Alite

Alite

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Alite is a name for tricalcium silicate, Ca3SiO5, sometimes formulated as 3CaO·SiO2 (C3S in cement chemist notation
Cement chemist notation
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.-Abbreviations of oxides:...

, CCN). It is the major, and characteristic, mineral phase in Portland cement
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...

. The name was given by Törneborn in 1897 to a crystal identified in microscopic investigation of Portland cement. Alite is a name in common use in the cement industry, although it is not a recognised mineral name.

Composition and structure



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 is:

Oxide Mass %
CaO 71.6
SiO2 25.2
Al2O3 1.0
Fe2O3 0.7
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
Clinker
Clinker may refer to:* Clinker , construction method for wooden boats* Clinker , waste from industrial processes* Clinker , a kilned then quenched cement product* Clinker brick, rough dark coloured bricks...

, subject to certain limits. Substitution of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 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
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...

 (SO42-) ions, provided that for each sulfate ion, two aluminate
Aluminate
An aluminate is a compound containing aluminium and oxygen. When precipitated from solution, the anion condenses with more electropositive elements. The generally accepted form of the aluminate is a mononuclear tetrahedral complex that is negatively charged, Al4− or AlO2−...

 (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
Belite is an industrial mineral important in Portland cement manufacture, a name for dicalcium silicate, Ca2SiO4, sometimes formulated as 2 CaO · 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
Belite is an industrial mineral important in Portland cement manufacture, a name for dicalcium silicate, Ca2SiO4, sometimes formulated as 2 CaO · 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


Which can also be written in the cement chemist notation
Cement chemist notation
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.-Abbreviations of oxides:...

 (CCN) as:
2 C3S + 6 H → C3S2H3 + 3 CH

2 alite + 6 H2O → C-S-H + 3 portlandite
Portlandite
Portlandite is a rare oxide mineral, the naturally occurring form of calcium hydroxide . It is the calcium analogue of brucite .-Occurrence:...



The hydrate is referred to as the calcium silicate hydrate – "C-S-H" – phase. It grows as a mass of interlocking needle
Needle
-Crafting:* Knitting needle, a tool for knitting, not as sharp as a sewing needle* Sewing needle, a long slender tool with a pointed tip* Upholstery needle, a tool for upholstery, generally thick and curved-Botany:* Pine needle, the adult leaf of a pine tree...

s that provide the strength
Strength
- Physical ability :*Physical strength, as in people or animals*Superhuman strength, as in fictional characters*A common character attribute in role-playing gamesConflict between persons or groups:*Virtue and moral uprightness...

 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.

Alite as precursor of silicate phases found in medieval lime mortar


The composition of alite rich in CaO (71.6 wt. %) and relatively poor in SiO2 (25.2 wt. %) (see the hereabove table) may help to understand why in particular conditions, if a sufficiently high temperature is reached in a lime kiln during enough time, alite can also be directly formed by pyrolizing only siliceous limestone (containing amorphous SiO2 impurities up to 25 – 30 wt. %). Hydraulic mortar or pre-Portland cement may have been occasionally produced on a small scale in this way during the medieval epoch in locations where limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 was cemented by amorphous silica or contained chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...

 nodule
Nodule (geology)
A nodule in petrology or mineralogy is a secondary structure, generally spherical or irregularly rounded in shape. Nodules are typically solid replacement bodies of chert or iron oxides formed during diagenesis of a sedimentary rock...

s or a lot of clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 impurities.

This is likely the reason why some old medieval lime mortar
Lime mortar
Lime mortar is a type of mortar composed of lime and an aggregate such as sand, mixed with water. It is one of the oldest known types of mortar, dating back to the 4th century BC and widely used in Ancient Rome and Greece, when it largely replaced the clay and gypsum mortars common to Ancient...

s used to build the Tournai cathedral
Tournai Cathedral
The Cathedral of Our Lady is Roman Catholic church, see of the Diocese of Tournai in Tournai, Belgium. It has been classified both as a Wallonia's major heritage since 1936 and as a World Heritage Site since 2000.-History:...

 (Belgium) exhibit an unexpected hydraulic character as revealed by a mineralogical study made by Mertens et al. (2006) who evidenced the presence of wollastonite
Wollastonite
Wollastonite is a calcium inosilicate mineral that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolostone is subjected to high temperature and pressure sometimes in the presence of silica-bearing fluids...

 and rankinite along with CSH
CSH
CSH is a three-letter acronym with multiple meanings:* Caesium hydride, a crystalline solid with the molecular formula CsH* Calcium silicate hydrate, or calcium silicate hydrogel, the main component of cement paste...

 phases in lime mortars. The only explanation for the discovery of these silicate phases not normally expected in lime mortar is that they have been formed by the hydration of calcium silicate such as Ca3SiO5 (C3S) or Ca2SiO4 (C2S) formed at high temperature along calcium oxide
Calcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....

 in the lime kiln. Indeed, in the area of Tournai
Tournai
Tournai is a Walloon city and municipality of Belgium located 85 kilometres southwest of Brussels, on the river Scheldt, in the province of Hainaut....

 (Belgium
Belgium
Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...

), the Tournaisian
Tournaisian
The Tournaisian is in the ICS geologic timescale the lowest stage or oldest age of the Mississippian, the oldest subsystem of the Carboniferous. The Tournaisian age lasted from 359.2 ± 2.5 Ma to 345.3 ± 2.1 Ma...

 limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

s are particularly rich in amorphous silica and exploited as building stone and for making lime mortar since very ancient ages. It is presently unknown if the cathedral
Cathedral
A cathedral is a Christian church that contains the seat of a bishop...

builders of this area were aware of the hydraulic properties of their lime mortar or intentionally developed its use after their fortuitous finding.