This document provides an overview of gypsum products, including their history, formation, classification, properties, and uses. It discusses the various types of gypsum products like impression plaster, dental plaster, dental stone, die stone, and high strength stone. It describes the manufacturing process of calcination and how properties like setting time, expansion and strength can be controlled through additives and processing parameters. The key applications and desirable qualities of different gypsum classifications are also highlighted.

1. Gypsum Products

2. CONTENTS
• Introduction
• Objectives
• History
• Formation of Gypsum
• Classification
• Desirable properties
• Proportioning and mixing
• Setting reaction

3. • Tests For Working And Setting Time
• Control Of Setting Time
• Setting expansion
• Accelerators and retarders
• Hygroscopic Setting Expansion
• Strength
• Caring of gypsum products

4. •Recycling
•Infection control
•Conclusion

5. INTRODUCTION
• Gypsum is a non-metallic mineral, found in rock form.
• Composed of 79.1% calcium sulphate and 20.9% water, by weight
• Very soft mineral composed of calcium sulfate dihydrate, with the
chemical formula Ca(SO)4.2H2O

6. HISTORY
•The word “gypsum” is derived from Greek word
gypsos which means "to cook" referring to the burnt
or calcined mineral

7. HISTORY
• First commercial gypsum operation began in 1841 by Daniel Ball and
Warren Granger
• Production of gypsum
mainly takes place
in USA and Iran.

8. CALCINATION
•Calcination is a process in which different gypsum products
are obtained from gypsum{Ca(SO)4.2H2O}
by heating it to certain temperatures( 1100-1300 C)
•It is of two types
Dry Calcination -β form
Wet Calcination - α form

9. CALCINATION
•Dry calcination
It is done in a rotary kiln or
vat which is open to
atmosphere
•Wet calcination
It is done under steam
pressure in closed rotary kiln

10. CaSO4.2H2O
GYPSUM
CaSO4.1/2H2O
STONE or PLASTER
CaSO4
HEXAGONAL ANHIYDRITE
CaSO4
ORTHORHOMBIC ANHYDRITE
1100-1300
C
1300-2000 C
2000-10000 C

11. CLASSIFICATION
Classification is done according to
1. MANUFACTURING METHOD
-Dry calcination
-Wet calcination
-Dehydration with chemicals
-Synthetic
2. CRYSTAL STRUCTURE
-β spongy irregular monoclinic (type I , II)
-α prismatic regular monoclinic (type III,IV,V)

12. CLASSIFICATION
3. APPLICATTIONS
-type I Impression plaster
-type II Dental plaster
-type III Dental stone
-type IV Die stone
-type V High strength stone
4. EXPANSION
-type I, III, IV (minimum setting expansion)
-type II , V (large setting expansion)

13. DESIRABLE PROPERTIES
• Accuracy
• Dimensional stability
• Ability to reproduce fine detail
• Strength
• Resistance to abrasion

14. DESIRABLE PROPERTIES
• Compatibility
• Color
• Biologic safety
• Ease of use
Unfortunately not all gypsum products display all
these properties equally

15. TYPES OF GYPSUM
IMPRESSION
PLASTER
DENTAL
PLASTER
DENTAL
STONE
DIE
STONE HIGH
STRENGTH
STONE
IMPRESSION
PLASTER
β form α form

16. IMPRESSION PLASTER
•Composition
-It is dry calcined, β Ca(SO)4.1/2H2O ( type I )
-Impurities – uncalcined dihydrate
-Chemical retarders (borax,etc) and accelerators
(K2SO4)
-Potato starch
-Coloring material
β

17. IMPRESSION PLASTER
•Advantages
- Mucostatic impression
- Accurate reproduction of finer details
- Negligible dimensional change
•Disadvantages
- Non elastic cannot be used for dentulous areas
- Bad taste
- Messy work

18. DENTAL PLASTER
•Type II β CaSO4 hemihydrate
•Composition
- Dry calcined, β-calcium sulphate hemihydrate
- Uncalcined gypsum – impurity
- Chemical retarders
- Chemical accelerators

19. DENTAL PLASTER
•Uses
- Large size model or casts
- Mounting plaster in articulators
- Edentulous cast
bases
- Flasking

20. DENTAL STONE
•Type III α CaSO4 hemihydrate
- Manufactured by wet calcination
•Composition
- Wet calcined, α - CaSO4 hemihydrate
- Impurities – uncalcined gypsum
and anhydrite
- Chemical accelerators and retarders
- Colors ( green , yellow, pink) to identify margins

21. DENTAL STONE
• Uses
- Casts and models for dentulous
cases
- Final casts for edentulous cases
- Bases for stone dies
- Orthodontic study models

22. IMPROVED DENTAL STONE
•Type IV α CaSO4 hemihydrate( DIE STONE )
•Manufactured by wet calcination, by heating gypsum
in 30 % calcium chloride OR sodium succinate
solution (0.5%)
•Composition
- α CaSO4 hemihydrate
- uncalcined gypsum
- retarders, accelerators
- colouring agent

23. IMPROVED DENTAL STONE
• Used as common die material
as it has high strength
and resistance to abrasion
• Average dry
surface hardness is
92 ( Rockwell hardness )

24. HIGH STRENGTH,
HIGH EXPANSION DENTAL STONE
•Type V α CaSO4 hemihydrate
•Manufactured by adding small amounts of
surfactants i.e. lignin sulphonate
•Composition
- α CaSO4 hemihydrate
- retarders and accelerators
- Coloring agents

25. High Strength,
High Expansion Dental Stone
• Uses
-In die, as to compensate for solidification shrinkage
-Fabrication of cast crowns ( in case of inadequate
expansion)

26. COMPARISON AND DIFFERENCE
Properties Plaster Dental stone Die stone
type
water/powde
r
Setting time
Setting exp.
Strength
type II( )
0.45-0.50
12 ± 4
Max – 0.30
9 mpa
low
type III ( )
0.28- 0.30
12 ± 4
Max - 0.20
20.7 mpa
60 RHN
type IV( )
0.22 -0.28
12 ± 4
Max – 0.10
34.5 mpa
high
Type
Water/Powder
Setting time
Setting exp.
Strength
Hardness
type II (β)
0.45-0.50
12 ± 4min
Max – 0.30%
9 MPa
Low
type III (α)
0.28- 0.30
12 ± 4min
Max - 0.20%
20.7 MPa
60 RHN
type IV(α)
0.22 -0.28
12 ± 4min
Max – 0.10%
34.5 MPa
High

27. SPECIAL GYPSUM PRODUCTS
• Synthetic Gypsum Products
•These are obtained as the by products of manufacturing process of
phosphoric acid .
•Production is expensive in comparision to natural gypsum products.
• Extra white, smooth dental stone
For special orthodontic models

28. PROPORTIONING AND MIXING
•Proportioning
- Measure water
- Weigh powder
- Add water to bowl
- Slowly add powder

29. Powder to Water OR Water to Powder

30. PROPORTIONING AND MIXING
• Mixing
- Stir powder in water
- Mix vigorously for 60 sec
- Vibrate or tap
- Visually inspect mix

31. PROPORTIONING AND MIXING
•Mixing can be
mechanical or by
hand spatulation
•Longer mixing time
can reduce working
time

32. PROPORTIONING AND MIXING
•Entrapment of air should
be avoided to prevent
porosity
•Automatic vibrator can
be used to reduce
entrapment of air
•Over mixing should be
avoided

33. PROPORTIONING AND MIXING
• Pouring cast
- Drip in molar areas
- Vibrate anteriorly
- Fill the teeth imprints
- Place blebs on surface
- Prepare second mix
- Invert impression on
second mix
- Allow to set

34. PROPORTIONING AND MIXING
•Clean up
-Remove bulk excess with spatula
-Wipe insides of the bowl with a paper towel
-Wipe spatula clean
-Clean bowl and spatula in sink with plaster trap

35. SETTING OF GYPSUM
•It is the reverse reaction to that of calcination
(CaSO4)2H2O +3H2O
2CaSO4.2H2O +Unreacted CaSO4.1/2H2O +HEAT
Value of heat is almost same that is required during
Calcination process i.e. 3900 cal/gm mol

36. SETTING OF GYPSUM
•There are 3 theories which describe the setting
reaction of calcium sulfate hemihydrate
•These are
1. Colloidal theory
2. Hydration theory
3. Dissolution precipitation theory

37. •COLLOIDAL THEORY When mixed with water,
plaster enters into colloidal state through sol-gel
mechanism.
•HYDRATION THEORY
Rehydrated particles join together through
hydrogen bonding to the sulfate groups to form a set
material

38. DISSOLUTION PRECIPITATION THEORY
Water Plaster
Fluid and Workable Suspension
Saturated solution of hemihydrate
Supersaturation of dihydrate , so precipitation
Hemihydrate Saturation lost, so more dissolution
More precipitation of dihydrate & crystal growth occurs

39. WATER/POWDER RATIO
The ratio of water to hemihydrate powder is
expressed as W/P (Water/Powder) ratio OR the
quotient obtained when the weight of the water is
divided by weight of the powder e.g..
60 ml of water
100 gm of powder
The water powder ratio will be 0.6

40. WATER/POWDER RATIO
•As W/P increases ,
setting time increases
and strength and setting
expansion decreases
•More water powder
ratio more will be the
porosity

41. WATER/POWDER RATIO
•Water powder ratio for
type I 0.40 - 0.75
type II 0.45 - 0.50
type III 0.28 - 0.30
type IV 0.22 - 0.24
type V 0.18 - 0.22

42. • β form requires more water than α form as the crystals
are large, irregular shaped and porous whereas in α
form crystals are small and regularly shaped
• Surface area of β is more than α form
• β form require more water to float its particles
• Adhesion between particles which can be increased by
surface –active materials eg. Gum Arabic plus calcium
carbonate

43. TESTS FOR WORKING AND SETTING TIME
• MIXING TIME
It is defined as the time from the addition of powder to the
water until the mixing is complete.
It is 1 minute for hand spatulation
• WORKING TIME
It is the time available to use a workable mix, one that
maintains its consistency to perform one or more tasks.
A 3 minute working time is adequate

44. TESTS OF SETTING TIME
•SETTING TIME
It is defined as the time that elapses from the
beginning of mixing until the mix hardens
•TESTS
Loss of gloss test for initial set
Vicat's test
Gillmore test
For initial set
For final set

45. LOSS OF GLOSS
As the reaction proceeds
some of the water is taken up
in formation of dihydrate so
mix looses its gloss . It occurs
approximately at
9 minutes

46. INITIAL GILLMORE TEST
•Smaller Gillmore needle with
weight of ¼ pounds and tip
diameter 1/12 (inch) is used for
this.
•Plaster water mixture is spread
out and the needle is lowered
onto the surface . The time when
it no longer leaves a impression
is called initial set Gillmore needles

47. FINAL GILLMORE TEST
• Larger Gillmore needle with
weight of 1 pounds and tip
diameter 1/24 (inch) is used for this.
• The time elapsed when the needle
leaves a barely perceptible mark
on the surface is called as final
setting time

48. VICAT'S TEST
•Soon after the gloss is lost the
needle with weighted plunger,
which was held in contact, is
released. The time elapsed until
the needle no longer penetrates to
the bottom of the mix is known as
the setting time
•Weight -300gms, diameter -1 mm
Vicat's
penetrometer

49. Relation between
Time and
Compressive strength
showing:
- Mixing time
- Working time
- Initial Gillmore
- Vicat’s
- Final Gillmore and
- Ready for use

50. CONTROL OF SETTING TIME
There are 3 methods to achieve that
• Solubility of hemihydrate can be increased or
decreased
• Number of nuclei of crystallization can be increased or
decreased
• Addition of accelerators and retarders

51. FACTORS AFFECTING SETTING TIME
• Impurities
• Fineness
• Water/Powder Ratio
• Spatulation Time
• Temperature
• Accelerators And Retarders

52. SETTING EXPANSION
•The dimensional increase which occurs concurrent
with the hardening of different materials such as
dental plaster and dental stone.
•Depending upon composition it varies from 0.06%
to 0.5%

53. •According to the volumetric calculations a volumetric
contraction occurs but on the basis of crystallization
mechanism , setting expansion is observed
Net change in volume is -7.11% i.e.shrinkage occurs
(CaSO4)2.H2O + 3H2O 2CaSO4.2H2O
Mol. Mass 290.284 54.048 344.332
Density 2.75 0.997 2.32
Eq. volume 105.556 54.211 148.405
Total volume 159.767 148.405

54. CRYSTALLIZATION MECHANISM
•This process involves
outgrowth of crystals from
nuclei of crystallization.
•Crystals growing from one
nuclei impinges on adjacent
and obstructs its growth.
Growth of crystals from nuclei

55. CRYSTALLIZATION MECHANISM
•When this mechanism is repeated by thousands of
nuclei, an outside thrust or stress is developed which
causes expansion of total mass i. e setting expansion

56. •This impingement and
movement leads to formation
of micro pores
•These micro pores contain
extra water, which when dries,
makes the cast porous, so
external volume is more than
the crystalline volume
CRYSTALLIZATION MECHANISM

57. CONTROL OF SETTING EXPANSION
• Factors affecting setting expansion
-Water / powder ratio
setting expansion
-Mixing time
-Can be reduced by addition of chemicals like
potassium sulfate and sodium chloride

58. ACCELERATORS AND RETARDERS
Accelerators and retarders
must together be added so
that optimum strength and
working time can be
obtained
Compressive strength of
plaster against time

59. ACCELERATORS
•Most commonly used is potassium sulfate (K2SO4)
•It increases the solubility of hemi hydrate without
affecting the solubility of dihydrate
•Some accelerators can also act as retarders when
increased in amount like sodium chloride is
accelerator up to 2 % but act as a retarder when used
in higher concentration
•Powdered gypsum can be used as a accelerator

60. RETARDERS
•Action
It forms a coating on the hemihydrate particles and
decreases its solubility
•Citrates , borates and acetates generally retard a
reaction
Operator should not add other ingredients as this may
counteract with the effects of premixed chemicals

61. HYGROSCOPIC SETTING EXPANSION
•The expansion that occurs under water is called
hygroscopic setting expansion
•Under water the crystal growth occurs more freely as it is
not constrained by surface tension.
•A cast of dental stone may show a hygroscopic expansion
of 0.30 %
•W/P Hygroscopic expansion

62. HYGROSCOPIC SETTING EXPANSION
•Stage I
Initial mix represented by
few particles
•Stage II
Initial crystal growth occurs
and particles are drawn
closer by surface tension

63. HYGROSCOPIC SETTING EXPANSION
•Stage III
Water around particles
decreases, particle
contraction is opposed by
outward thrust of growing
crystals
•Stage IV
Crystals become more
intermeshed and entangled

64. HYGROSCOPIC SETTING EXPANSION
•Stage V
In the termination stage
the effect becomes more
marked
In hygroscopic expansion
crystals are allowed to
grow freely

65. STRENGTH
•Wet strength (
green strength)
The strength obtained
when excess water than
required for hydration is left
•Dry strength
The strength obtained
when the excess water is
driven off by drying

66. STRENGTH
•Strength is affected by:
-W/P ratio strength
-Overmixing decreases strength
-Retarders and accelerators decreases both wet
and dry strength

67. CARING OF THE CAST
•Casts should not be stored at high temperatures
•Suitable precautions should be followed for infection
control
•Gypsum products should be stored in dry atmosphere
•Only required amounts should be taken out at a time
•Large stocks should not be purchased

68. RECYCLING OF GYPSUM
• Research was carried out to prepare
dental stone from the previously
fabricated stone casts.
• The study included reheating
of the used casts at different
conditions in an autoclave.
• Results indicated that calcium sulphate
hemihydrate can be reproduced using
the previously fabricated stone casts.

69. INFECTION CONTROL
•Infection control in necessary to prevent cross
contaminations of microorganisms, these can be
controlled by various methods
•0.525% sodium hypochlorite and 0.1% povidone
iodine can be used to disinfect dental gypsum with
no significant effect on the dimensional accuracy and
reproducibility of the resultant casts

70. INFECTION CONTROL
•Under the described conditions, microwave-
irradiated gypsum casts satisfy current disinfection
requirements
•Four readily available disinfecting solutions
(glutaraldehyde, povidone-iodine, chlorhexidine
and sodium hypochlorite) can be added to the die
stone mix used to pour up the impressions.

71. CONCLUSION
•Considering the different
properties of different type of
gypsum products we should be
able to judge the particular
material for a particular task

72. REFERENCES
Phillips' Science of Dental Materials (11th Edition) by By Kenneth J
Anusavice
Applied Dental Materials (7th Edition) by
J F McCabe & A W G Wells
Dental Materials: Properties and Manipulation (8th Edition)by Robert
Craig
Clinical Aspects of Dental Materials by Marcia Gladwin & Michael
Bagby
Materials in dentistry :principles & application by Jack L Ferracanes
 J Prosth Dent. 2004 Jul;92(1):27-34
Int J Prosthodont. 2005 Nov-Dec;18(6):520-5

73. THANK YOU

74. EFFECT OF WATER/POWDER RATIO ON THE COMPRESSIVE STRENGTH OF
MODEL PLASTER, DENTAL STONE , AND HIGH STRENGTH DENTAL STONE
Material W/P ratio
(ml/g)
Compressive strength
(MPa)
Model stone 0.45
0.50
0.55
12.5
11.0
9.0
Dental stone 0.27
0.30
0.50
31.0
20.5
10.5
High strength
dental stone
0.24
0.30
0.50
38.0
21.5
10.5