Calcium buildup on water neck: how do I fix that?

[In The Ten Ring]

First off, how does this form? My truck's original owner told me that he only put distilled water into the radiator but if so, what is this gunk? The water hoses have it on the inside and I am going to assume so does the water jacket. The outside of the radiator has a heavy buildup outside as well so I bought a new radiator.

How do I get this out of my cooling system?