This document discusses different types of gypsum plasters used in construction. It describes four classes of gypsum plaster (A, B, C, D) produced by heating gypsum rock to different temperatures, which results in materials with varying water content and setting times. Class A sets very quickly while Class B has retarded setting times adjusted for use in undercoats versus finishing coats. Class C and D set more slowly and require accelerators. The document also provides application guidelines for different plaster mixes and backgrounds.

1. PLASTERING AND DRY LINING
Building Construction III
Spring 2012
Dr.Hikmat Hammad
Lecture 4

2. Gypsum Plasters
• In recent years the use of Gypsum plasters increased
greatly and in most cases replaced the cement plasters
in internal applications.
• The advantages of Gypsum is that it expands slightly on
setting and drying prevents cracking
• Gypsum is a common mineral available as calcium
sulphate dehydrate CASO4 2H2O in the nature.
• It is usually combined with some improve clay,
limestone, and iron oxides with pink or gray color.
• Gypsum used in plastering is obtained from this raw
material driving some or all of its water off by heating in a
kiln.

3. • The characteristics of the resultant materials depends on
heating process and the following classes of Gypsum is
obtained:
– Class A: Hemi-hydrate (CaSo41/2H2O) (plaster of Paris):
– This material is produced by heating the row material up to 150-
170 C.
– At this temperature 75% of the molecular water evaporates.
– Resulting Gypsum is very fast setting material.
– It sets in 5-10 minutes by regaining the lost molecular water.
– The use of this plaster is limited only filling cracks and repair
works, and casting in mold for ornamental works.
– Wet plaster mix is poured into the moulds and reinforced with
glass fiber.
– It does not permit to use trowel for spreading and smoothing the
mortar.

4. • Class B: Retarded Hemi-hydrated plasters:
• This type is obtained from the hemi-hydrated plaster by the addition
of retarder to control the setting time.
• Setting times are adjusted for the types to be used in undercoat and
to be used in finishing coats.
• For undercoat plasters more time is required to spread and level the
plaster so they should have slower setting time than finishing coats.
• There are two types specified in BS1191: Part 1:
– Type a: A slower setting plaster to be used in undercoats:
– It is designed to be used with sand in ratios 1:2 or 1:3 plasters:
sand.
– Generally light weight aggregate such as perlite or vermiculite is
preferred.
– Increasing sand content in the mix has the effect of accelerating
the setting time
– Usual setting time is 2-3 hours.

5. – Type b: Slightly faster setting plaster to be used in finishing
coats:
– It can be used neat or by the addition of hydrated time up to 25%
by weight.
– The effect of hydrated lime is accelerating the setting time and
providing workability.
– The usual setting times are in the range of 1-1.5 hours.
– Class B type plasters should not be re-tempered once the setting
has started.
– Premixed plasters made by the addition of lightweight aggregate
to class B plasters are now widely used.

6. • Class C: Anhydrous Gypsum Plaster:
• This plaster is obtained by heating the raw material to a higher
temperature than class B plasters, about 220C.
• At this temperature most of molecular water evaporates resulting is
the formation of anhydrous calcium sulphate.
• Still some hemi-hydrate plasters may remain in the heating process.
• This resulting materials is too slow setting that an accelerator is
added to improve the setting time.
• Due to the presence of hemi-hydrate component this plaster has two
setting times.
• the initial set is rapid and not suitable for application.
• After re-tempering the mix during the first half hour we have enough
time to work with mortar until the second setting.
• This plaster is currently used as the finishing plaster coat on
cement/sand undercoats.
• It is not suitable on plasterboards or any other soft background.
• It can be used neat or by the addition of 25% lime by weight.

7. • Class D: Keene’s Plasters:
• This plaster is obtained by heating the gypsum rock to a
temperature sufficient enough to drive off all the molecular water.
• The setting process is too slow and needs the addition of
accelerators.
• The resulting plaster is very hard and can be worked to a perfectly
true surface.
• This plasters is always is used neat and applied as a finishing coat
of 3 mm thick to a hard strong background such as cement and
sand.
• It is recommended for sharp corners, and surfaces of squash court
walls where high impact resistance is required.
• It can not be used on plaster boards, insulating boards or any such
soft background.
• It should not be mixed with lime.

8. Application of various types of undercoat plasters
Premixed
lightweight
gypsum
plaster mixes
Cement/Lim
e/Sand
Mixes
Gypsum/sand
plaster mixes
Type of backgroundN
o.
Bonding
plaster
1:1:51:1.5Dense Concrete1
:1:1:51:1.5Concrete brick2
:1:1:51:1.5Engineering brick3
:1:1:51:2Concrete blocks4
Browning plst.1;1:61:2Lightweight agg. concrete5
:1:1:61:2Low strength concrete6
:1:1:61:2Ordinary clay brick7
Metal lathing
plast.
1:1:61:2Metal lathing/wire mesh8
Bonding plst.--plasterboard9

13. Important points to be considered in gypsum
plasters:
1. Do not mix different classes of plasters
or different brands of the same kind
2. Sand should be clean and free from clay
3. Sand content should be less for
application on plasterboard or concrete
4. Never mix cement with gypsum
5. Always use clean water