I'm not sure why I remember cement to be water proof. Its possible I infered from how its been used around me. I have seen cement very often used where water contact is expected (bathrooms etc). I have also seen brick walls plastered with an extra layer of cement as putty considered to be rain proof and any leaks were thought of as poor workmanship rather than poor material choice.
P.S. I'm not a builder. I've taken an interest in the topic a few months ago.
https://www.quikrete.com/productlines/hydraulicwaterstopceme...
But I wouldn't want to make any sort of building that only relied on it for waterproofing.
Normal cement, concrete .. Is a different story.
'Concrete' is a combination of sand, cement, water and aggregate, that cures together to form.
When the water migrates away from the curing concrete, it leaves open pores behind that future moisture can move through.
It is generally macro-waterproof, however, unless there are large cracks.