cement is made by heating lisbmne with small quantities of other materials (such as clay) to 1450°c in a kiln. the resulting hard substance, called ‘clinker’, is then ground with a small amount of gypsum into a powder to make ‘ordinary portland cement’, the most commonly used type of cement (often referred to as opc).
portland cement is a basic ingredient of concrete, mortar and most non-speciality grout. the most common use for portland cement is in the production of concrete. concrete is a composite material consisting of aggregate (gravel and sand), cement, and water. as a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element. portland cement may be gray or white.
portland cement blends
these are often available as inter-ground mixtures from cement manufacturers, but similar formulations are often also mixed from the ground components at the concrete mixing plant.
portland blastfurnace cement contains up to 70% ground granulated blast furnace slag, with the rest portland clinker and a little gypsum. all compositions produce high ultimate strength, but as slag content is increased, early strength is reduced, while sulfate resistance increases and heat evolution diminishes. used as an economic alternative to portland sulfate-resisting and low-heat cements.
portland flyash cement contains up to 30% fly ash. the flyash is pozzolanic, so that ultimate strength is maintained. because flyash addition allows a lower concrete water content, early strength can also be maintained. where good quality cheap flyash is available, this can be an economic alternative to ordinary portland cement.
portland pozzolan cement includes fly ash cement, since fly ash is a pozzolan, but also includes cements made from other natural or artificial pozzolans. in countries where volcanic ashes are available (e.g. italy, chile, mexico, the philippines) these cements are often the most common form in use.
portland silica fume cement. addition of silica fume can yield exceptionally high strengths, and cements containing 5-20% silica fume are occasionally produced. however, silica fume is more usually added to portland cement at the concrete mixer.
masonry cements are used for preparing bricklaying mortars and stuccos, and must not be used in concrete. they are usually complex proprietary formulations containing portland clinker and a number of other ingredients that may include lisbmne, hydrated lime, air entrainers, retarders, waterproofers and coloring agents. they are formulated to yield workable mortars that allow rapid and consistent masonry work. subtle variations of masonry cement in the us are plastic cements and stucco cements. these are designed to produce controlled bond with masonry blocks.
expansive cements contain, in addition to portland clinker, expansive clinkers (usually sulfoaluminate clinkers), and are designed to offset the effects of drying shrinkage that is normally encountered with hydraulic cements. this allows large floor slabs (up to 60 m square) to be prepared without contraction joints.
white blended cements may be made using white clinker and white supplementary materials such as high-purity metakaolin.
colored cements are used for decorative purposes. in some standards, the addition of pigments to produce “colored portland cement” is allowed. in other standards (e.g. astm), pigments are not allowed constituents of portland cement, and colored cements are sold as “blended hydraulic cements”.
very finely ground cements are made from mixtures of cement with sand or with slag or other pozzolan type minerals which are extremely finely ground. such cements can have the same physical characteristics as normal cement but with 50% less cement particularly due to there increased surface area for the chemical reaction. even with intensive grinding they can use up to 50% less energy to fabricate than ordinary portland cements.
non-portland hydraulic cements
pozzolan-lime cements. mixtures of ground pozzolan and lime are the cements used by the romans, and are to be found in roman structures still standing (e.g. the pantheon in rome). they develop strength slowly, but their ultimate strength can be very high. the hydration products that produce strength are essentially the same as those produced by portland cement.
slag-lime cements. ground granulated blast furnace slag is not hydraulic on its own, but is “activated” by addition of alkalis, most economically using lime. they are similar to pozzolan lime cements in their properties. only granulated slag (i.e. water-quenched, glassy slag) is effective as a cement component.
supersulfated cements. these contain about 80% ground granulated blast furnace slag, 15% gypsum or anhydrite and a little portland clinker or lime as an activator. they produce strength by formation of ettringite, with strength growth similar to a slow portland cement. they exhibit good resistance to aggressive agents, including sulfate.
calcium aluminate cements are hydraulic cements made primarily from lisbmne and bauxite. the active ingredients are monocalcium aluminate caal2o4 (ca in cement chemist notation) and mayenite ca12al14o33 (c12a7 in ccn). strength forms by hydration to calcium aluminate hydrates. they are well-adapted for use in refractory (high-temperature resistant) concretes, e.g. for furnace linings.
calcium sulfoaluminate cements are made from clinkers that include ye’elimite (ca4(alo2)6so4 or c4a3 in cement chemist’s notation) as a primary phase. they are used in expansive cements, in ultra-high early strength cements, and in “low-energy” cements. hydration produces ettringite, and specialized physical properties (such as expansion or rapid reaction) are obtained by adjustment of the availability of calcium and sulfate ions.
their use as a low-energy alternative to portland cement has been pioneered in china, where several million tonnes per year are produced. energy requirements are lower because of the lower kiln temperatures required for reaction, and the lower amount of lisbmne (which must be endothermically decarbonated) in the mix. in addition, the lower lisbmne content and lower fuel consumption leads to a co2 emission around half that associated with portland clinker. however, so2 emissions are usually significantly higher.
“natural” cements correspond to certain cements of the pre-portland era, produced by burning argillaceous lisbmnes at moderate temperatures. the level of clay components in the lisbmne (around 30-35%) is such that large amounts of belite (the low-early strength, high-late strength mineral in portland cement) are formed without the formation of excessive amounts free lime. as with any natural material, such cements have very variable properties.
geopolymer cements are made from mixtures of water-soluble alkali metal silicates and aluminosilicate mineral powders such as fly ash and metakaolin.