Building Materials, Third Edition

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Anyhydrite cement is obtained by burning natural dihydrate gypsum at a temperature of
about 700°C and then grinding the product together with hardening catalyzers (lime, mixture
of sodium sulphate with green or blue vitriol, burned dolomite, granulated basic blast-furnace
slag, etc).
A typical anhydrite binder may be of the following composition: lime, 2–5%; a mixture of
sodium bisulphate or sulphate with green or blue vitriol in amounts of 0.5 to 1% each; dolomite
burned at 800–900°C, 3–8%; granulated basic blast-furnace slag, 10–15%. Green and blue
vitriols consolidate the surface of hardened anhydrite cement, so that the catalyzers do not
seep out and discolour the item’s surface. The action of the catalyzers is due to the ability of
anhydrite to form complex compounds with various salts in the form of an unstable multiple
hydrate, which then decomposes yielding CaSO 4
2H 2 O. Anhydrite cement can also be obtained
by grinding natural anhydrite with the above additives.
Anhydrite cement is a slowly setting binder; its setting starts not earlier than in 30 min and
ends not later than in 24 hours. It is used for preparing brick-laying and plastering mortars,
concretes, heat insulating materials, artificial marble and other ornamental items.
A variety of anhydrite cements is the high-burned gypsum (estrich gypsum). It is
manufactured by burning natural gypsum or anhydrite at a temperature between 800 to
1000°C followed by fine grinding. This results not only in complete dehydration but also in
partial decomposition of anhydrite with the formation of CaO(3–5%) according to the reaction
CaSO 4 = CaO + SO 3. When estrich-gypsum is mixed with water, CaO acts as a catalyzer which
promotes the hardening of the anhydrite cement in a manner discussed above.
High-burned gypsum is used to prepare brick-laying and plastering mortars, to build mosaic
floors, to manufacture artificial marble, etc. Items from high-burned gypsum have low heat
and sound conductivity, higher frost and water resistance and a smaller tendency to plastic
deformation than products from building gypsum.

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It is produced by incompletely dehydrating pure finely ground gypsum at a temperature some

what lower than 185°C. Most plasters theoretically approach — CaSO + H 421 O
2

— which
contains about 6.2 per cent of water.
The setting of plaster of paris is attributed to the formation of gypsum crystals from a
supersaturated aqueous solution. When substances of colloidal nature (for example glue) are
mixed with the plaster the formation of crystals is hindered and the time of set retarded. In
hardening, Plaster of Paris first shrinks then expands. The latter property makes the material
suitable for making casts, since a sharp impression of the mould can be secured. For the same
reason it forms an excellent material for filling cracks, holes in the plastered surfaces and also
on the wooden surfaces before painting/polishing.
Owing to the rapidity of set and difficulty in working, its use in structures is limited to
ornamental works. Being unstable in water it should be used for indoor works only.

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1. White in colour


2. Setting time is 5 to 10 minutes


3. Specific gravity is 2.57.