New coolant for water spray boilers used to cool the APU's

I've been listening to the audio from NASA TV and have heard them talking
about testing a new coolant mixture for the water spray boilers used to cool
the APU's. It sounded like the old coolant was simply water, but the new
coolant sounds like a mixture of water and something else.

I was wondering what the issue was here. If the water freezes in the water
spray boiler, does that mean that you can't start the APU until the ice
sublimates away?

Thanks,
Jeff
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