Alite is a name for Tricalcium Silicate, Ca3SiO5, sometimes formulated as 3CaO.SiO2 (CCN Cement chemist notation was developed to simplify the formulas cement chemists use on a daily basis. It is a "short hand" way of writing the chemical formula of oxides of calcium, silicon, and various metals C3S). It is the major, and characteristic, mineral in Portland cement Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout. It is a fine powder produced by grinding Portland cement clinker (more than 90%), a limited amount of calcium sulfate (which controls the set time) and up to 5% minor. The name was given by Törneborn in 1897 to a crystal identified in microscopic investigation of portland cement.[1] Alite is a name in common use in the cement industry, and is not a recognised mineral name.

Contents

Composition and structure

Simplified Crystal Structure of Alite

The alite found in Portland cement differs in composition from simple tricalcium silicate. It is a solid solution and contains minor amounts of other oxides besides CaO and SiO2. A typical composition[2] is:

Oxide Mass %
SiO2 25.2
Al2O3 1.0
Fe2O3 0.7
CaO 71.6
MgO 1.1
Na2O 0.1
K2O 0.1
P2O5 0.2

Based on this, the formula can be expressed as Ca2.90Mg0.06Na0.01Fe0.03Al0.04Si0.95P0.01O5. In practice, the composition varies with the bulk composition of the clinker, subject to certain limits. Substitution of calcium ions or orthosilicate ions requires that electric charges be kept in balance. For instance, a limited number of orthosilicate (SiO44-) ions can be replaced with sulfate (SO42-) ions, provided that for each sulfate ion, two aluminate (AlO45-) ions are also substituted.

Polymorphs

Tricalcium silicate is thermodynamically unstable below 1250°C, but can be preserved in a metastable state at room temperature by fast cooling: on slow cooling it tends to revert to belite Belite is an industrial mineral important in Portland cement manufacture, a name for dicalcium silicate, Ca2SiO4, sometimes formulated as 2CaO.SiO2 (Ca2SiO4) and CaO.

As the temperature changes, it passes through several polymorphic states:

Temp° Name Crystal
>1070 R Rhombohedral
1060-1070 M3 Monoclinic
990-1060 M2 Monoclinic
980-990 M1 Monoclinic
920-980 T3 Triclinic
620-920 T2 Triclinic
<620 T1 Triclinic

The polymorphs differ structurally by minor deformations from the basic hexagonal structure.

Hydration

Alite is the mineral in Portland cement responsible for setting and development of "early" strength. The other silicate, belite Belite is an industrial mineral important in Portland cement manufacture, a name for dicalcium silicate, Ca2SiO4, sometimes formulated as 2CaO.SiO2 contributes "late" strength, due to its lower reactivity. Alite is more reactive because of its higher Ca content, and the presence of an oxide ion in the lattice. It reacts with water (roughly) according to the reaction:

2Ca3SiO5 + 6H2O → 3CaO.2SiO2.3H2O + 3Ca(OH)2

The hydrate is referred to as the "C-S-H" phase. It grows as a mass of interlocking needles that provide the strength of the hydrated cement system. High alite reactivity is desirable in Portland cement manufacture, and this is achieved by retaining, as far as possible, high temperature polymorphs, in crystals that are small, distorted and highly defective. Defects provide sites for initial water attack.

Detection

See the article on belite Belite is an industrial mineral important in Portland cement manufacture, a name for dicalcium silicate, Ca2SiO4, sometimes formulated as 2CaO.SiO2.

Notes

  1. ^ Jean-Pierre Bournazel, Yves Malier, Micheline Moranville Regourd, 1998, Concrete, from Material to Structure RILEM Publications, ISBN 2912143047
  2. ^ H F W Taylor, Cement Chemistry, Academic Press, 1990, ISBN 0-12-683900-X, pp 10-11

Note:The word Alite is also used by fashion designer Thony C Anyiam to describe his Afrocentric line of men's formal wear. As he described it Alite stands for top of the line or class.website:www.anyiams.com

Categories: Silicates | Calcium compounds For chemical compounds containing calcium. For an alternative listing please see inorganic_compounds_by_element#Calcium | Cement

 

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