GYPSUM.pptx

Gypsum Products
Presented By :
Dr. AKANKSHA
1st year PG
Department of Prosthodontics and Crown & Bridge
Seminar 2

TABLE OF CONTENTS
Introduction
History
Applications
Dental applications
Calcination

Setting of gypsum
products
Classification
Properties
Manipulation
Infection control
Conclusion

INTRODUCTION
• Dihydrate form of calcium sulfate
• White to milky yellowish
• Supplied as fine hemihydrate particles
• Commercial importance as Plaster of Paris
• Diversified use
• By-product of some chemical processing operations
• Auxiliary material in clinical and laboratory procedure
Craig’s Restorative Dental Materials. 14th Edition. Elsevier Pub. Chapter 12. Replicating Materials: Impression and Casting, Gypsum Products 252-60

9000
350-40
2800
7000
Oldest traces of plaster
found in Anatolia & Syria
In building, within the city
and roads at Sumerian
and Assyria periods
Coloured wall plaster by soil
mixing in Pharaohs graves
at Giza Pyramid
Wall and wooden ceiling
plaster in ancient Greece and
Rome civilizations

1700
Walls of wooden houses
were covered with plaster as
protection against fire in
Paris
Plaster of Paris
1786
First scientific study on
plaster by French scientist
Lavoisier
1820
Plaster war
as plaster was regarded
as miraculous fertiliser
Ottawa Indian discovered a rock outcrop
where Plaster creek enters the Grand River
and took a sample of the soft, sugary
material, which was identified gypsum
1827

1827
Le chatelier studied hydration
mechanisms of plaster
Crystalline theory
1889
Plaster used in interior walls
and ornamental work
1889
Construction of Gypsum mine below the
bed of Grand river by William T. Powers
1893
Colloidal theory
1912
Use of gypsum as
medicine in Japan

APPLICATIONS
Building
constructions
Soil conditioning Food
additives
Pharmaceuticals Medical
devices
Dental
application
Annusavice KJ. Phillip’s Science Of Dental Materials. Twelfth Edition, Elsevier Pub. Chapter 9 Gypsum Products. 182-93.

DENTAL APPLICATIONS
Impression material
Constructing cast and die
Mount cast on articulator
Dental investments
Bite registration
Orthodontic models

CALCINATION
PRODUCTION OF CALCIUM SULFATE HEMIHYDRATE
Process in which different gypsum products are obtained from
heating gypsum to certain temperatures
Annusavice KJ. Phillip’s Science Of Dental Materials. Eleventh Edition, Elsevier Pub. Chapter 10 Gypsum Products. 255-83.

MONOCLINIC HEXAGONAL ORTHORHOMBIC
Calcium sulfate dihydrate
Calcium sulfate hemihydrate
Soluble Anhydrite Insoluble Anhydrite
Cole Wf. Lancucki CJ. A Refinement Of The Crystal Structure Of Gypsum. Acta Crystallographics B 24:1968.

Dry Calcination
Synthetic methods
Dehydration by
boiling with chemicals
Wet Calcination

DRY CALCINATION
• gypsum mineral is heated in an open kettle at a temperature of
about 110 to 120°C
• β- calcium sulfate hemihydrate produced
• Type I and type II gypsum products

WET CALCINATION
• Gypsum is ground and steam heated to a temperature of
110-130 °C in a closed kiln, kettle or autoclave
• -calcium sulfate hemihydrate produced
• Type III gypsum product (Hydrocal)

DEHYDRATION BY BOILING WITH CHEMICALS
• Boiling gypsum rock in 30% CaCl2solution .
• The chlorides are then washed away with 0.5 % Sodium
Succinate and the material is ground to the desired fineness.
• Modified -hemihydrate obtained
• Type IV and V high strength dental stone ( Densite )

SYNTHETIC METHODS
•  and  hemihydrate formed from by-products or waste
products of Phosphoric Acid
• More expensive than other gypsum products

 Crystal size LARGER SMALLER
 Shape IRREGULAR PRISMATIC
 Packing LOOSELY PACKED
AMPLE SPACE B/W
CRYSTALS
CLOSELY PACKED
LITTLE SPACE B/W
CRYSTALS
 W/P Ratio MORE LESS
 Strength LESS MORE
 Surface area/wt. MORE LESS
 Example IMPRESSION PLASTER
DENTAL PLASTER
DENTAL STONE
 HEMIHYDRATE
 HEMIHYDRATE

Powder particles of Plaster of Paris (-hemihydrate)

Powder particles of Dental Stone (-hemihydrate)

Powder particles of Improved stone (modified -hemihydrate)

SETTING OF GYPSUM PRODUCTS
Advanced Dental Biomaterials. 2019 Elsevier Ltd. Chapter 3:Dental gypsum and investments 37-52
EXOTHERMIC

SETTING REACTION
THEORIES OF SETTING REACTION
Colloidal theory
Hydration theory
Crystalline theory

COLLOIDAL THEORY
• Mahaelis in 1893
• Mixed with water, plaster enters into the colloidal state through
the sol-gel mechanism.
• In the sol state, hemihydrate particles are hydrated to form
dihydrate, thereby entering into an active state. As the
measured amount of water is consumed, the mass converts to
a solid gel.

HYDRATION THEORY
• Hemihydrate undergoes hydration
• Rehydrated plaster particles join together through hydrogen
bonding to the sulfate groups to form the set material.

CRYSTALLINE THEORY
• Proposed in1887 by Henry Louis Le Chatelier
• In 1907, it received the full support of Jacob Henricus vant’s
Hoff
• Also called Dissolution-precipitation theory
• The difference in the solubilities of calcium sulfate dihydrate and
hemihydrate causes the setting of these materials.

STAGES OF CRYSTALLIZATION
Dissolution
Suspension
Saturation
Supersaturation
Nuclei Formation
Growth Of Nuclei
Spherulite Formation

Winkler MM, Monaghan P, Gilbert JL, Lautenschlager EP. Freeze-drying and scanning electron microscopy of setting dental gypsum. Dent Mater. 1995 Jul;11(4):226-30. doi:
10.1016/0109-5641(95)80053-0. PMID: 8621042

Dental Materials and Their Selection, William.J.O’Brein, Fourth Edition, Quintessence Publishing Co, Inc, Chapter 4:Gypsum Products 38-61

STAGES IN SETTING
Important
physical changes
recognized :
• Fluid
• Plastic
• Friable
• Carvable

WATER REQUIREMENTS
• Result of differences in the apparent density of the powder
• Higher apparent densities, require less water

Excess
unreacted
water in set
mass
Water
evaporation
when dried
Microscopic
porosities
Weaking of
cast
100 g
Calcium Sulfate
Hemihydrate
18.6 g
Water
Calcium Sulfate
Dihydrate

Required and Excess Water for Gypsum
Materials
Gypsum
Mixing Water
(mL/100 g of
Powder)
Required Water
(mL/100 g of
Powder)
Excess Water
(mL/100 g of
Powder)
Model plaster 37-50 18.6 18-31
Dental stone 28-32 18.6 9-13
High strength
dental stone
19-24 18.6 0-5

WATER-POWDER RATIO (W/P)
• Amount of water in millilitres added to
100 g of powder
• Expressed as a fraction
• Determines physical and chemical
properties of set gypsum product.
45/100
or 0.45
45 mL of water is
added to 100 g of
powder

W/P RATIO
EFFECT OF HIGH AND LOW W/P RATIO
Slow setting
Low expansion
More porosity
Less strength
Less surface
hardness
Quick setting
High expansion
Less porosity
More strength
More surface
hardness
THIN
MIX
(
high
w/p
ratio)
THICK
MIX
(low
w/p
ratio)

TEMPERATURE
• Water used for mixing and environment temperatures
EFFECT ON SETTING REACTION
change in the relative solubilities of calcium
sulfate hemihydrate and calcium sulfate
dihydrate which alters the reaction
change in ion mobility with temperature

CHANGE IN THE RELATIVE SOLUBILITIES OF CALCIUM SULFATE HEMIHYDRATE
AND CALCIUM SULFATE DIHYDRATE, WHICH ALTERS THE RATE OF THE REACTION
Temperature
increases
solubility ratio
decreases
Setting reaction
slowed
Setting time
increases

CHANGE IN ION MOBILITY WITH TEMPERATURE
Temperature
increases
Mobility of
calcium and
sulfate ions
increases
Rate of reaction
increases
Setting time
decreases

EFFECT OF TEMPERATURE ON SOLUBILITY
Little change in setting time occurs
between 0 °C and 50 °C.
Temperature of the plaster-water mixture
exceeds 50 °C, a gradual retardation
occurs.
At 100 °C, no reaction takes place
COMPLEX REACTION FOR GYPSUM PRODUCTS

HUMIDITY
relative humidity
increases to 70%
and above
moisture in the
air
conversion of
hemihydrate to
dihydrate
more nuclei for
crystallization
Setting
reaction
accelerated
EFFECT ON SETTING REACTION

Therefore all gypsum products should be kept in a closed
container and well protected from moisture in the air.

CLASSIFICATION OF
GYPSUM PRODUCTS

ACCORDING TO International Organization for Standardization
(ISO 6873:2013)
Type 1: Dental plaster for impression
Type 2: Dental Plaster
Class 1 - for mounting
Class 2 - for models
Type 3: Dental Stone
Type 4: Dental Stone, high-strength,
low-expansion for dies
Type 5: Dental Stone, high-strength,
high-expansion for dies

ACCORDING TO ADA SPECIFICATION 25
Type 1 : Impression Plaster
Type 2 : Model Plaster
Type 3 : Dental Stone
high strength
high strength ,high expansion

IMPRESSION PLASTER
• Composed of plaster of Paris (β-hemihydrate)
• Seldom used , replaced by less rigid materials
COMPOSITION
• Dental plaster
• K2SO4
• Borax
• Coloring and flavoring agents

USES
• For making impressions in complete denture and maxillofacial
prosthetics
• Bite registration material.

MODEL PLASTER
COMPOSITION
•  hemihydrate
• Modifiers
USES
• Diagnostic cast
• Mounting casts on articulator
• Artistic position of study cast
• Flasking procedures
MODEL PLASTER, LABORATORY PLASTER, MOUNTING
PLASTER

DENTAL STONE
COMPOSITION
•  hemihydrate
• 2-3 % coloring agents
• K2SO4
• Borax
USES
• Preparation of master cast
• Orthodontic models
• Complete and partial denture models
CLASS I STONE OR
HYDROCAL

DENTAL STONE HIGH STRENGTH
USES
• Model bases
• Dies for fabricating inlay, crown and
bridge wax pattern
• CAD/CAM dies
CLASS II STONE, DIE STONE, DENSITE,
IMPROVED STONE

DENTAL STONE HIGH STRENGTH
HIGH EXPANSION
EXAMPLES
• Hard Rock, Jade Rock, Resinrock XL5 (Whipmix),
Denflo-HX, Supra stone (Kerrlab)
USES
• In die, to compensate for solidification shrinkage
• Fabrication of cast crowns ( inadequate expansion)

PROPERTIES OF GYPSUM PRODUCTS
MIXING TIME,
WORKING TIME,
SETTING TIME
SETTING EXPANSION
STRENGTH OF SET
GYPSUM PRODUCTS
SURFACE HARDNESS
AND ABRASION
RESISTANCE
REPRODUCTION OF
DETAILS

MIXING TIME
• Time from addition of powder to water until mixing is completed
• Mechanical mixing : 20-30 sec
• Hand spatulation : a minute

WORKING TIME
• Time from start of mixing to the point where the consistency is
no longer acceptable
• 3 minute working time is adequate

SETTING TIME
• Time that elapses from the beginning of mixing until the material
hardens
INITIAL SETTING TIME
time required for gypsum products to reach a certain
arbitrary stage of firmness in their setting process
FINAL SETTING TIME
time at which the material can be separated from the
impression without distortion or fracture

LOSS OF GLOSS TEST
As reaction
proceeds
Excess water
taken up
Formation of
dihydrate
Mix looses its
gloss
TEST FOR INITIAL SET

Initial needle
113.4 ± 0.5 g
Final needle
453.6 ± 0.5 g
Lower arm
preferably
adjustable for height
Cross arms must be
designed to prevent their
rotation about the vertical
shaft

PASTE PAT
Base diameter 76 ± 13 mm
Top diameter 50 ± 13 mm
Center thickness 13 ± 3 mm
PLANE NON- ABSORPTIVE
PLATE
100 ± 5 mm square
PAT WITH TOP SURFACE FLATTENED FOR DETERMINING TIME
OF SETTING BY GILLMORE METHOD

Replaceable tips may be made of stock drill rod
or wire tempered after shaping and held by
suitable chuck or other fastener
TIP DIAMETER
Initial 2.12 ± 0.05 mm
Final 1.06 ± 0.05 mm
DETAIL OF GILLMORE APPARATUS NEEDLE TIPS

Mixture is
spread out
Smaller needle
lowered on
surface
No longer
leaves an
impression Initial set
INITIAL GILLMORE TEST
• Mass has still no measurable compressive strength
• Cannot be safely removed from the impression

VICAT TEST FOR SETTING TIME
Diameter 1mm

Vicat

FINAL GILLMORE TEST
Mixture is
spread out
Larger needle
lowered on
surface
No longer
leaves an
impression Final set

COMPRESSIVE STRENGTH
OF A TYPE-II MODEL PLASTER
DURING SETTING

CONTROL OF SETTING TIME
Solubility of the hemihydrate
Number of nuclei of crystallization
Rate of crystal growth

Solubility of the hemihydrate
solubility of the
hemihydrate
increased
supersaturation
of the dihydrate
is achieved
faster
accelerates the
rate of dihydrate
crystal
deposition
Increases the
setting reaction
Decreases
setting time

Number of nuclei of crystallization
greater the
number of nuclei
of crystallization
faster the
dihydrate
crystals
formation
Increased setting
reach
Decreased
setting time

Rate of crystal growth
• Increasing or decreasing the rate of crystal growth will
accelerate or retard the setting time.

FINENESS
finer
particle size
More no. of
nuclei
rate of the
hemihydrate
dissolution
increased
rapid rate of
crystallization
faster mix
hardens

IMPURITIES
calcination
incomplete/
manufactures
addition
gypsum
particles
remain
increase nuclei
of crystallization
Faster
setting
reaction
setting time
decreased

WATER-TO-POWDER RATIO
more water
used for mixing
fewer nuclei
per unit
volume
Slower
reaction
Setting time
prolonged

SPATULATION ( MIXING )
mixing
begins
formation of
crystals
increases
Rate of
spatulation
increases
breaks
down to
smaller
crystals
new centers
of
nucleation
calcium
sulfate
dihydrate
precipitated
Setting time
prolonged

MODIFIERS FOR CONTROLLING SETTING TIME
Organic material :
Glue, Gelatine
and gums
Salts : Borax,
potassium citrate,
NaCl (20%)
Organic material :
Blood, Saliva
Salts : Potassium
sulfate (>2%),
NaCl (2%),
Sodium sulfate (3-
4%), Slurry water
(gypsum <20%),
Tera alba
SETTING
TIME
SETTING
TIME
RETARDERS ACCERELATORS

SETTING EXPANSION
• Theoretically, calcium sulfate hemihydrate should contract
volumetrically during the setting process.
• However, experiments have determined that all gypsum
products expand linearly during setting
• linear expansion may be as low as 0.06% or as high as 0.5%

Mahler and Bruce Ady, 1960; Lyon et al., 1955
Hemihydrate
surrounded by
water molecules
Water around particles
reduced due to hydration
and particles drawn closer
Crystals become
entangled and
intermeshed
Water around particles
exhausted and crystals tend to
shrink which is opposed by
outward thrust of crystal growth
NORMAL SETTING EXPANSION

Mahler and Bruce Ady, 1960; Lyon et al., 1955
HYGROSCOPIC SETTING EXPANSION
Hemihydrate
surrounded by
water molecule
Since water of hydration
replace by other water
molecule, particles
remains at same place
Lost water molecule replaced by
new molecules, growth not opposed
by shrinkage and crystals grow
freely
Intermeshing of crystals
tales long time, expansion
goes undisturbed
Crystal expansion
continues

Graphic representation

Initial mix
Initial crystal growth
Solid phase contact
Expansion
Termination

CONTROL OF SETTING EXPANSION
Spatulation
Water powder ratio
Modifiers
( accelerators and retarders )

STRENGTH OF SET GYPSUM PRODUCTS
• expressed - compressive strength
• free-water content of set product affects its strength
DRY STRENGTH
excess water has been driven off by drying
WET STRENGTH
when water in excess of that required for
hydration of hemihydrate is left

FACTORS AFFECTING STRENGTH
Loss of excess water
Water-to-powder ratio
Manipulation
Additives

Effect of loss of excess water on compressive strength of dental
stone.

Compressive strength as a function of W/P ratio for five types of
gypsum products

SURFACE HARDNESS AND ABRASION RESISTANCE
• related to compressive strength
• increases at a faster rate than compressive strength as surface
of the hardened mass reaches a dry state earlier than the inner
portion of the mass

• Mixing high-strength dental stone with a commercial hardening
solution containing colloidal silica (about 30%) improves the
surface hardness of the set gypsum
• Abrasion resistance of 15% to 41% were observed for a high-
strength dental stone impregnated with epoxy resins or a light
cured dimethacrylate resin

Onizuka T, Kamimura N, Kajiwara H, Nakashima A, Suenaga K, Jimi T. Influence of boiling water treatment on surface roughness and surface microstructure of set gypsum
(dental stone). Dent Mater J. 1995 Dec;14(2):245-55. doi: 10.4012/dmj.14.245. PMID: 8940563.

Tripathi A, Gupta A, Bagchi S, Mishra L, Gautam A, Madhok R. Comparison of the Effect of Addition of Cyanoacrylate, Epoxy Resin, and Gum Arabic on Surface Hardness of
Die Stone. J Prosthodont. 2016 Apr;25(3):235-40. doi: 10.1111/jopr.12314. Epub 2015 Dec 29. PMID: 26713763.

REPRODUCTION OF DETAILS
TYPES 1 AND 2
reproduce a groove 75 μm in width
TYPES 3, 4, AND 5
reproduce a groove 50 μm in width
ANSI/ADA SPECIFICATION NO. 25

Gypsum dies do not reproduce
surface detail as well as
electroformed or epoxy dies
because the surface of the set
gypsum is porous on a
microscopic level
SCANNING ELECTRON PHOTOMICROGRAPH OF THE
SURFACE OF A SET HIGH-STRENGTH STONE DIE

Type W/P Ratio Setting Time ( min )
Two hour
setting
expansion (%)
0ne hour
Compressive
Strength
(Mpa)
Min Max
I. Plaster, impression 0.50-0.75 4 ± 1 0.00 0.15 4.0
II. Plaster, Model 0.45-0.50 12 ± 4 0.00 0.30 9.0
III. Dental stone 0.28-0.30
12 ± 4
0.00 0.20 20.7
IV. Dental stone, high
strength
0.22-0.24
12 ± 4
0.00 0.10 34.5
V. Dental stone, high
strength, high expansion
0.18-0.22
12 ± 4
0.10 0.30 48.3
PROPERTIES OF 5 TYPES OF GYPSUM PRODUCTS

MANIPULATION
OF GYPSUM
PRODUCTS
PROPORTIONING
MIXING
POURING

PROPORTIONING
Recommended W/P
ratio should be used
Weighing balance
for weight of powder
Powder should not be
measured by volume (as
by using a scoop)
Accurate graduated
cylinder for the water
volume

ADD POWDER TO WATER
water should be
added first
Powder sprinkled slowly
over a period of about
20-30 seconds
This technique allows any air entrapped in the powder to
escape, which reduces the number of bubbles in the final mix.

MIXING
Flexible rubber
mixing bowl
Metal spatula
with a stiff blade Bowl should be
parabolic in shape,
smooth, and
resistant to abrasion
Handle that is
convenient to hold

designed to promote release of
bubbles in gypsum mix and to
facilitate pouring of the impression
VIBRATOR

smooth mix
obtained
Spatulation
: 1 minute
at 2
revolutions
per second
wiping
inside
surfaces of
the bowl
with
spatula
stirring the
mixture
vigorously
V I B R A T O R

POWER-DRIVEN MECHANICAL
SPATULATOR WITH A VACUUM
ATTACHMENT

Smooth mix
placed on
vibrator
mix
spatulated
for 20
seconds on
low-speed
drive of
mixer
powder
initially wet
by water as
with hand
mixing
Vacuuming during mixing reduces the air
entrapped in the mix

POURING
Impression tray is held in one hand against the vibrator

Surface of the impression should be free of excess water

With metal spatula, a small amount of dental stone is
added to one open end of the impression

The impression may be tilted to control the movement of
the stone into the tooth depression

Once the tooth depressions are completely filled, larger amounts of dental
stone can be added under light vibration to fill the remaining impression

The impression is then briefly placed on the vibrator for a few
seconds to distribute the stone evenly across the impression

The dental stone model should be left undisturbed for 45 to 60 min until
the material has set completely

Dental stone model is now separated from the impression

• Cross-contamination to dental office personnel via dental
impressions.
• Dental casts come into direct contact with impression materials
and other items that are contaminated by saliva and blood from
a patient's mouth, leaving the casts susceptible to cross-
contamination.

• If an impression not disinfected, it is necessary to disinfect the
stone cast.
Models disinfected by immersion in 1:10 dilution of
sodium hypochlorite for 30 minutes or with a spray of
iodophor following manufacturer’s instructions.

Abdelaziz KM, Combe EC, Hodges JS. The effect of disinfectants on the properties of dental gypsum: 1. Mechanical properties. J Prosthodont. 2002 Sep;11(3):161-7. PMID:
12237796.

Abdelaziz KM, Combe EC, Hodges JS. The effect of disinfectants on the properties of dental gypsum, part 2: surface properties. J Prosthodont. 2002 Dec;11(4):234-40. doi:
10.1053/jpro.2002.129049. PMID: 12501136.

Robati Anaraki M, Lotfipour F, Moslehifard E, Momtaheni A, Sigari P. Effect of different energy levels of microwave on disinfection of dental stone casts. J Dent Res Dent Clin Dent Prospects.
2013;7(3):140-146. doi:10.5681/joddd.2013.022

CONCLUSION
A wide choice of gypsum
product is available
One should choose the one or
the combination which is most
suitable for our desired
requirements

THANK YOU FOR
YOUR ATTENTION !

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