concrete

Building material in use since Roman times, made up of four main ingredients: coarse aggregate (gravel, usually between 0.5 to 1.5 inch in diameter), fine aggregate (sand, usually between 0.005 and 0.25 inch in diameter), Portland cement, and water. Air too plays an important part, and often special additives (called admixtures) are also added to improve or modify the concrete's properties. Reinforced concrete is made by embedding steel rods in concrete that improve its tensile strength.