Hey Otto et al,
It's been a while since I was on the forum, so hello.
First off, let's just start by saying that an air lime like Type S lime is sufficient for any mortar or lime render that you need to plaster less than 1/2" as long as you give it time to cure. It is recommended to protect from sun and wind for a week or more if you have the ability, and hanging burlap works great. If that is unreasonable, just misting the walls several times a day will still give you great results.
There are many options for pozzolans in order to make your lime hydraulic. The key thing you are looking for is that the pozzolan contains amorphous silica and aluminate particles, hopefully close to 50/50. Trans Minerals offers bagged Natural Hydraulic Lime products (St. Austier) which are pricy, ($40 something a bag), and is shipped all the way from France, but it is a great product. Metakaolin is a great natural choice, but again will depend on our location whether or not you can find it locally. Rice husk ash works great, but good luck finding it in the US. Pumice works as well, and you can find it in the US. Perlite can work, but you want the fines, the waste from the plant operations. Brick dust works awesome, but it's hard to come by. Fly ash works amazingly, but it may contain heavy metals, as does blast furnace slag.
The more hydraulic the lime, the lower the permeability, the higher the strength, and the more brittle it is. Hydraulic lime will almost never be as strong as a cement-based mortar or concrete. But compressive strength doesn't mean it is better. The beauty of lime, especially hydrated lime (aka air lime aka non-hydraulic) is that is is much less brittle and can shift with the building. Concrete is so strong that it buckles upon itself. That is why control joints are necessary when using cement based products, to control where it cracks. Lime is self-healing on a microscopic level due to the free lime moving around inside the carbonated lime. This makes it much more suitable for softer brick, adobe, or any softer material or older building that may shift around.
I guess what I am getting at is unless you plan to use the lime as a foundation, or a thick render, then maybe consider using an air lime like a Type S lime. It has lower embodied energy, higher permeability, and more flexibility. Again, it totally depends on your situation. There is a lot of misinformation out there about lime and it is easy to get misled. Especially if you are getting your information from Europe where they use beautiful aged lime putty and their bagged products are inferior to our ASTM standard products here in the US.
That's more than my 2 cents, but hopefully, it helps. Good luck!
