Why use concrete?
The fountain in the center of the
courtyard is concrete
(This is from the Palace
of Domitian in Rome)
Concrete is a kind of man-made stone. It's cheaper and easier to get than natural stone, and it's easier to shape it. You make it by mixing lime and clay and sand and gravel and water. Lime is basically the ashes of limestone; it's what you get when you burn limestone or marble. Lime and clay and water mixed make cement, and then you mix the cement with sand and gravel to make concrete.
People have been using concrete for a long time; the earliest use we know about was to make a hard floor for a house in eastern Europe (modern Serbia) about 5600 BC. The Egyptians, Mesopotamians, Chinese, and Greeks knew how to make concrete but they didn't use it very much. The Romans began to use a lot of concrete in their buildings about the time of the Emperor Nero, 60 AD, and they used it more and more from then on, usually with a brick facing to cover it and make it look prettier.
Basilica of Maxentius and Constantine, Rome (312 AD)
You can make reinforced concrete by putting metal rods inside the concrete. This makes it a lot stronger. Most concrete buildings today are reinforced with steel. The Romans were the first to use reinforced concrete, mainly in aqueducts.