eli5 how does cement and concrete become hard if it’s only rocks and water
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It’s not only rocks and water. Concrete is cement + rocks + water. The key part there is the cement…that’s not inert, it’s a chemical mix called Portland cement that’s basically a really cheap glue that reacts when it hits water.
You can mix just cement and water and it will set into a solid, but that’s (relatively) expensive and not that strong. But if you use the cement to glue rocks together, you get concrete, which is really strong in compression but really weak in tension. So if you stick steel bars in it you get re-enforced concrete, which is really strong in compression and tension.
There are other cements than Portland cement but they’re pretty rare; Portland cement is really well understood and ubiquitously available all over the world. It reacts with water to form a solid.
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