https://www.backyardchickens.com/forum/viewtopic.php?pid=483040
"Hydrated lime is sold in feed stores, lumber stores, etc. and is a finely powdered cooked limestone that is slightly caustic. Not anywhere near as caustic as quicklime, but a little. It has uses around poultry:
As a litter amendment. The original deep-litter bedding system for poultry involved stirring hydrated lime into the litter at the rate of ten pounds per hundred square feet of henhouse. You just scatter it over the top and then stir it in with a hoe or turn the litter over with a spading fork. It reduces the amount of caking, makes the litter seem a lot drier, and is hostile to coccidiosis.
As a deoderant and fly repellant. If you have a smelly heap of chicken manure (for example, under your roosts or in an outdoor heap after you've shoveled out the chicken house), sprinkling it with hydrated lime will kill the smell and also drive off the flies. Of course, if we were perfect farmers, we'd never have smelly heaps of anything, but in real life it's nice to have a sack of hydrated lime handy. It also seems to keep the raccoons away from our compost piles.
As hen repellant. As far as I know, this is my own invention. Hens don't like hydrated lime. If they're nesting, dust-bathing, or foraging where you don't want them to, you can dust the area heavily with hydrated lime, and they usually avoid it thereafter. I've watched them, and while they clearly don't like the stuff, they don't find it distressing or anything. (In fact, the effect is mild enough that it doesn't always work.) I've used this to keep them from nesting in odd corners and foraging in smelly manure heaps. It would probably also be at least partly effective in keeping them out of your garden beds, though I won't make any promises." R. Plamondon