system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is concrete so strong if it is basically just water, sand, and cement that has dried?

At the molecular level, what makes concrete so strong that we can make enormous buildings and support so much weight on it?
gpt
Concrete (mostly Calcium Oxide, CaO) doesn't "dry" in the conventional sense: the water doesn't leave, it chemically binds with the CaO and CO2 from the air to form interlocking crystals of Calcium Carbonate. This reaction even happens underwater.