Filling spaces between chimney liner and masonry

Spaces between the lining and the surrounding masonry should not be filled with ordinary mortar. In the absence of the liner manufacturer's instructions, the space should be filled with one of the following weak insulating concrete mixes – (a) one part ordinary Portland cement to 20 parts suitable lightweight expanded clay aggregate, minimally wetted; (b) one part ordinary Portland cement to 6 parts Vermiculite; or (c) one part ordinary Portland cement to 10 parts Perlite.