Calcium aluminate sulfate

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Crystal structure
Crystal structure of calcium aluminum sulfate hydrate (ettringite)
Crystal structure of calcium aluminum sulfate hydrate in a projection along c
General
Surname Calcium aluminate sulfate
other names
  • Calcium sulfoaluminate
  • Hexacalciumhexaoxotris [sulfato (2 -)] dialuminate (12-)
Ratio formula Ca 6 Al 2 O 6 (SO 4 ) 3
Brief description

non-flammable solid

External identifiers / databases
CAS number
  • 12004-14-7
  • 12252-12-9 (ettringite or unspecified hydrate)
EC number 234-448-5
ECHA InfoCard 100.031.303
Wikidata Q1026338
properties
Molar mass
  • 678.62 g mol −1
  • 1255.11 g mol −1 (26-hydrate)
Physical state

firmly

density
  • 1.80 g cm −3
  • 1.30–1.40 g cm −3 (hydrate)
Melting point
  • > 400 ° C
  • 80 ° C (beginning of decomposition with loss of crystal water) (hydrate)
solubility
  • poorly soluble in water (0.62 g / l at 20 ° C)
  • approx. 1 g / l in water (hydrate)
safety instructions
GHS labeling of hazardous substances
no GHS pictograms
H and P phrases H: no H-phrases
P: no P-phrases
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Calcium aluminate sulfate is a chemical compound from the group of calcium aluminates with the ratio formula Ca 6 Al 2 O 6 (SO 4 ) 3 . It occurs mainly as a hydrate.

Calcium aluminate sulfate slurry , also called casul or calcium sulfoaluminate, is a calcium aluminate sulfate in suspended form. The empirical formula of the associated solid compound is Ca 6 Al 2 [(OH) 12 (SO 4 ) 3 ] · 24 H 2 O or Ca 6 Al 2 [(OH) 12 (SO 4 ) 3 ] · 26 H 2 O. In construction chemistry, the oxidic sum formula is usually used instead as notation : 3CaO · Al 2 O 3 · 3CaSO 4 · 32H 2 O.

In the case of the compound with 26 mol of water of crystallization and a molar mass of 1255.11, 32 mol of water or 46 percent by weight can be released when heated due to the additional decomposition of the hydroxide ions . The water solubility of calcium aluminate sulfate hydrate is about 1 g / liter and is thus about half as great as the solubility of calcium sulfate .

Occurrence

The salt, which contains water of crystallization, also occurs naturally as mineral ettringite , especially in volcanic deposits, for example in the volcanic Eifel.

Synthetic manufacture

When producing calcium aluminate sulphate slurry, it is important to generate particularly finely divided ettringite crystals with a short reaction time, because only these are relevant for industrial use. Naturally "grown" ettringite is not suitable for industrial marketing.

Two processes are known today for the synthesis:

This process produces a finely crystalline, bright white precipitate which is suspended with suitable dispersants such as polyacrylates or casein to form slurries with 30% to 50% solids.

A particularly highly concentrated slurry with up to 70% solids can be produced using this patented process. This reaction also plays an important role in cement solidification .

  • Impure ettringite is produced from wastewater containing sulphate by adding aluminates and lime.

Special properties

SEM image of Casul

Calcium aluminate sulfate is a particularly white product. It only arises at a pH value above 12 and breaks down into the components calcium aluminate and calcium sulphate at a pH value below 9. Due to its needle-like crystal habit, it tends to matt paint, which is shown by a strong adhesive effect, which was previously felt to be a disadvantage, so that satin white was also called "devil white" or "satan white" in the paper industry. The adhesive effect is used today as a positive property, especially in paints such as emulsion paints and liquid plasters.

Although the refractive index with n D = 1.49 for a white pigment is very low and the wet opacity is correspondingly weak (similar to alabaster ), the dry opacity in coatings containing calcium aluminate sulfate is due to the reflection and the density-related light absorption coefficient (according to Kubelka-Munk ), very good. Finely divided calcium aluminate sulfate (mean particle size <0.5 µm) is therefore particularly suitable as a gloss-imparting component of coating slips for paper coating. The coated and additionally satined papers are characterized above all by high whiteness, opacity, smoothness and gloss and give the paper a pleasant feel . They are used as art paper with an excellent print image and for high-quality food packaging papers.

Because of the high pH value and the slightly dissolved aluminum ions, calcium aluminum sulfate has an antibacterial effect, so that paints and wall paints are protected against bacteria and mold even without preservatives .

application

Individual evidence

  1. ↑ a b c d e f Entry on Hexacalciumhexaoxotris [sulfato (2 -)] dialuminate (12-) in the GESTIS substance database of the IFA , accessed on February 28, 2017(JavaScript required) .
  2. ↑ a b Safety data sheet Remondis Lippewerk
  3. ↑ see under Ettringite
  4. ↑ G: Römpp Lexikon Chemie, 10th edition, 1996-1999 Volume 2: Cm - G . Georg Thieme Verlag, 2014, ISBN 3-13-199971-3 , p. 1245 ( limited preview in Google Book search).
  5. ^ AE Moore, HFW Taylor: Crystal structure of ettringite . In: Acta Crystallographica, Section B . 26, 1970, pp. 386-393. doi : 10.1107 / S0567740870002443 .
  6. ↑ paper dictionary; Gernsbach 1999; ISBN 3-88640-080-8
  7. ↑ Patent EPA 98 963 436.5; Remondis Lippe Plant
  8. ↑ see under Ettringite
  9. ↑ Conversion of dissolved sulfates from process and waste water to secondary raw materials; WLB 6/1996