Structural engineer here. Much of what is said in the other responses is correct, but I will try to explain it as structural engineers do and also use industry-

Structural engineer here. Much of what is said in the other responses is correct, but I will try to explain it as structural engineers do and also use industry-standard terms. There are many things called concrete. I am going to use the word "concrete" to mean "Portland cement concrete". That's what you are describing in your questions. Concrete is the stuff sidewalks are made of. Some people use the word "cement" when describing sidewalks. But that is akin to calling bread yeast. Sure, there is cement in concrete like there is yeast in bread but concrete is more than cement as bread is more than yeast. Cement and yeast are the active ingredients only. Hardened concrete is a composite material made up of aggregates and a binding matrix that holds the aggregates together. Aggregates in concrete include sand (fine aggregate) and gravel or crushed stone (course aggregate). The matrix consists of cement particles and minerals that grow out of those particles as the cement reacts with water: a process we call hydration. These minerals or "hydration products" grow around the aggregates and give the concrete its strength. What we call cement is rightly called Portland cement. It is added to a concrete mix in the form of a very fine powder. The cement is manufactured from limestone (primarily) that has been fired in a kiln before being ground to that extremely fine powder. The kiln treatment is how it becomes hydraulically reactive. Material that reacts with water similar to Portland cement also occurs naturally around volcanoes. I believe that is where the Romans got the cement for their concrete. But nowadays we manufacture Portland cement it instead. Hardened concrete is strong in compression (when squeezed) but weak in tension (when pulled apart). When we need tensile strength, we add steel reinforcement. The resulting composite material is referred to as reinforced concrete. Concrete can also be used without steel reinforcement when it is not subject to tensile loads. This we call either plain concrete or unreinforced concrete. The word "rock" or "stone" encompasses a wide range of naturally-occurring solid materials made up of various minerals. Limestone, granite, marble, and sandstone are common examples. Most of the solid rock you see around your world is much stronger than concrete. Concrete is an imitation of rock. In fact, the name Portland cement was given to it because the concrete resulting from its use supposedly looks similar to natural stone quarried on the Isle of Portland in the United Kingdom. I hope this helps. In short: Portland cement, water, and aggregate (sand and crushed natural rock or gravel) are mixed or "batched" and harden by a chemical reaction called hydration to produce a solid material similar to natural stone but with many practical advantages over rock.