eli5. when concrete powder gets wet it becomes a paste but when it drys and you pour water on it it stays the same?

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eli5. when concrete powder gets wet it becomes a paste but when it drys and you pour water on it it stays the same?

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It stays the same as what? A paste?

When concrete powder gets wet, it doesn’t just dry out, it undergoes a chemical reaction and becomes a waterproof solid. Some types of concrete can even cure while underwater. Once the chemical reaction is complete, the concrete is “set” and is no longer dissoluble in water. Even if you were to take the set concrete and grind it into a powder similar in appearance to the original uncured concrete, the powdered set concrete wouldn’t “reset”.

Concrete is a mixture of multiple materials, but in modern concrete one of them is usually Portland Cement, which is the material which actually undergoes the chemical reaction causing it to become hard and waterproof. The other ingredients help increase the strength, or add volume and texture. The chemical reactions which cause Portland Cement to cure are very complex and definitely not ELI5-able. Just consider it like a permanent glue, when it is wet you can clean it up, but once it dries it becomes permanently hard and waterproof.

concrete hardens due to a chemical reaction, when it “dries” the water doesn’t evaporate it being used up on that reaction to make it stone like

The concrete become hard not becouse just dry, but for chemical reactions with water and oxygen, during the solidification the concrete can reach high temperature for the chemical reaction

As an exemple, if you wet a cloth, then dry it, then do it again you can go from wet to dry, to wet again, etc.

But if you cook an egg, you can not go back to raw again.

Cement doesn’t dry, it cures and becomes solid, very hard, and also waterproof.