Sealing a chimney in a stone rubble-fill wall

[redbumperman]

Does anyone know if you can get a heatproof expanding foam?
I've cut a duct for an extractor fan at the top of the house through a 2ft stone wall, and accidentally caught the side of the chimney, opening it to the rubble cavity. I want to keep the duct in this place. I can't use sand & cement to seal the cavity because of its awkward shape and the fact that filling needs to be done above the duct too. Foam or some kind of wadding is needed to stuff the space and seal it.
Any help gratefully received.

 

[noseall]

asbestos rope and pyruma.

 

[redbumperman]

Thanks, Noseall. The gaps are too big for that, though, and it's the space above the duct that's going to be tricky. Any other thoughts, anyone?

 

[JohnD]

there is a fire-resistant foam used for sealing round doorframes. Don't know how suitable it is for your job though. I would have tried to use sand and cement, possibly stuffing the gap with handfuls of fibreglass insulation to give it some support while it sets.

 

[redbumperman]

Thanks, JohnD. The fibreglass idea sounds just the job.