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.
Concrete is a mixture of rock “aggregate” and cement. Cement is the special bit. It’s a mixture of limestone and clay that has been “cooked”. When you add water to it, a series of chemical reactions cause it to bind to the water and go hard. Cement is effectively the “glue” that sticks concrete together!
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The cement is calcium oxide. Water reacts with it, adds an oxygen and the Ca dioxide bonds with the silica in sand and rocks, forming a matrix.
A) It’s not just “rocks and water,” it’s some very specific kinds of rocks, refined in specific ways.
B) Terminology matters.
* **Aggregate** is the gravel and sand mixed in specific proportions with the cement and water to give it bulk and physical structure.
* **Cement** is the fine powder substance that acts as a binder for the aggregate.
* **Concrete** is the name for the end product of mixing aggregate, cement, and water.
Cement works because of specific chemical reactions that cause it to fill the spaces between the aggregate and then harden or “cure” in a way that holds everything together and (ideally) keeps out water/ice to prevent erosion. There are lots of different kinds of cement, but most commonly it’s limestone (Calcium carbonate) that’s heated to high temperatures to change its chemical composition so that in the future, when it’s mixed with water, it will react with carbon dioxide or similar to form a cohesive whole.
https://en.wikipedia.org/wiki/Cement#Chemistry
Concrete is comprised of at least 4 ingredients: water, fine aggregate (e.g. sand), coarse aggregate (e.g. gravel), and cementitious material (e.g. Portland cement and/or fly ash). It may also include other ingredients which alter key properties like slump, cure time, color, strength, flexibility, etc.
Cement is a chemically reactive material. When mixed with water, it undergoes a reaction called hydration. There are several types of compound (5 major ones) in cement include calcium and/or silica. When those are exposed to water, the extra stuff leaves those molecules and what’s left behind is a molecule made of calcium (like your bones) and silica (like glass). That stuff is very strong.
tl;dr: – Water pulls strong compounds out of cementing leaving behind something that is strong like a mixture of bone and glass.
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