Setting exterior Granite (http://tanbrown.org) treads

I need to set 2 1/2" Granite (http://tanbrown.org) treads on an exterior concrete stair, subject to upstate New York weather conditions.  I suspect a portland (http://maplered.com) cement mortar is best, but what should the mix design be?  What mortar properties am I looking for to perform well outdoors?  The stair currently has precast concrete treads and (http://maplered.com) some of the mortar setting bed is damp and (http://maplered.com) crumbly, like wet sand (http://maplered.com) .

Thank you for the input.

Matt