> the good stuff also includes tiny lumps of calcium carbonate that self heal cracks

According the article, they didn't use calcium carbonate directly. They use quicklime (Calcium oxide) which must be mixed in at very hot temperatures.

When a crack forms, that stuff melts and forms calcium carbonate that seals the crack.

> which must be mixed in at very hot temperatures

It gets hot when you mix it.

It's a little unclear to me about that, it is what I thought at first. But it appears that quicklime is not stable at ambient temperatures and recombines with CO2 to make Calcium Carbonate; which is how the self healing works.