Civil Engineering Reference
In-Depth Information
excessively exposed to liquid water. The pH of the pore solution in hardened pastes made
from gypsum plasters is around 7. Thus it must be expected that a steel reinforcement, if
used in combination with this type of binder, will undergo corrosion. On the other hand,
gypsum plasters are well compatible with glass or cellulose fibers.
11.2.1
Hemihydrate gypsum plaster (plaster of Paris)
Plaster of Paris is produced by a “dry” dehydration (calcination) of calcium sulfate
dihydrate, which is available either as natural gypsum or as a by-product of the chemical
industry. Increasing amounts of dihydrate are also produced as flue gas gypsum in the
desulfurization of flue gas in power plants that use sulfur-containing fossil fuels as source
of energy. A number of technologies are available to produce plaster of Paris.
The main or sole constituent of plaster of Paris is calcium sulfate hemihydrate. If
natural gypsum is used as the starting material the plaster may also contain some
anhydrite, calcite, or clay minerals. The binder consists of individual particles whose size
and shape corresponds to those of the starting dihydrate. They exhibit a distinct internal
porosity and a relatively large specific surface area as a result of the dry dehydration
process.
Pastes made from plaster of Paris exhibit fast setting and hardening, and the hydration
process is typically completed within hours. Small amounts of chemical additives may be
added to the calcined material to modify its properties. These may include set
accelerators (such as soluble sulfates or prehydrated plaster), set retarders (such as citric
acid or ker-atin), or modifiers of rheology (such as methyl cellulose or carboxy methyl
cellulose).
The strength of the hardened paste depends mainly on its overall porosity, which is
determined by the initial water/binder ratio. To a lesser extent the strength may also be
affected by the size and shape of the formed dihydrate crystals, which may be altered by
the hydration temperature and by chemical additives (Rössler and Odler, 1989).
Very high-strength hardened gypsum pastes can by obtained by mechanical
compaction of flue gas desulfurization gypsum (Stoop
et al.,
1996). By applying
pressures between 50 and 500 MPa the porosity of the material can be reduced to 21-26
vol.%, resulting in compressive strengths of up to 96 MPa. To achieve acceptable results
a few per cent of water must be added to the original desulfurization gypsum powder
prior to compacting. The microstructure of such a material is characterized by the
presence of granular crystals, aligned boundary to boundary without much intergrowth.
Plaster of Paris is the calcium sulfate based binder that is produced and used in the
largest amounts by far. Its main uses include:
• production of gypsum based boards of different kinds;
• gypsum based wall plaster compositions;
• protection of steel and concrete structures from fire damage (fire protective boards or
plasters);
• thermal protection (lightweight boards and mortar compositions).
