Portland cement, when combined with water, forms a paste that binds sand, gravel, and stone into a rocklike mass called concrete. Portland Cement Portland cement powder is a mixture of calcium, silica, iron, and aluminum minerals. Portland cement and water hydrate, or react chemically, to form cementing compounds of the water, calcium, silica, iron, and aluminum. Hydration produces some heat. The natural or raw materials from which portland cement is made are materials containing calcium and silica.
Limestone is the calcium-containing material most used to make cement.
Clay is the silica-containing material most used. Chalk or marl is sometimes used in place of limestone, and shale or slate in place of clay. Blast furnace slags, which contain both calcium and silica, are also used. Portland cement powder is so finely ground that it can sift through a screen that has 40,000 holes to the square inch. Five types of portland cement are made in the United States.