A versatile building material composed of sand, water, cement (a powdery substance made of calcined lime and clay) and aggregate (rubble, gravel, crushed stone).  When these ingredients are mixed together, they form a fluid mass that is easily molded into shape and sets into a hard, durable material.  The ancient Romans pioneered the use of concrete in architecture (web resource here).  Reinforced concrete, used widely today, has steel reinforcing bar (rebar) embedded in it.

Pantheon_domeThe concrete dome of the Pantheon, Rome. Ca. 126 CE.