Physical and chemical properties
The chemical composition for the variant ultramarine blue dark is Na8Al6Si6O24S4,so it is the polysulphide of a sodium aluminium silicate and thus belongs to the few inorganic pigments that do not have a heavy metal atom.
The blue colour is produced by polysulphide anions in the cavities of a lattice structure of aluminium, silicon, and oxygen atoms.
Temperature resistance
The temperature stability of ultramarine blue is very good, it is stable up to approx. 900 °C. The particle size of the primary particles is between 300 and 8000 nm, depending on the type. The particles are therefore larger than iron oxides and are very well fixed in a concrete matrix, for example. Electrostatic binding analogous to iron oxides also takes place.
Ultramarine blue is insoluble in water and organic solvents. An essential difference to other inorganic pigments is the low stability against acids. It is already destroyed by dilute or weak acids. To prevent this, coated acid-stable grades are available on the market.
Reactions with cement / lime
Although ultramarine is described in the literature as "alkali-resistant", it can be destroyed during concrete production. The cations of the so-called clathrate structure are formed by sodium ions, which can be exchanged by the calcium present in the cement. This exchange of the metal atom in the crystal lattice leads to colour loss. Furthermore, alkalis attack the colouring polysulphide by OH-. This also leads to colour loss or an alteration of shade.
On a large scale, these reactions can take place during the curing phase of the concrete. These reactions are also possible after the hardening process, especially if the concrete product is exposed to free weathering. In principle, the reactions are possible as long as free calcium hydroxide (from cement) and moisture are present in the concrete block.
