Cement
A cement mithraeum at Ostia, in Italy (about 200 AD)
Cement is a very hard kind of rock which people make themselves. People all over Asia and North Africa used cement as a mortar to stick bricks together, or to make a hard floor, from the Stone Age onward, for instance in the Great Wall of China (about 200 BC). But concrete was not used as a building material until the Roman Empire. You start with lime and powdered clay, and you add water. You mix it all up, and it turns into this soft squishy stuff, a lot like gray Playdough. Then you make it into whatever shape you want, or pour it into a mold, and let it dry. When it is dry it will be hard (like Playdough) and even if you get it wet it won't get soft again.
If you want a stronger building material, you can add sand or gravel, and that makes concrete.
Architects in the Roman Empire used cement and concrete as a main building material beginning in the reign of Nero, about 60 AD, to build palaces like Nero's Golden House, temples like the Pantheon, and shopping malls like Trajan's Market. By using cement, people could build huge domes and big open spaces with barrel vaults or groin vaults over them.
Outside the Roman Empire, though, people did not use cement as a main building material. They continued to use cement as a mortar to stick stones or bricks together.
After the fall of Rome, about 500 AD, people didn't build any big buildings in Europe or North Africa for several hundred years, and they stopped using cement or concrete except as mortar to hold bricks together. Even in West Asia, Late Roman or Byzantine churches like Hagia Sophia, and early mosques like the Dome of the Rock, use cement only as a mortar. In the Middle Ages, builders used cement mortar to build castles and cathedrals, but they didn't use cement as a main construction material.
