Cement is a binder substance used for constructions which sets, hardens, and adheres materials to be bound together. Although it is used individually, it is rather the substance which binds sand and gravel together. If mixed with fine aggregate, a mortar for masonry will be produced and by mixing it with sand and gravel, concrete will be produced.
The Cements used in construction are usually inorganic, mostly with lime or calcium silicate base and depending on its ability to set in presence of water may be characterized as hydraulic or non-hydraulic.
Hydraulic cements (such as Portland cement) set and become adhesive due to a chemical reaction 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 to be set 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 used volcanic ash (pozzolana) with added lime (calcium oxide).
Non-hydraulic cement, such as slaked lime (calcium oxide mixed with water), hardens by carbonation in the presence of carbon dioxide, naturally present in the air. First calcium oxide (lime) is produced from calcium carbonate (limestone or chalk) by calcination at temperatures above 825 °C for about 10 hours at atmospheric pressure:
CaCO3 → CaO + CO2
The calcium oxide is then spent 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 time consuming, because the partial pressure of carbon dioxide in the air is low. 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.
Conversely, hydraulic cement hardens by hydration when water is added. Hydraulic cements (such as Portland cement) are made of a mixture of silicates and oxides, the four main components being:
Belite (2CaO·SiO2); Alite (3CaO·SiO2); Tricalcium aluminate (3CaO·Al2O3); Brownmillerite (4CaO·Al2O3·Fe2O3).
The silicates are responsible for the cement’s mechanical properties—the tri-calcium aluminate and brown millerite are essential for formation of the liquid phase during the kiln sintering (firing).
Types of Portland cement
The ASTM has designated five types of Portland cement, designated Types I-V. Physically and chemically, these cement types differ primarily in their content of C3A and in their fineness. In terms of performance, they differ primarily in the rate of early hydration and in their ability to resist sulfate attack. The general characteristics of these types are listed in Table below.
General features of the main types of Portland cement.
|General purpose||Fairly high C3S content for good early strength development||General construction (most buildings, bridges, pavements, precast units, etc.)|
|Moderate sulfate resistance||Low C3A content (<8%)||Structures exposed to soil or water containing sulfate ions|
|High early strength||Ground more finely, may have slightly more C3S||Rapid construction, cold weather concreting|
|Low heat of hydration (slow reacting)||Low content of C3S (<50%) and C3A||Massive structures such as dams. Now rare.|
|High sulfate resistance||Very low C3A content (<5%)||Structures exposed to high levels of sulfate ions|
|White color||No C4AF, low MgO||Decorative (otherwise has properties similar to Type I)|
The differences between these cement types are rather subtle. All five types contain about 75wt% calcium silicate minerals, and the properties of mature concretes made with all five are quite similar. Thus these five types are often described by the term “ordinary Portland cement”, or OPC.
Types II and V OPC are designed to be resistant to sulfate attack. This is an important phenomenon which may cause severe damage to concrete structures, a chemical reaction between the hydration products of C3A and sulfate ions that enter the concrete from the outside environment. This reaction’s products have a larger volume than the reactants, and this creates stresses that force the concrete to expand and crack. Although hydration products of C4AF are similar to those of C3A, they are less vulnerable to expansion, so the designations for Type II and Type V cement focus on keeping the C3A content low. There is actually little difference between a Type I and Type II cement, hence it is common to see cements meeting both designations labeled as “Type I/II”.
Type III cement is designed to develop early strength more quickly than a Type I cement. This is useful for maintaining a rapid pace of construction, since it allows cast-in-place concrete to bear loads sooner and it reduces the time that precast concrete elements must remain in their forms. These advantages are particularly important in cold weather, which significantly reduces the rate of hydration (and thus strength gain) of all Portland cements. The downsides of rapid-reacting cements are a shorter period of workability, greater heat of hydration, and a slightly lower ultimate strength.
Type IV cement is designed to release heat more slowly than a Type I cement, meaning of course that it also gains strength more slowly. A slower rate of heat release limits the increase in the core temperature of a concrete element. The maximum temperature scales with the size of the structure, and Type III concrete was developed because of the problem of excessive temperature rise in the interior of very large concrete structures such as dams. Type IV cement is rarely used today, because similar properties can be obtained by using a blended cement.
White Portland cement (WPC) is made with raw ingredients that are low in iron and magnesium, the elements that give cement its grey color. These elements contribute essentially nothing to the properties of cement paste, so white Portland cement actually has quite good properties. It tends to be significantly more expensive than OPC, however, so it is typically confined to architectural applications. WPC is sometimes used for basic cements research because the lack of iron improves the resolution of nuclear magnetic resonance (NMR) measurements.
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