Cement

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A cement is a binder material, a substance used in construction that sets, hardens and adheres to other materials, binding them together.

While still debated, the main difference between cement and glue is that the former is mainly inorganic, while the latter is most often an organic compound/mixture. Other definitions consider that a glue is a binding material used to adhere two items together without occupying too much of the space between the two items, while a cement is used to encapsulate two items, forming a significant bond. However, many definitions don't consider to be any difference between the two.

Properties

Cements are solid powdered or emulsion materials, usually inorganic, often lime or calcium silicate based, and are characterized in two main categories:

  • Hydraulic 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. Portland cement is the most common hydraulic cement.
  • Non-hydraulic cement will not set in wet conditions or underwater; rather, it sets as it dries and reacts with carbon dioxide in the air. It is resistant to attack by chemicals after setting. Slaked lime (calcium hydroxide-water suspension)

Heating water containing cements will cause them to crack or more often to spall.

Cements generally resist the attack of oxidizers, but many corrosive reagents like acids, bases or halogens will corrode them, the speed depends on the corrosive reagent used and the type of cement.

Hydraulic cements

Portland cement

Main article: Portland cement

Portland cement is a mixture of silicates and oxides, the four main components being alite, belite, brownmillerite and tricalcium aluminate, (historically, and still occasionally, called 'celite'). The silicates are responsible for the mechanical properties of the cement, the tricalcium aluminate and the brownmillerite are essential to allow the formation of the liquid phase during the kiln sintering (firing).

Salt-concrete

Used to reduce the water inflow in mining shafts in salt mines. It is composed of 16% cement, 39% halite, 16% limestone powder, 14% water and 15% sand.

Non-hydraulic cements

Dental cement

Various types of cement materials, classified based on the type of their matrix, like:

  • Phosphate: Water-based cements, like zinc phosphate;
  • Polycarboxylate: Zinc polycarboxylate, glass ionomer;
  • Phenolate: Zinc oxideeugenol, 2-ethoxybenzoic acid;
  • Resin: Acrylate or methacrylate polymeric resins, often UV-light activated

Hempcrete

Made from hemp hurds and lime, works similar to slaked lime, though it gives a lighter and stronger cement.

Slaked lime

Consists of a suspension of calcium hydroxide in water. Setting occurs when the cement dries and reacts with the carbon dioxide and/or sulfur dioxide from air to form calcium carbonate or calcium sulfate.

Sorel cement

Consist of magnesium oxide (burnt magnesia) with magnesium chloride, corresponding to a weight ratio of 2.5–3.5 parts MgO to one part MgCl2.

A zinc variant uses zinc oxide with zinc chloride instead of the magnesium compounds.

Water glass

Main article: Sodium silicate

An aqueous solution of sodium silicate, will harden as the water evaporates.

Xylitol-quicklime

An interesting type of putty/cement experimented by Sciencemadness user deltaH, it is made by mixing four cups of xylitol with two cups of calcium oxide and one cup of water. As the mixture is stirred, the temperature rapidly increases giving off steam and foaming, causing the mixture to thicken, and as it cools it will turn into a plasticine like materials, which, when it hardens at room temperature is very strong. While not hygroscopic, it is not resistant to water, and will break down in the presence of water. It also melts when heated.

Handling and safety

Cements in their powdered form are harmful and/or caustic and proper protection should be worn when working with them.

See also

References

Relevant Sciencemadness threads