Calcium aluminates are obtained by having lime and alumina react at high temperature. Lime (or calcium oxide) generally comes from limestone, alumina (or aluminium oxide) is contained in natural minerals such as bauxite, or is refined beforehand in the form of Bayer alumina. The product of this reaction between lime and alumina, after cooling, is a hard ore called calcium aluminate clinker.
This clinker when ground to a powder is called calcium aluminate cement (CAC). Depending on the proportion of alumina and lime as well as impurities in the minerals used, these powders may vary in colour from the whitest white via all shades of beige to jet black.
When mixed with water, these cements take on a pasty texture to rapidly harden forming a rigid solid, thus their generic hydraulic binder name meaning a powder that reacts with water to give a "cement".
Historically, calcium aluminates were first invented at the start of the 20th century to be used as hydraulic binders due to their improved corrosion resistance properties compared to other cements. They were largely developed between the two world wars because of their quick hardening property, even in cold weather, to rapidly recommission civil works. Lastly, their resistance to heat saw the rapid development of castable refractory concrete as a complement to or replacement of fire bricks.
Calcium aluminate cements can also be combined with other fine components to obtain hydraulic binders with new properties. In this case, calcium aluminate cements are both a complex hydraulic binder and a mineral reagent interacting with the other components in the mixture. In modern castables with low cement content (LCC), calcium aluminate cements are combined with either ultrafine silica (silica fume) or ground calcined alumina. These concretes are more temperature and corrosion-resistant. Their plasticity during pouring is also better. By mixing Portland cement with a calcium aluminate cement, a rapid setting hydraulic binder is obtained. According to the proportions of the two cements, setting time can vary from a few minutes to an hour. When combined with calcium sulphate (gypsum, plaster or anhydrite), calcium aluminate cements obtain an "ettringite" hydraulic binder, i.e. which in the presence of water rapidly forms a hydrated mineral called ettringite. Ettringite binders have generated a vast family of products for civil works and construction, in particular technical mortars, floor resurfacing mortars and tiling cement. Organic additives are used in most of these complex hydraulic binders called formulations to improve their properties.
Calcium aluminates can also be used as reagents in non-hydraulic systems.
This is the case in metallurgical treatments (iron and steel industry, foundry) where certain special grades of calcium aluminate are used for their low-temperature melting properties and trapping molten metal impurities.
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