Mortar mix for fireplace cheeks

[John Rainford]

I'm replacing the side cheeks of a 1960s open fire. They have crumbled away due to wear and tear over the last 60 years. The fire back is fine and will stay in place. Can anyone tell me what to buy and mix for fire resistant mortar. I also need to buy the side cheeks which are quite big, 50 cm x 20 cm x 5 cm. I can't seem to find any this size on line. I also need to know what material they need to be. I have seen the term refractory being used with reference to side cheeks. Any help would be appreciated.