There is mortar specially made for glass blocks for a reason !

Most mortars and grouts depend on some absorption-ability of the units you're applying it to, so the brick/block/tile and the mortar/grout grab each other and bond firmly.

Glass Block has ZERO absorption !!!

Its best to use Glass Block Mortar and nothing else. It is specially made to hold strong and not slip or stay separate , unbonded, with the glass blocks. For safety, and durability, better use Glass Block Mortar.