material to fill in part of fireplace

[mattnsiony]

hi, I am currently working on re-installing a solid fuel fire place in my new house, I have an insert here to put in to put in, or I may go or a stove,

anyway I have been told I need to fill in the sides of the fireplace as it is quite a wide hole inside, what would be the best material to use to block up the bits that need blocking?

Thanks

 

[cotswoldbuilders]

Get a bag of Micafil, mix the whole bag to a bag of cement to form a light weight fireproof concrete.