looking for some advise if poss, i have just had my colant flushed and renewed on my 1.9 petrol Transporter, drove for 20 miles and checked the expansion tank and found the level to be very low if not nearly empty, so i topped it up with water (plain) not much approx 3 mug fulls, drove back 20 miles all good running well, i checked it this morning after a very cold night and the water in the expansion tank had frozen

my question is: is this because it is now too weak? or matbe because the water i added has just sat in the expansion tank and not mixed with the rest of the system?
i started the van up and it seemed to run ok, im just worried this might damage the system.
any advise would be greatly appreciated,
many thanks