Concrete is made up of five main ingredients. These ingredients are cement, often referred to as Portland cement, a coarse aggregate such as a limestone or granite, a fine aggregate like sand or manufactured sand, water, and chemical admixtures.
In some mixes Portland cement can also be partially replaced with other cementitious elements such as fly ash and slag cement. Fly ash is the by-product that is produced by coal fired electric generating plants. Slag is the by-product of steel production. Fly ash can replace Portland cement by up to 60% and slag can replace Portland cement by up to 80%.