Sandstone - Mortar bed

[triggers]

Hi, everyone, i am going to have some indian sandstone paving laid soon, but what i would like to know is can i just use a standard 3-1 mortar mix using normal builders sand which my builder says will be ok, but every where i read it says to use a 6-1 sharp sand and cement mix or a sort of half and half of the 2 sands. What is the reason for using sharp sand. Thanks.

[lutonlagerlout]

sharp(concreting sand) is much much stronger at 30-50mm thick
building sand is only good for bedding bricks with 10mm joints
and is no good for patios

sharp sand has bigger aggregate and makes a much stronger mortar
LLL