Fascinating piece on ancient Roman concrete -- wait, no, don't leave! -- in Ars Technica.

Apparently the #Roman #AncientEngineers mixed their #concrete with #quicklime at high temperatures. This formed veins of calcium carbonate in the concrete. The veins were stress lines, so that if the concrete cracked, it did so along the veins. But the veins could sort of melt and reform, if they got wet. So after a few rains, the cracks in the concrete would heal!

https://arstechnica.com/science/2023/01/ancient-roman-concrete-could-self-heal-thanks-to-hot-mixing-with-quicklime/