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

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.