A cement is a binder, a

chemical substance A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent Chemical element, elements by physical separation m ...

used for construction that sets, hardens, and adheres to other

material Material is a substance or mixture of substances that constitutes an object. Materials can be pure or impure, living or non-living matter. Materials can be classified on the basis of their physical and chemical properties, or on their geolo ...

s to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (

aggregate Aggregate or aggregates may refer to: Computing and mathematics * collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the consistency of changes being made within the ...

) together. Cement mixed with fine aggregate produces mortar for masonry, or with

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...

and

gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classif ...

, produces

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...

. Concrete is the most widely used material in existence and is behind only water as the planet's most-consumed resource. Cements used in construction are usually

inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemis ...

, often lime or

calcium silicate Calcium silicate is the chemical compound Ca2SiO4, also known as calcium orthosilicate and is sometimes formulated as 2CaO·SiO2. It is also referred to by the shortened trade name Cal-Sil or Calsil. It occurs naturally as the mineral larnite ...

based, which can be characterized as hydraulic or the less common non-hydraulic, depending on the ability of the cement to set in the presence of water (see hydraulic and non-hydraulic lime plaster). Hydraulic cements (e.g.,

Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th cen ...

) set and become adhesive through a

chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...

between the dry ingredients and water. The chemical reaction results in mineral hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet conditions or under water and further protects the hardened material from chemical attack. The chemical process for hydraulic cement was found by ancient Romans who used

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

(

pozzolana Pozzolana or pozzuolana ( , ), also known as pozzolanic ash ( la, pulvis puteolanus), is a natural siliceous or siliceous- aluminous material which reacts with calcium hydroxide in the presence of water at room temperature (cf. pozzolanic reactio ...

) with added lime (calcium oxide). Non-hydraulic cement (less common) does not set in wet conditions or under water. Rather, it sets as it dries and reacts with

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

in the air. It is resistant to attack by chemicals after setting. The word "cement" can be traced back to the Ancient Roman term , used to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick supplements that were added to the burnt lime, to obtain a

hydraulic binder Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counte ...

, were later referred to as , , ''cäment'', and ''cement''. In modern times, organic polymers are sometimes used as cements in concrete. World production is about four billion tonnes per year, of which about half is made in China. If the cement industry were a country, it would be the third largest carbon dioxide emitter in the world with up to 2.8 billion tonnes, surpassed only by China and the United States. The initial

calcination Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), gene ...

reaction in the production of cement is responsible for about 4% of global emissions. The overall process is responsible for about 8% of global emissions, as the

cement kiln Cement kilns are used for the pyroprocessing stage of manufacture of portland and other types of hydraulic cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates. Over a billion tonnes of ...

in which the reaction occurs is typically fired by coal or

petroleum coke Petroleum coke, abbreviated coke or petcoke, is a final carbon-rich solid material that derives from oil refining, and is one type of the group of fuels referred to as cokes. Petcoke is the coke that, in particular, derives from a final cracki ...

because a luminous flame is required to heat the kiln by radiant heat transfer. As a result, the production of cement is a major contributor to

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

Chemistry

Cement materials can be classified into two distinct categories: hydraulic cements and non-hydraulic cements according to their respective setting and hardening mechanisms. Hydraulic cement setting and hardening involves hydration reactions and therefore requires water, while non-hydraulic cements only react with a gas and can directly set under air.

Hydraulic cement

By far the most common type of cement is hydraulic cement, which hardens by hydration of the

clinker Clinker may refer to: *Clinker (boat building), construction method for wooden boats *Clinker (waste), waste from industrial processes *Clinker (cement), a kilned then quenched cement product * ''Clinkers'' (album), a 1978 album by saxophonist St ...

minerals when water is added. Hydraulic cements (such as Portland cement) are made of a mixture of silicates and oxides, the four main mineral phases of the clinker, abbreviated in the

cement chemist notation Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals. Abbreviations of oxides The main ...

, being: :C₃S:

Alite Alite is an impure form of tricalcium silicate, , sometimes formulated as ( in cement chemist notation), typically with 3-4% of substituent oxides. It is the major, and characteristic, phase in Portland cement. The name was given by Törnebohm i ...

(3CaO·SiO₂); :C₂S:

Belite Belite is an industrial mineral important in Portland cement manufacture. Its main constituent is dicalcium silicate, Ca2SiO4, sometimes formulated as 2 CaO · SiO2 (C2S in cement chemist notation). Etymology The name was given by Törnebohm i ...

(2CaO·SiO₂); :C₃A: Tricalcium aluminate (3CaO·Al₂O₃) (historically, and still occasionally, called ''celite''); :C₄AF: Brownmillerite (4CaO·Al₂O₃·Fe₂O₃). The silicates are responsible for the cement's mechanical properties — the tricalcium aluminate and brownmillerite are essential for the formation of the liquid phase during the

sintering Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...

(

firing Dismissal (also called firing) is the termination of employment by an employer against the will of the employee. Though such a decision can be made by an employer for a variety of reasons, ranging from an economic downturn to performance-related ...

) process of clinker at high temperature in the

kiln A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay int ...

. The chemistry of these reactions is not completely clear and is still the object of research. First, the

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...

(calcium carbonate) is burned to remove its carbon, producing lime (calcium oxide) in what is known as a

reaction. This single chemical reaction is a major emitter of global carbon dioxide emissions. :CaCO3 -> CaO + CO2 The lime reacts with silicon dioxide to produce dicalcium silicate and tricalcium silicate. :2CaO + SiO2 -> 2CaO.SiO2 :3CaO + SiO2 -> 3CaO.SiO2 The lime also reacts with aluminum oxide to form tricalcium aluminate. :3CaO + Al2O3 -> 3CaO.Al2O3 In the last step, calcium oxide, aluminum oxide, and ferric oxide react together to form cement. :4CaO + Al2O3 + Fe2O3 -> 4CaO.Al2O3.Fe2O3

Non-hydraulic cement

A less common form of cement is non-hydraulic cement, such as

slaked lime Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has ma ...

(

calcium oxide Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "'' lime''" connotes calcium-containing inorganic ...

mixed with water), which hardens by

carbonation Carbonation is the chemical reaction of carbon dioxide to give carbonates, bicarbonates, and carbonic acid. In chemistry, the term is sometimes used in place of carboxylation, which refers to the formation of carboxylic acids. In inorganic ...

in contact with

, which is present in the air (~ 412 vol. ppm ≃ 0.04 vol. %). First

(lime) is produced from calcium carbonate (

limestone Limestone ( calcium carbonate ) is a type of carbon