I need to do several jobs requiring cement around a fireplace and chimney

I need to repoint the chimney right through to the flue in places!!
I need to flaunch around the pot, install fireback with vermiculite/cement
mortar, smooth out a throat when I've done and no doubt other associated
tasks.

Where do I need to use sulphate resistant cement?

Oh and can I use vermiculite from a garden centre, I'm thinking they may
sell smaller quantities, though maybe I need a giant bag of the stuff, it's
all new ground for me here.

It is for a decorative fuel effect fire, but I'm trying to do the work up to
spec of open fire.

TIA


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Mike W