Cement is the common name for Portland cement, the most important modern construction material, notably as a constituent of concrete. In the manufacturing process, limestone is ground into small pieces (about 2 centimeters). To provide the silica (25%) and alumina (10%) content required, various clays and crushed rocks are added, including iron ore (about 1%). This material is ground and finally burned in a rotary kiln at up to 1500°C, thus converting the mixture into clinker pellets. About 5% gypsum is then added to slow the hardening process, and the ground mixture is added to sand (for mortar), gravel and crushed rock (for concrete). When water is added, the cement solidifies gradually, undergoing many complex reactions. The name "Portland" cement arises from a resemblance to stone quarried at Portland, England.