While the terms are sometimes used interchangeably, concrete and cement are not the same. Concrete is a building material, a composite of aggregates including sand and gravel, plus cement, water, and other materials. Cement is a key ingredient of concrete, typically making up 10% to 12% of the volume.
Cement does what its name implies-it cements the aggregates and other ingredients together. A fine powder that is usually gray or white in color, cement is hydraulic, meaning it chemically reacts with water. As the concrete components are mixed, cement helps turn the mixture into a flowable, formable emulsion, finally binding the components, as the concrete cures, into the rock-like substance used for everything from simple sidewalks to sophisticated skyscrapers.
Portland cement is a typical ingredient of concrete, and the most widely used type of cement. It was invented in the early nineteenth century and named after the fine building stones it resembled that were quarried in Portland, England. The innovation of Portland cement marked a milestone in the construction history, as it created a far stronger bond than the plain crushed limestone of the day. Today it remains the best-performing and most economical binder used in concrete.
SCMs are used in concrete as cement replacement and/or to modify the properties of the fresh or hardened mixture. The ingredients are typically the by-products of other industrial processes, including fly ash, which is left over from coal burning power plants, and slag, which is produced during the production of steel. Other examples include silica fume and calcined clays.
As industrial by-products, some SCMs may not be part of an ideal future. As sustainable development extends to other industries, less and less of these materials may be available to be recycled into concrete. In the meantime, SCMs offer an opportunity to improve concrete performance with a recycled material that would otherwise have to be disposed of in landfills.
SCMs work with cement to bind the aggregates and other concrete ingredients, and can improve concrete's fresh properties as well as its strength and durability. Light-colored SCMs, such as white silica fume or metakaolin, are used in architectural-face concretes. Certain SCMs, such as fly ash, may alter the color of the concrete or delay set times, which may be offset by chemical accelerating admixtures. SCMs work through either hydraulic or pozzolanic reactions.
These terms describe how concrete sets and then hardens. Hydraulic reactions occur when a reactive ingredient is mixed with water. Cement is hydraulic, and so are Class C fly ash and certain types of ground-granulated blast-furnace slags. Pozzolanic reactions occur in the presence of calcium hydroxide (Ca(OH)²), which is a by-product of the hydration of cement. Class F fly ash, silica fume, calcined clays, and most slags are pozzolanic.
Both hydraulic and pozzolanic reactions increase the strength and durability of finished concrete, and alter the fresh properties of concrete