The thing about Ca(OH)2 is that you don't need much of it. Suppose, for example, that you were to make a beer with a typical ale malt and use insane levels of black malt and dark crystal malt to the extent of 1/6 the of the weight of the grist each. IOW 1/3 of the grist is very acidic colored malt. With typical water to grist ratio you'd need about 110 mg/L Ca(OH)2 for a proton absorbing capacity about 3 mEq/L at mash pH. Were you mashing 15 lbs of grain with 5 gal water that would amount to 2.2 grams Ca(OH)2. Adding it to 5 gal of DI water would raise the pH to around 11.6. Now that's alkaline but not highly alkaline so I question whether you would have a repeat of Lakehurst in your kitchen were you to mix that much lime with that much water in an aluminum pot. Yes, the aluminum would pit and corrode but I'm guessing that the reaction would be quite slow i.e. that you might see some discoloration over night or the course of a few days but I haven't tried this. You could try filling your pot and stirring in some lime at the rate of 110 mg/L. I'm guessing that you won't see much, if any, visible reaction. In this case I'd think if you measured out what you need, make a lime slurry out of it by shaking it up in a small container with say a quart of water, heat the strike water to strike temperature and then dump the slurry in with a quick stir just before adding to the grains (or the grains to the water) that you should be fine. If you are still worried about the aluminum reaction pull out a gallon of the hot liquor and dump the slurry into that. Stir and add to the grains or add the grains to the 4 gal remaining and then add the treated gallon. Stir thoroughly. With a liquid to liquid mix of those volumes you should get good mixing pretty quickly.
Lime does pick up CO2 from the air. If it didn't we wouldn't have mortar would we?. It is easy to check on the purity of you lime, however. First put a bit in a cup or on a dish and add a couple of drops of vinegar. If it fizzes then obviously it is largely converted. If it doesn't, mix a little into some DI water and measure the alkalinity using a test kit. The purity (the ratio of hydroxyl to carbonate equivalence) is 100*(2*P - M)/M in which P is the P alkalinity and M the M (total) alkalinity. As this is a ratio it doesn't matter what units your test kit uses.
If you have a crucible handy you can always purify a small quantity of your lime by heating it to orange to yellow heat with a torch (propane torch should do but MAPP or acetylene would be better). Be aware that what's left after the mass cools is quick lime and it will get very hot if exposed to water and that the numbers must be redone to reflect CaO rather than Ca(OH)2.
