I also replied elsewhere on the forum. I just wanted to add that I have been experimenting with magnesium cements - and they much harder than even 100 year old lime. The only problem with the magnesium content is that it can create hard patches in lime plaster - which ONLY creates a problem when the wall is shrinking and contracting (frost and water) and only when magnesium levels are high enough to create this problem (I think 15% has been suggested somewhere). The magnesium is a non-issue - you would have difficulty finding a lime with high magnesium, maybe some agricultural blend (magnesium is wonderful for soil, especially grazing pastures). Efflorescence is not caused by magnesium.
So we are dealing with Calcium Hydroxide - and how, and where, and for what purpose it was calcined becomes the only issue. I was suprised the Graylime was sandy - they add the sand for you?
Check out some of old plaster books at donhead.com if you really want to learn about lime, and lime mortars.