I get where you are coming from, and some careful calculations are needed. But it's important to note this isn't replacing sand, it increasing the strength, you need 30% less concrete if it's 30% stronger.
The energy needed to extract sand and gravel from the ground, the logistics around moving heavy bulk material, crushing and preparing it, uses a staggeringly large amount of fuel. This is potentially up to 30% saving on that energy use.
The energy needed to extract sand and gravel from the ground, the logistics around moving heavy bulk material, crushing and preparing it, uses a staggeringly large amount of fuel. This is potentially up to 30% saving on that energy use.