Properties
Pure tricalcium aluminate is formed when the appropriate proportions of finely divided calcium oxide and aluminium oxide are heated together above 1300 °C. The pure form is cubic, with unit cell dimension 1.5263 nm and has density 3064 kg·m-3. It melts with decomposition at 1542 °C. The unit cell contains 8 cyclic Al6O1818− anions, which can be considered to consist of 6 corner sharing AlO4 tetrahedra.
In Portland cement clinker, tricalcium aluminate occurs as an "interstitial phase", crystallizing from the melt. Its presence in clinker is solely due to the need to obtain liquid at the peak kiln processing temperature (1400–1450 °C), facilitating the formation of the desired silicate phases. Apart from this benefit, its effects on cement properties are mostly undesirable. It forms an impure solid solution phase, with 15-20% of the aluminium atoms replaced by silicon and iron, and with variable amounts of alkali metal atoms replacing calcium, depending upon the availability of alkali oxides in the melt. The impure form has at least four polymorphs:
| Alkali % m/m | Designation | Crystal |
|---|---|---|
| 0-1.0 | CI | Cubic |
| 1.0-2.4 | CII | Cubic |
| 3.7-4.6 | O | Orthorhombic |
| 4.6-5.7 | M | Monoclinic |
Typical chemical compositions are:
| Oxide | Mass % Cubic | Mass % Orthorhombic |
|---|---|---|
| SiO2 | 3.7 | 4.3 |
| Al2O3 | 31.3 | 28.9 |
| Fe2O3 | 5.1 | 6.6 |
| CaO | 56.6 | 53.9 |
| MgO | 1.4 | 1.2 |
| Na2O | 1.0 | 0.6 |
| K2O | 0.7 | 4.0 |
| TiO2 | 0.2 | 0.5 |
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