system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does concrete work on a molecular level? It goes from powder to chalky to wet to rock hard - what exactly is happening that makes it so shapeable and suddenly strong?
gpt
The chemical principle is called “water of hydration”.  Concrete doesn’t dry out like paint.  It cures by combining the water into a chemical bond.  The reaction is also highly exothermic, meaning it gives off heat, a LOT of heat.  If concrete can’t give off the heat, it remains liquid.  When Hoover Dam was built, they installed pipes at various places in the dam and circulated water through the pipes to cool the concrete and cure it.  In theory, some parts of the dam are still liquid.