I know this is an old thread and Jyn has probably solved the problem, but here’s what I use for lime mortar, whether interior or exterior…
3 parts masonry sand…
1 part hydrated lime
1/2 part diatomaceous earth (or whatever pozzolan you have available, brick dust, metakaolin, VCAS, volcanic ash, etc., but diatomite is easy to find and results in a really brilliant white plaster that glazes well and doesn’t adulterate pigments)
For lathe coats, add some nylon fiber
The pozzolan isn’t absolutely necessary for a number of reasons, but it will give the plaster some hydraulic curing and much more early strength. Hydrated lime is calcium hydroxide, which reacts with silicon oxides and water to form tricalcium silicate hydrate, which is the crystal lattice that provides most of the early strength in the hydration of Portland cement. The reaction is much slower with lime plaster and you may never notice any exothermic heat, but there will be a large difference in strength after 48 hours.
I have done some experiments with acrylic fortifiers and it does result in something a lot more like Portland cement concrete after about 10 days (holding water in the plaster keeps the tricalcium silicate hydrate reaction going longer) but it will also REALLY slow down your operations. If you want a less permeable surface, like for a countertop, then an acrylic may be a good admixture decision, but otherwise I wouldn’t bother.
Practice before you start on a surface that matters. Put some lathe over a sheet of plywood and experiment to see what works. It will be a worthwhile investment of time in your learning curve.
