cement not bringing water temp down

I have a 5 gallon bucket dwc setup going on in the basement. I read that a cement floor would help keep it cold, but the water temp is steady at 78 F

Now I know I could add a bottle of frozen water in there, but with it floating around wouldnt it freeze the roots?

i dont know ah bot frezzing them but you could give them frost bite maybe one taped to the bottom where they cant tuch but still be close if you know what i mean ,,you could maybe tape it or some thing like that ....iam just saying....hopes this helps