Ultramarine blue is an inorganic pigment. The chemical composition reads Na8Al6Si6O24S4, thus it is the polysulphide of a sodium aluminium silicate. It is one of the few inorganic pigments that do not contain any heavy metal atom.
Ultramarine blue is the synthetic equivalent to the natural semi-precious stone "lapis lazuli". The synthesis is achieved by melting down clay, quartz, soda, sulphur and charcoal.
|Temperature resistance||very high, up to approx. 900 °C stable|
|Particle size||The size of the primary particle is between 300 and 8000 nm, depending on the type. Therefore, the particles are larger than that of iron oxides and bond very well in the concrete matrix. An electrostatic bond is also analogous to iron oxides.|
|Solubility||insoluble in water and organic solvents|
|Chemical stability, acids||Normal Ultramarine is already destroyed by diluted and weak acids. There are coated acid-stable types.|
Reaction with cement / lime:
Although literature describes Ultramarine as "resistant to alkali", it can be destroyed in the concrete production process:
- The calcium of the cement replaces the sodium ion. The replacement of the metal atom in the crystal lattice results in a loss of colour.
- OH replaces the polysulphide group. This again results in a loss of colour or an alteration of shade.
To a large extent, these reactions may occur during the concrete hardening process. However, these reactions may also occur after the hardening process, in particular, when the concrete product is exposed to outdoor weathering. In principle, the reactions are as long possible as there is free calcium hydroxide (from cement) and moisture in the concrete block.