
I just cut some cement board (durock) not to long ago and was a bit surprised to find it is bonded together using what looks like nylon lath. If that is indeed the case (someone on the board can confirm), i doubt it is
fire resistant. I would think naively that the nylon would melt and the board would then just come apart. Hardibacker might work, though, it does not seem to have this nylon reinforcement.
Of course you could always take a plumber's torch to a scrap sheet of this stuff and do your own flamability test. Won't give you a definitive answer, but should give you a pretty good idea. I quickly determined using this method that fiberglass insulation was not acceptable. Neither was the foil backed insulation board (which the stove store sold me saying that is what they use for covering the damper area after putting a insert!)
I used a peice of Wonderboard (cement board) as a shield placed in the doorway of my stove while cleaning the glass. Used only when there was very little embers left like at beginning of the day. The board would get very hot, and after a while the fibers were getting quite brown and smelled. So, I'd say No, it's not fireproof, but resistant. My problems were solved by covering the piece with aluminum foil. now it doesn't get hot at all.
If using it this way, I would be careful and check local building codes or call Durock or wonderboard for their official word on this.
basically the single wall pipe passes though a thimble bricked out from the masonry chimney. the chimney is interior and was closed in by plaster wall on all sides with framing aroun it. to do it (make the header, brick out the thimble, etc) I had to take down the plaster wall on one side. So I want to replace the wall, all clearances to stove, veritcal run of pipe, etc are fine. The surface of the bricked out area is 1 inch sunken from the original wall surface. I will replace plaster with either hardibackerboard and skimcoat it, or perlite-mixed plaster on wire lath. nothing will come in contact with the pipe, its just a matter of how close I can get. 2 inches? 10 inches? the larger the clearance, the bigger the collar I have to put on it to cover the gap, the sillier it looks. don't want to do anything dangerous, just thught that if it is the equivalent of masonry, there's no reason that it can't have the same/somewhat less proximity to the pipe as the brick. but if it smokes, then it must not be. thoughts one this or alternative set-ups?
also, any good suggestions for finding collars of various sizes online? thanks