Easy Key Lime Pie

I've been craving something tangy and this pie is very tart. It didn't firm up as much as I thought it would, though I am not sure why. I may try it frozen too because that would be delicious as well.

Easy Key Lime Pie
from Real Simple
1 14-ounce can sweetened condensed milk
1 6-ounce can frozen limeade concentrate (not thawed)
1 cup heavy cream
1 6-ounce graham cracker pie crust
Zest of 1 lime

1. Place the condensed milk, limeade concentrate, and cream in the bowl of an electric mixer fitted with the whisk attachment. Beat until fluffy and soft peaks form.
2. Pour the filling into the pie crust and smooth the top.
3. Refrigerate until chilled and loosely set, at least 4 hours (and up to overnight).
4. Sprinkle the pie with the zest before serving.