fire cement setting

[visionset]

Not used fire cement before, the stuff I have is "thermo setting resins and inorganic fillers (sodium silicate)" Do I have to apply and heat in a shortish time frame, or can I allow it to dry out and then heat at a later date, say 24 hours or so later? I've let a bit go off in the sun, and it is hard but I guess is unlikely to be very strong. I'd like to assemble the flue gradually rather than be stressed out and risk a bodge. Do you think I can get a join hot enough with a heat gun for it to be strong enough to withstand the inevitable flexing as other bits are fitted and before the final heavo to pop the pipe in the stove?

[dom]

IME fire cement is very brittle - flexing during final assembly doesn't sound good unless it's all still wet. My technique has always been to dry fit everything, then slap on plenty inside the joints and fire it up in fairly short order.

[visionset]

I feared as much. I'm thinking with one joint that is vitreous pipe to s/s connector that I could perhaps do that one and heat it up with a gun. The stuff does seem to dry out very quickly. When you've got 100kg of stove to man handle, speed isn't really something I can do.

[cynic]

I have found that some brands of fire cement become crumbly if given excessive heat too soon. The water turns to steam, expands and breaks up the structure. Allowing to dry out and set first results in a stronger result

[Harry Bloomfield]

visionset presented the following explanation :

The ideal procedure is to bring it up to maximum temperature gradually, over a period of 24 hours or so - not very practical to do that though.