Soft or sharp sand for (sort of) drystone wall

[Joe Shmoe]

I have a load of this quartize drystone walling for which I will be making a retaining wall for my flower border (approx 18" high).

I'm not making it really wide as I don't have the space so can't really do this as a genuine drystone wall as I don't think such a thin wall will be stable enough (especially with my amateur walling skills), so was intending to use mortar.

Would I be better off using sharp sand as the blocks are bigger and heavier than normal bricks?

If so, what strength mix do I need and do I add a plastisier?

Thanks for any guidance, I have limited bricklaying skill.

 

[pgrbff]

When I was taught about the use of lime and lime mortars it was always correctly graded sharp.