Bagged lime powder is usually hydraulic, that is, it sets quickly once dampened, and doesn't need exposure to the air. I *like* the slow setting nature of non-hydraulic lime. (Hydraulic lime also doesn't have the same self-healing properties - it all sets, rather than leaving a core of unset, which can set after the building has shifted.)
Also, I don't think it's a matter of "ready-mixed" putty. That's how it is turns out after you slake it. Slaking quicklime to give a dry powder is much more tricky.