QLD Kev & Darkie replied:

Jeff,

Use "Hebel Blocks",
They are lighter and can be cut with an old hand saw and have very good fire resistance ;))
They can be rendered very easily as well and are glued together with a heat resistant glue. The bottom course is the only one that needs a motar bed.
You can get them in a range of widths and lengths. Average length is 600mm

Their only downfall is that they are not good for sub soil applications.

Cheers Kev