100% antifreeze will freeze as solid as a brick. Antifreeze has to be
mixed with water to the right proportions. Technically its called the
molal freezing point depression of the 'mixture'.
What happened is that the water vapor saturated air condensed on top of
the antifreeze, didnt mix with the antifreeze and formed a skin of
water ice on top of the antifreeze.
Ive done the same thing .... ;-) .... thats why I know the answer.




In article , Roger Long
wrote:

I went to check our boat today. It was 50 degrees here on the coast of
Maine. That should make anyone who ever spent a winter in this part of
the world believe in global warming.

When I winterized the boat, I pumped three gallons of antifreeze into
the bilge by turning the wrong valve and then forgot to pump it out.
When I looked in today, there were clumps of white crust floating
around on top of it that turned out to be ice.

What’s going on here? The bilge was pretty dry last fall so it isn’t
diluted and I can see that no water has been getting into the boat by
the level. I sure hope it isn’t freezing in the engine.