Seems like the #1 reason we are running out of sand, is concrete doesn't last very long. The #1 reason for concrete not lasting long that is rebar. Sure rebar lets you use less concrete, but then it doesn't last nearly as long. Without rebar concrete can last 1000s of years (like the roman aqueducts.)
I'm fairly certain Roman concrete also relied on chemistry as an alternative to thermal energy in its manufacturing process, as thermal energy simply wasn't as abundant to the Romans.
The longevity may simply be a fortuitous consequence, though a highly significant one.
Roman engineering was also rather more overengineered. You compare their columns to modern or say gothic ones, and they clearly have a lot of excess strength.
