Question on Coolant Expansion Tank Replacement

I'm having the coolant being forced out of the expansion tank by popping the cap and sometimes just coming out with the cap still on. I have been topping up with coolant and water but unsure of the mixture %. Is the 50-50 mix essential and as I'm not using it is this causing my problem?
I've looked at the articles and tried checking those I'm able to rule them out. I noticed that another reason for this is the fan not kicking in. Mine kicks in but I'm not sure of it's a stage one fan or stage 2. I used the diagnostic test to check the temp and when the fan kicks in and it comes on at 117'C. I think this maybe stage 2 as I assume it should kick in earlier to stop overheating. So at what temp should stage one kick in?
My car is a 02 Cooper.

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This post was auto-generated based upon a question asked on our tech article page here:

[Nick at Pelican Parts]

The fan turns on at 105°c and high speed kicks in at 115°c.

Coolant mixture will not cause this issue.

- Nick