Physical and chemical properties
The chemical composition of the dark ultramarine blue variant is Na₈Al₆Si₆O₂₄S₄; it is therefore the polysulphide of a sodium aluminium silicate and is thus one of the few inorganic pigments that do not contain any heavy metal atoms.
The blue colour is produced by polysulphide anions in the voids of a lattice structure composed of aluminium, silicon and oxygen atoms.
Temperature resistance
Ultramarine blue has very good temperature resistance and remains stable up to approximately 900 °C. The particle size of the primary particles ranges from 300 to 8,000 nm, depending on the type. The particles are therefore larger than iron oxides and are very well fixed, for example, in a concrete matrix. Electrostatic binding, similar to that of iron oxides, also occurs.
Ultramarine blue is insoluble in water and organic solvents. A key difference from other inorganic pigments is its low stability against acids. It is destroyed even by diluted or weak acids. To prevent this, acid-stable, coated types 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 in the so-called clathrate structure are formed by sodium ions, which can be replaced by the calcium present in the cement. This replacement of the metal atom in the crystal lattice leads to colour loss. Furthermore, alkalis leach out the colour-giving polysulphide via OH⁻. This, too, leads to colour loss or colour change.
On a large scale, these reactions can take place during the hardening phase of the concrete. These reactions are also possible after the hardening process, particularly if the concrete product is exposed to the elements. In principle, the reactions can occur as long as free calcium hydroxide (from the cement) and moisture are present in the concrete block.
