Alite
Alite is an impure form of
Composition and structure
The alite found in Portland cement differs in composition from simple
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
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 (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
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.[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 the cement chemist notation as:
- 2C3S + 6H → C3S2H3 + 3CH
- + 6H2O → C-S-H +
The hydrate is referred to as the
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
impurities.This is likely the reason why some old medieval
Detection
See the article on belite.
See also
- Cement kiln – High temperature rotating oven used for producing clinker
- Cement chemist notation – Abbreviated notation for chemical formulas of common oxides
Notes
- ^ OCLC 38207086.
- ISBN 2-912143-04-7..
- ^ "Hatrurite". mindat.org. Hudson Institute of Mineralogy. Retrieved November 5, 2020.
- OCLC 925061061.
- .
- ISSN 1960-1360. Retrieved 2010-07-21.