Alite is an impure form of tricalcium silicate, , sometimes formulated as ( in

cement chemist notation Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals. Abbreviations of oxides The main ...

), typically with 3-4% of substituent oxides. It is the major, and characteristic, phase in

Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th cen ...

. The name was given by Törnebohm in 1897 to a crystal identified in microscopic investigation of Portland cement. Hatrurite is the name of a mineral that is substituted .

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 . A typical composition is: Based on this, the formula can be expressed as . In practice, the composition varies with the bulk composition of the

clinker Clinker may refer to: *Clinker (boat building), construction method for wooden boats *Clinker (waste), waste from industrial processes *Clinker (cement), a kilned then quenched cement product * ''Clinkers'' (album), a 1978 album by saxophonist St ...

, subject to certain limits. Substitution of

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

ions or orthosilicate ions requires that electric charges be kept in balance. For instance, a limited number of orthosilicate () ions can be replaced with

sulfate The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...

() ions, provided that for each sulfate ion, two aluminate () 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 () and CaO. As the temperature changes, it passes through several polymorphic states: The polymorphs differ structurally by minor deformations from the basic hexagonal structure.

Hydration steps

Alite is the major phase in Portland cement responsible for setting and development of "early" strength. The other silicate, belite 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. During clinker grinding, first step of partial dissolution of C₃S involves hydration of superficial oxide ions and leads to a hydroxylated C₃S surface. : It reacts with water (roughly) according to the reaction: : Which can also be written in the

as: : : + C-S-H + The hydrate is referred to as the

calcium silicate hydrate Calcium silicate hydrate (or C-S-H) is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials (e.g. concrete). Preparation When water is added to cement, each of the compounds u ...

– "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.

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 A lime kiln is a kiln used for the calcination of limestone ( calcium carbonate) to produce the form of lime called quicklime ( calcium oxide). The chemical equation for this reaction is : CaCO3 + heat → CaO + CO2 This reaction can ta ...

during enough time, alite can also be directly formed by pyrolizing only

siliceous limestone Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

(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 In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire a ...

epoch in locations where

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...

was cemented by amorphous silica or contained

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...

nodules or a lot of

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay part ...

impurities. This is likely the reason why some old medieval lime mortars used to build the

Tournai cathedral The Tournai Cathedral, or Cathedral of Our Lady (french: Notre-Dame de Tournai, nl, Onze-Lieve-Vrouw van Doornik), is a Roman Catholic church, see of the Diocese of Tournai in Tournai, Belgium. It has been classified both as a Wallonia's ma ...

(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 is a calcium inosilicate mineral ( Ca Si O3) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolomite is subjected to high temperature ...

and rankinite along with

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 (C₃S) or (C₂S) formed at high temperature along

calcium oxide Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "'' lime''" connotes calcium-containing inorganic ...

in the

.{{Cite journal , last1 = Mertens , first1 = Gilles , last2=Elsen , first2=Jan , last3=Laduron , first3=Dominique , last4=Brulet , first4=Raymond , date = 2006-12-31 , title = Minéralogie des silicates de calcium présents dans des mortiers anciens à Tournai , journal = ArchéoSciences , issue = 30 , pages = 61–65 , series = Varia , doi = 10.4000/archeosciences.150 , issn = 1960-1360 , url = http://archeosciences.revues.org/index150.html , accessdate = 2010-07-21 , doi-access = free In the area of

Tournai Tournai or Tournay ( ; ; nl, Doornik ; pcd, Tornai; wa, Tornè ; la, Tornacum) is a city and municipality of Wallonia located in the province of Hainaut, Belgium. It lies southwest of Brussels on the river Scheldt. Tournai is part of Eu ...

(

Belgium Belgium, ; french: Belgique ; german: Belgien officially the Kingdom of Belgium, is a country in Northwestern Europe. The country is bordered by the Netherlands to the north, Germany to the east, Luxembourg to the southeast, France to ...

), the Tournaisian

s are particularly rich in amorphous silica and exploited as building stone and for making lime mortar since very ancient ages. It is unknown if the

cathedral A cathedral is a church that contains the ''cathedra'' () of a bishop, thus serving as the central church of a diocese, conference, or episcopate. Churches with the function of "cathedral" are usually specific to those Christian denominatio ...

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

Detection

See the article on belite.

Notes