Fireplace query

Hello, I am preparing a fireplace to receive a gas firebox (Stovax Victorian Tiled insert with Gazco convector). I am building a rectangular brick opening to the dimensions of the box and was going to render the surface to seal any gaps and provide a flat surface to seal against the cast-iron insert. Although not exposed to naked flame the opening should be fire-resistant. Is this true of a concrete render? Will cracks open up with time allowing fumes to escape? Thanks, Ian