I've been doing some preliminary testing on materials and methods for lining a fiber blanket system, primarily using the furnace cement I got from Lowe's along with the Roxul. This is Hercules regular body high-heat furnace cement, nominally rated for 3000˚F. Here is a picture showing some results.
The tube fragment on the right was made with about a half-half mix of the furnace cement with some gray modeling clay from my stockpile of random junk I've accumulated over the years smeared on a cardboard tube, rolled in some perlite, left to dry for a few hours and then tossed into the fireplace The resulting tube had the thickness and the fragility of a thick egg shell or a tortilla chip. There is also a pita bread effect, with raised bubbles. Messing around with it in the fire, it soon acquired a lengthwise crack, maybe due to the ends expanding and contracting differently when it was used as a chimney. It's been kicking around for weeks now and I haven't totally destroyed it.
The piece on the left I made more recently, just smearing some of the cement like peanut butter on a piece of the ceramic fiber blanket. I let this piece dry longer, but it was still somewhat pliable, like stiff leather, when I put it in the microwave. The pita bread effect was dramatic.
As this stuff dries, surfaces exposed to air develop a tough skin that slows further drying and traps water vapor, leading to big bubbles if brought above boiling.