A cement is a binder, a substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry, or sand and gravel, produce concrete. Concrete is the most widely used material in existence and is behind only water as the planet’s most-consumed resource.
In this post, Pritish Halder takes a brief look at Cement is a finely milled mineral powder, usually grey in colour.
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 reaction in the production of cement is responsible for about 4% of global CO2 emissions. The overall process is responsible for about 8% of global CO2 emissions, as the cement kiln in which the reaction occurs is typically fired by coal or petroleum coke due to the luminous flame required to heat the kiln by radiant heat transfer. As a result, the production of cement is a major contributor to climate change.
Chemistry
Cement materials can be classified into two distinct categories: non-hydraulic cements and 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 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, being:
C3S: Alite (3CaO·SiO2);
C2S: Belite (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 (firing) process of clinker at high temperature in the kiln. The chemistry of these reactions is not completely clear and is still the object of research.
First, the limestone (calcium carbonate) is burned to remove its carbon, producing lime (calcium oxide) in what is known as a calcination 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
Calcium oxide obtained by thermal decomposition of calcium carbonate at high temperature (above 825 °C).
A less common form of cement is non-hydraulic cement, such as slaked lime (calcium oxide mixed with water), hardens by carbonation in contact with carbon dioxide, which is present in the air (~ 412 vol. ppm ≃ 0.04 vol. %). First calcium oxide (lime) is produced from calcium carbonate (limestone or chalk) by calcination at temperatures above 825 °C (1,517 °F) for about 10 hours at atmospheric pressure:
CaCO3 -> CaO + CO2
The calcium oxide is then spent (slaked) mixing it with water to make slaked lime (calcium hydroxide):
CaO + H2O -> Ca(OH)2
Once the excess water is completely evaporated (this process is technically called setting), the carbonation starts:
Ca(OH)2 + CO2 -> CaCO3 + H2O
This reaction is slow, because the partial pressure of carbon dioxide in the air is low (~ 0.4 millibar). The carbonation reaction requires that the dry cement be exposed to air, so the slaked lime is a non-hydraulic cement and cannot be used under water. This process is called the lime cycle.
Modern cements
Modern hydraulic development began with the start of the Industrial Revolution (around 1800), driven by three main needs:
1)Hydraulic cement render (stucco) for finishing brick buildings in wet climates
2)Hydraulic mortars for masonry construction of harbor works, etc., in contact with sea water
3)Development of strong concretes
Portland cement
Portland cement, a form of hydraulic cement, is by far the most common type of cement in general use around the world. This cement is made by heating limestone (calcium carbonate) with other materials (such as clay) to 1,450 °C (2,640 °F) in a kiln, in a process known as calcination that liberates a molecule of carbon dioxide from the calcium carbonate to form calcium oxide, or quicklime, which then chemically combines with the other materials in the mix to form calcium silicates and other cementitious compounds.
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-specialty grout. The most common use for Portland cement is to make concrete. Concrete is a composite material made of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape, and once it hardens, can be a structural (load bearing) element. Portland cement may be grey or white.
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