Cement
Binder that sets, hardens, and binds other materials together.
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Cement is a construction binder that hardens and sticks to other materials, joining them together. It is rarely used by itself; instead, it binds sand and gravel (called aggregate). When mixed with fine aggregate, it makes mortar for masonry; with sand and gravel, it becomes concrete. Concrete is the most widely used material on Earth, second only to water in global consumption. Construction cements are typically inorganic, based on lime or calcium silicate, and are either hydraulic or, less often, non-hydraulic.
Hydraulic cements, like Portland cement, set through a chemical reaction between dry ingredients and water. This produces mineral hydrates that are not very water-soluble, allowing setting in wet conditions or underwater and protecting the hardened material from chemical attack. The ancient Romans discovered this process using volcanic ash (pozzolana) with added lime. Non-hydraulic cement does not set in wet conditions or underwater; instead, it hardens as it dries and reacts with carbon dioxide in the air. After setting, it resists chemical attack.
The word "cement" comes from the Roman term *opus caementicium*, describing masonry like modern concrete made from crushed rock and burnt lime. Volcanic ash and crushed brick added to burnt lime for a hydraulic binder were later called *cementum*, *cimentum*, *cäment*, and finally *cement*. Today, organic polymers are sometimes used as cements in concrete.
World cement production is about 4.4 billion tonnes per year (2021 estimate), with roughly half made in China, followed by India and Vietnam. The production process causes nearly 8% (2018) of global CO₂ emissions, from heating raw materials in a kiln and releasing CO₂ from calcium carbonate (calcination). However, hydrated products like concrete gradually reabsorb atmospheric CO₂ (carbonation), offsetting about 30% of initial emissions.
**Chemistry**
Cements fall into two categories based on how they set and harden: hydraulic and non-hydraulic. Hydraulic cements require water for hydration reactions; non-hydraulic cements react only with a gas and set in air.
**Hydraulic cement**
The most common type is hydraulic cement, which hardens by hydration when water is added to clinker minerals. Hydraulic cements (like Portland cement) are a mix of silicates and oxides. The four main clinker mineral phases, in cement chemist notation, are: - C₃S: alite (3CaO·SiO₂) - C₂S: belite (2CaO·SiO₂) - C₃A: tricalcium aluminate (3CaO·Al₂O₃) - C₄AF: calcium aluminoferrite (4CaO·Al₂O₃·Fe₂O₃)
The silicates give cement its mechanical properties; tricalcium aluminate and brownmillerite help form a liquid phase during high-temperature sintering in the kiln. The exact chemistry is still being researched. First, limestone (calcium carbonate) is burned to remove carbon, producing lime (calcium oxide) in a calcination reaction—a major source of global CO₂ emissions: CaCO₃ → CaO + CO₂
Lime then reacts with silicon dioxide to form dicalcium silicate and tricalcium silicate: 2CaO + SiO₂ → 2CaO·SiO₂ 3CaO + SiO₂ → 3CaO·SiO₂
Lime also reacts with aluminium oxide to form tricalcium aluminate: 3CaO + Al₂O₃ → 3CaO·Al₂O₃
Finally, calcium oxide, aluminium oxide, and ferric oxide react to form calcium aluminoferrite: 4CaO + Al₂O₃ + Fe₂O₃ → 4CaO·Al₂O₃·Fe₂O₃
**Non-hydraulic cement**
A less common form is non-hydraulic cement, such as slaked lime (calcium oxide mixed with water). It hardens by carbonation when exposed to carbon dioxide in the air (about 0.04% by volume). First, calcium oxide (lime) is made from calcium carbonate by calcination above 825°C (1,517°F) for about 10 hours at atmospheric pressure: CaCO₃ → CaO + CO₂
The calcium oxide is then slaked with water to make calcium hydroxide: CaO + H₂O → Ca(OH)₂
After excess water evaporates (called setting), carbonation begins: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
This reaction is slow because the partial pressure of CO₂ in air is low (~0.4 millibar). The dry cement must be exposed to air, so slaked lime is non-hydraulic and cannot be used underwater. This is called the lime cycle.
**History**
The earliest known cement deposit dates from about twelve million years ago, formed when oil shale adjacent to limestone burned naturally. These deposits were studied in the 1960s and 1970s.
**Alternatives to cement used in antiquity**
Cement, chemically, is a product with lime as the main binding ingredient, but it was not the first material used for cementation. Babylonians and Assyrians used bitumen (asphalt or pitch) to bind burnt brick or alabaster slabs. In Ancient Egypt, stone blocks were joined with mortar made from sand and roughly burnt gypsum.
- field
- Construction materials
- known_for
- Binding agent in concrete, the most widely used material after water
- annual_production
- About 4.4 billion tonnes (2021 estimate)
- top_producer
- China (about half of world production)
- co2_emissions_share
- Nearly 8% of global CO2 emissions (2018)
- carbonation_reabsorption
- Approximately 30% of initial CO2 emissions reabsorbed by hydrated products
Lore & Background
The word 'cement' traces back to the Ancient Roman term opus caementicium, describing masonry made from crushed rock with burnt lime as binder. Volcanic ash and pulverized brick added to burnt lime for hydraulic binder were later called cementum, cimentum, cäment, and cement. The earliest known occurrence of cement is from twelve million years ago, formed after oil shale adjacent to limestone burned by natural causes. Ancient Romans used volcanic ash (pozzolana) with added lime to create hydraulic cement that could set under water. The Greeks used volcanic tuff from the island of Thera as pozzolan, and Romans used crushed volcanic ash from Pozzuoli. In the absence of pozzolanic ash, Romans used powdered brick or pottery. The huge dome of the Pantheon and the Baths of Caracalla are examples of ancient structures made from these concretes.
Reader's Guide
Cement is fundamental to modern construction, serving as the binding agent in concrete, the most consumed material on Earth after water. Its significance lies in its versatility: hydraulic cements like Portland cement set through chemical reactions with water, allowing use in wet conditions and underwater, while non-hydraulic cements set by reacting with carbon dioxide in air. The production process, however, carries substantial environmental impact, responsible for nearly 8% of global CO2 emissions in 2018, primarily from calcination of limestone and fuel combustion. Notably, concrete gradually reabsorbs about 30% of these emissions through carbonation. The chemistry involves four main mineral phases: alite, belite, tricalcium aluminate, and calcium aluminoferrite. Historically, Roman engineers demonstrated the durability of hydraulic cement in structures still standing today. Modern production exceeds 4.4 billion tonnes annually, with China producing about half. The material's legacy includes both ancient Roman engineering and contemporary global infrastructure, though its environmental footprint remains a challenge.
Did You Know?
- Concrete is the most widely used material in existence, behind only water as the planet's most-consumed resource.
- The earliest known occurrence of cement is from twelve million years ago, formed by natural burning of oil shale adjacent to limestone.
- The word 'cement' comes from the Ancient Roman term opus caementicium.
- Hydrated products of cement, such as concrete, gradually reabsorb atmospheric CO2, compensating for approximately 30% of initial CO2 emissions.
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Frequently Asked Questions
Who is Cement?
Cement is an inorganic binding agent—typically lime- or calcium-silicate based—that hardens over time and glues other materials into a single solid mass. It almost never works alone; instead, it serves as the glue that locks sand, gravel, and other aggregates into a unified structure.
What are Cement's powers/role?
Its core ability is to set, harden, and create an adhesive bond that fuses separate particles into one cohesive unit. When paired with fine aggregate it becomes mortar for brickwork, and when combined with sand and gravel it becomes concrete—the single most-produced material on Earth after water.
How does Cement's story end?
Rather than a dramatic finale, Cement's life concludes through a slow chemical process called carbonation, during which its hydrated products gradually reabsorb roughly 30% of the CO₂ originally released when it was manufactured. In this way, its 'ending' partially offsets the environmental cost of its creation.
Why is Cement important?
It underpins the most widely used material in human history—concrete—making it foundational to virtually every modern structure, road, and infrastructure project. Global output sits at roughly 4.4 billion tonnes per year, with China alone accounting for about half of that total.
What's Cement's biggest controversy?
The production process is responsible for nearly 8% of all global CO₂ emissions, making it one of the single largest industrial sources of greenhouse gas release. This has driven ongoing research into low-carbon alternatives and supplementary cementitious materials.
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