Pozzolans are finely divided siliceous or siliceous and aluminous materials that are by themselves of little or no cementitious value but that react with calcium hydroxide in the presence of water at ordinary temperatures to form cementitious compounds. They include natural materials such as some types of volcanic tuff, opaline chert, shale, or calcined clay and artificial materials such as precipitated silica and some types of fly ash. Pozzolans are used mainly as admixtures in portland-cement concrete, particularly in massive structures, in proportions ranging from 5 per cent to more than the amount of portland cement. Their use may effect reduction in cost of construction, improve workability, decrease permeability, reduce heat of hydration, increase resistance to sulfate waters, and reduce expansion caused by alkali-aggregate reaction. Concrete made with portland-pozzolan cement generally has lower early strength and takes longer to cure than that made only with portland cement, but under favorable conditions it often has higher ultimate strength.