https://newatlas.com/materials/concrete-steel-recycle-cambridge-zero-carbon-cement/
For the new study, Cambridge researchers investigated how waste concrete could be converted back into clinker, the dry component of cement, ready to be used again...
An electric arc furnace needs a “flux” material, usually lime, to purify the steel. This molten rocky substance captures the impurities, then bubbles to the surface and forms a protective layer that prevents the new pure steel from becoming exposed to air. At the end of the process, the used flux is discarded as a waste material.
So for the Cambridge method, the lime flux was swapped out for the recycled cement paste. And sure enough, not only was it able to purify the steel just fine, but if the leftover slag is cooled quickly in air, it becomes new Portland cement. The resulting concrete has similar performance to the original stuff.