Is there some flexible cement-based mortar?
I'd opt for a fire resistant silicone sealant such as this one >>
http://www.bostik.co.uk/construction/product/bostik/Fireseal-Silicone-Joint-Sealant/340

That's from a UK site, but I'm sure there are similar products in your part of the world.

When it comes to cement then flexibility is usually imparted by including lime in the mortar - although that's a slow process involving the exposed lime within a crack recalcifying over time so not really suitable in your scenario.