Alite
Aliteis an impure form oftricalcium silicate,Ca3SiO5,sometimes formulated as3CaO·SiO2(C3Sincement chemist notation), typically with 3-4% of substituent oxides.[1]It is the major, and characteristic, phase inPortland cement.The name was given byTörnebohmin 1897 to a crystal identified in microscopic investigation of Portland cement.[2]Hatrurite is the name of a mineral that is substitutedC3S.[1][3]
Composition and structure[edit]
The alite found in Portland cement differs in composition from simpletricalcium silicate.It is a solid solution and contains minor amounts of other oxides besides CaO andSiO2.A typical composition is:[4]
| 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 asCa2.90Mg0.06Na0.01Fe0.03Al0.04Si0.95P0.01O5.In practice, the composition varies with the bulk composition of theclinker,subject to certain limits. Substitution ofcalciumions or orthosilicate ions requires that electric charges be kept in balance. For instance, a limited number oforthosilicate(SiO4−4) ions can be replaced withsulfate(SO2−4) ions, provided that for each sulfate ion, twoaluminate(AlO5−4) ions are also substituted.
Polymorphs[edit]
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 tobelite(Ca2SiO4) and CaO.
As the temperature changes, it passes through several polymorphic states:
| Temp (°C) | 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 steps[edit]
Alite is the major phase in Portland cement responsible for setting and development of "early" strength. The other silicate,belitecontributes "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.[5]
- 3Ca+2+ SiO4−4+ O2−+ H2O → 3Ca2++ SiO4−4+ 2OH−
It reacts with water (roughly) according to the reaction:
- 2Ca3SiO5+ 6H2O → 3CaO·2SiO2·3H2O + 3Ca(OH)2
Which can also be written in thecement chemist notationas:
- 2C3S + 6H → C3S2H3+ 3CH
- +6H2O →C-S-H +
The hydrate is referred to as thecalcium silicate hydrate– "C-S-H-" – phase. It grows as a mass ofinterlockingneedles that provide thestrengthof 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[edit]
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 alime kilnduring enough time, alite can also be directly formed by pyrolizing onlysiliceous limestone(containing amorphous SiO2impurities up to 25 – 30 wt. %). Hydraulic mortar or pre-Portland cement may have been occasionally produced on a small scale in this way during themedievalepoch in locations wherelimestonewas cemented byamorphous silicaor containedchertnodulesor a lot ofclayimpurities.
This is likely the reason why some old medievallime mortarsused to build theTournai cathedral(Belgium) exhibit an unexpected hydraulic character as revealed by a mineralogical study made by Mertenset al.(2006) who evidenced the presence ofwollastoniteandrankinitealong withcalcium silicate hydratephases 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 asCa3SiO5(C3S) orCa2SiO4(C2S) formed at high temperature alongcalcium oxidein thelime kiln.[6]In the area ofTournai(Belgium), theTournaisianlimestonesare particularly rich in amorphous silica and exploited as building stone and for making lime mortar since very ancient ages. It is unknown if thecathedralbuilders of this area were aware of the hydraulic properties of their lime mortar or intentionally developed its use after their fortuitous finding.
Detection[edit]
See the article onbelite.
See also[edit]
- Cement kiln– High temperature rotating oven used for producing clinker
- Cement chemist notation– Abbreviated notation for chemical formulas of common oxides
Notes[edit]
- ^abTaylor, H.F W. (1997).Cement Chemistry(2nd ed.). London: T. Telford.ISBN0-7277-2592-0.OCLC38207086.
- ^Bournazel, Jean-Pierre; Malier, Yves; Regourd, Micheline Moranville (1998).Concrete, from material to structure.Rilem Publications.ISBN2-912143-04-7..
- ^"Hatrurite".mindat.org.Hudson Institute of Mineralogy.RetrievedNovember 5,2020.
- ^Taylor, H.F.W. (1990).Cement Chemistry.Academic Press. pp. 10–11.ISBN0-12-683900-X.OCLC925061061.
- ^R. K. Mishra; R. J. Flatt; H. Heinz (2013). "Force Field for Tricalcium Silicate and Insight into Nanoscale Properties: Cleavage, Initial Hydration, and Adsorption of Organic Molecules".Journal of Physical Chemistry C.117(20): 10417–10432.doi:10.1021/jp312815g.
- ^Mertens, Gilles; Elsen, Jan; Laduron, Dominique; Brulet, Raymond (2006-12-31)."Minéralogie des silicates de calcium présents dans des mortiers anciens à Tournai".ArchéoSciences.Varia (30): 61–65.doi:10.4000/archeosciences.150.ISSN1960-1360.Retrieved2010-07-21.