Gypsum
Gypsum is a soft sulfate mineral composed of calcium sulfate
dihydrate, with the chemical formula CaSO4·2H2O. It is
widely mined and is used as a fertilizer and as the main constituent in many
forms of plaster, blackboard/ sidewalk chalk, and drywall.
It contains 79.1 % calcium sulfate and 20.9% water. Its specific gravity is 2.3. It is a white crystalline substance. It is not available in nature in pure form but contains impurities such as alumina, calcium carbonate, magnesium carbonate and silica. It is a white crystalline substance less soluble in water but soluble in hydrochloric acid. As a binding material, it sets and hardens quickly. Its initial setting time after addition of water is about 4 to 6 minutes and its final setting time is about 30 minutes.
Effect of heat and moisture on gypsum -
This plaster is mixed with a small of accelerator (i.e., alum
potassium sulphate, raw gypsum) to make anhydrous or Keene’s gypsum plaster.
GYPSUM PLASTER -
Gypsum is used as a binding material instead of Portland cement. Gypsum plaster is a white cementing material made by partial or complete dehydration of the mineral gypsum. When dry gypsum powder is mixed with water it gets hardened. This material can be applied over block, brick or concrete surface to form a smooth surface.
A. Plaster of Paris: is formed by
incomplete calcination at 100- 190°C. No admixtures are found.
B. Hard Wall Plaster: Plaster of Paris +
Admixtures (Glue, Sand...)
Obtained by Complete Calcination (CaSO4)
1. Flooring Plaster (CaSO4 with
no impurities)
2. Hard Finish Plaster (CaSO4 + Al2(SO4)3)
(CaSO4 + Na2B4O7) -
When gypsum is burnt at considerably high temperature than that
for calcining of cement plaster, and treated with certain solutions like alum
and Glauber’s salt (Na2SO4), the plasters so produced show slow setting but
ultimately become very hard. Such plasters may be polished to form a smooth
surface and make a very satisfactory finish for interior walls.
Two commercial hard finish plaster cements are available.
Keene's cement is made by burning a
very pure rock gypsum at a red heat (700°C), cooling, and then adding 1.0 per
cent of potassium and aluminum sulphates to accelerate the set. Subsequently
the material is ground so that 90 per cent or more passes No. 100 sieve. It is
pure CaSO4 of pure white colour. Keene’s cement is not injured by storage and
mortars of it may be retempered. Set occurs between 20 minutes to 6 hours. At 7
days the tensile strength is 3.16 N/mm2. It is used as a finish plaster only
where a greater resistance to moisture and surface abrasion is required.
Mack's cementis made by burning
gypsum at a very high temperature and adding about 0.4 per cent of burnt
Glauber’s salt or potassium sulphate. It is said to form unusually hard, dense
and durable surface which will take paint very well.
Properties of Gypsum -
Following are the properties of gypsum :
(i) It is light in weight and unaffected by bacteria.
(ii) It shows good adhesion to fibrous materials.
(iii) It sets with less change in volume and shows negligible
shrinkage on drying.
(iv) Because of its quick drying property, finishing coats can
be applied immediately.
(v) A porous gypsum is an important sound-absorbing material.
(vi) Gypsum is usually white, colourless, or gray in colour. But
sometimes, it can also be found in the shades of pink, yellow, brown, and light
green, mainly due to the presence of impurities.
(vii) The water solubility of this mineral is affected by
temperature. Unlike other salts, gypsum becomes less soluble in water as the
temperature increases.
(viii) Its specific gravity is 2.3.
(ix) Its initial setting time after addition of water is about 4
to 6 minutes and its final setting time is about 30 minutes.
USES AND PROPERTIES OF GYPSUM PLASTER
Following are the properties and uses of the gypsum plaster.
1. Gypsum plaster is a fire resisting material, and it does not
allow heat to pass easily. Hence it is used as an insulating material to
protect wood or metal columns and beams from high temperature.
2. Gypsum plaster is light in weight and hence its usage for
plastering does not increase the structural load on the building.
3. It is practically not affected by bacteria.
4. It is slightly soluble in water, about 2 gm/litre. Hence it
cannot be adopted for damp conditions and external work. The bags containing
gypsum plaster should therefore be stored in dry place.
5. It is used for ornamental plaster work and for preparation of boards and blocks. The gypsum plaster boards are used for ceiling, for internal lining of wall and for partition walls. They are cheap, easy to work. light in weight and fire proof.
6. It requires small proportion of sand and other aggregates.
When used as base coat, the sand or light weight aggregate or wood fibre is
added. When used as final coat, the lime putty is added.
7. It sets by natural process of crystallization. Hence it can
be applied with case and without wastage.
8. It sets with little change in volume and hence Gypsum plaster
does not shrink during drying and hardening processes unlike cement plaster.
9. It shows good adhesion to the fibrous materials.
10. The plaster of paris is used in artwork, pottery, dentistry
and in surgery for the shaping fractured bones.
11. It is less prone to cracks.
12. Gypsum is non-combustible and contains high content of
crystal water. In the event of fire, it acts like a barrier and protects the
block work, concrete and steel.
13. Gypsum plaster has low thermal conductivity and ensures
energy and power saving.
14. Gypsum plaster gives high tensile and flexural strength.
Gypsum board -
Due to its inherent fire resistance, gypsum board, commonly
known as drywall is the premier building material for wall, ceiling, and
partition systems in residential, institutional, and commercial structures.
Designed to provide a monolithic surface when joints and fastener heads are
covered with a joint treatment system, gypsum products also provide sound
control, economy, versatility, quality, and convenience.
The standard size gypsum boards are 48 in. wide and 8, 10, 12, or 14 ft. long.
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