Waste steel is a manufacturing byproduct. There's lots of it. However, rust inside of concrete accelerates the decomposition of concrete. It says it's steel powder. What happens when all that steel oxidizes inside the concrete? While I believe it's a stronger material I question is longevity. Also, this waste steel is usually recycled like any other steel and tossed in during smelting. It's not like it's not used. It's openly bought on the market in bulk, by the ton.