Make low-carbon concrete by limiting the cement content — not just adding pozzolans.
Concrete is artificial rock that you make by mixing sand, gravel and a binder. Modern building codes require that a large part of that binder be Portland cement, a really fantastic binder that comes at a very, very high carbon cost — 5–7% of the carbon we put in the air every day comes from making Portland cement by baking limestone.
Which is funny, in a way, because the most famous concrete building in the world, the Pantheon, was made 1800 years ago with no Portland cement (it hadn’t been invented), and many modern buildings that DO have Portland do not perform so well.
You can make great concrete with industrial by-products (or “pozzolans”) like slag and fly ash, but to get the best effect and least carbon cost, you need to limit the Portland cement. The first generations of “green” concrete specified minimum amounts of low-carbon binders like fly ash, but concrete suppliers just added those binders on top of the Portland, giving people super strong concrete but no real energy improvement.