From the article:

> It’s actually created from waste steel dust which is normally discarded from industrial processes and silica from ground up glass. The iron within the steel dust reacts with C02 and rusts to form iron carbonate. It’s this that is fused into the matrix of Ferrock and, like concrete, after it’s dried, it cannot be melted back into a liquid form but retains its hard, rock-like qualities.

> ... But there’s skepticism from the cement industry that while Ferrock could be great for niche projects, it isn’t practical for large-scale industrial use, like highways for example. If steel dust suddenly goes from being a “waste” material to a highly-prized building resource, it’s price will increase exponentially and the costs of producing Ferrock may limit its application.

I read that the Sahara is useless, for example, because desert sand is rolled rounded. The only sand that concrete can use is the dug up, sharp cornered kind.

I heard that there are more stars in the universe than grains of sand on earth, so we could use stars instead..

You're forgetting that it's easier to create rock dust from scratch than it is to create iron dust from scratch. That's important when you're comparing the two, since you can't simply scoop up iron dust off beaches and river banks anyway. Given the choice between manufacturing rock dust and manufacturing iron dust, the choice is obvious.

To manufacture rock dust you:

  1. Quarry rock.
  2. Pulverize it.

To manufacture iron dust you:

  1. Quarry ore.
  2. Pulverize it.
  3. Smelt it (which requires coke, a basket of worms in itself.)
  4. Cast it.
  5. Grind it.

We may be running out of suitable sand to use for concrete, but that doesn't mean iron dust is more abundant.