Concrete is a composite material made from:

* Cement: A fine powder that acts as a binder.

* Aggregates: Larger particles like gravel or crushed stone that provide strength and durability.

* Water: Used to activate the chemical reactions that make concrete harden.

While sidewalks are most often concrete, some might be made of:

* Brick: Used for decorative or historic purposes.

* Stone: Can be used for sidewalks but is generally more expensive.

* Asphalt: Used more commonly for driveways and roads, but can be found in some sidewalks.

Let me know if you want to know more about concrete sidewalks!