Mortar suitable for Alphalite masonry can be any of those mixes defined in the Masonry Structures code: AS3700-2001, Table 10.1

The M3 mortars (general purpose) most commonly used are:-
1 : 5 (cement : sand) and
1 : 1 : 6 (cement : lime : sand)

The M4 mortars (exposure grade) most commonly used are:-
1 : 4 (cement : sand) and
1 : 0.5 : 4.5 (cement : lime : sand).

The non-lime mixes may have a cellulose-based water thickener added for workability and bond strength.