What type of Sand to mix with Portland Cement for filling gaps in a (irregular shape stones) Flagstone Patio?... and what ratio of Sand to Cement??
I was thinking of using Concrete (less fines) or Aphsalt Sand (more fines) and using about a 2-3 to one ratio............. gaps are anywhere from 1/2" to 1 1/2" in width.
Small disclaimer; I have never mixed mortar for flagstone but I have mixed my fair share for other projects. I did a quick google for "flagstone mortar" and it appears you were close with your ratio; The preference was for 2 parts sand to 1 part Portland. Evidently this ratio weathers well in freezing weather.
Ok with all that being said here's what I do know; The concrete sand being a little courser will also give the mortar mix more strength. Considering the gaps that you have to fill in, my personal preference would be for the concrete sand. Also here's a very informative article that addresses the optional usage of hydrated lime in the mix. I suppose I'm not very adventurous because I always buy mortar mix with the lime already in it.