Might be because it's water tolerant, i.e. it doesn't break down with exposure to water? Roman über-concrete stories are always full of "check out the concrete that's sitting in sea-water, it's still going strong! maybe it even helps!" cases too.

Hydraulic cement is made for some of those occasions:

https://www.quikrete.com/productlines/hydraulicwaterstopceme...

But I wouldn't want to make any sort of building that only relied on it for waterproofing.

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