GYPSUM

PRODUCTS
CONTENT
SIntroduction
Production of Gypsum
Products
Setting of Gypsum Products
Setting Expansion
Strength of Set Gypsum
Products
Types of Gypsum Products
Manipulation of Gypsum
Products
 INTRODUCTIO
N
Gypsum (CaSO4•2H2O; calcium sulfate dihydrate)
is a mineral mined in various parts of the world,
but it is also produced as a by-product of flue gas
desulfurization in some coal-fired electric power
plants.

TYPES
❑ Albaster- pure white, fine grained and
translucent.
❑ Satin spar - fibrous needle like with
silky lustre.

❑ Selenite - colourless, crystalline and

transparent.
USES
Building construction
Soil conditioning
Food additives
Pharmaceuticals
Medical devices
APPLICATION IN DENTISTRY
For cast preparation.
Models and dies.
Impression Material
As Investment Material
Mounting of Casts
As a mold material for processing of complete
REQUIREMENTS OF DENTAL CAST MATERIALS

• Dimensional accuracy
• Adequate mechanical properties
• Material should ideally be fluid at the time it is
poured into the impression
• A low contact angle between the model and
impression materials
• Set material should be sufficiently strong to resist
accidental fracture
• The material should be compatable with all the
other materials with which it comes into contact.
• It should give a contrast colour with the various
waxes
 PRODUCTION OF GYPSUM
PRODUCTS
These materials are produced by calcining calcium
sulfate dihydrate (gypsum).

Commercially, the gypsum is ground and subjected to

temperatures of 110 °C to 130 °C (230 °F to 266 °F) in
open containers to drive off part of the water of
crystallization.
 Calcination may be carried out in the presence or absence of water
(wet or dry calcination respectively).

b-Hemihydrate (dental plaster) is produced by the dry calcination of ground

gypsum in open containers (pans, kettles, or rotary kilns) to a temperature
of about 120°C. The resulting particle is a fibrous aggregate of fine crystals
with capillary pores known as plaster of Paris or dental plaster in
dentistry.
When gypsum is heated in a kettle, vat, or rotary kiln
that maintains a wet environment; a crystalline
hemihydrate called dental stone is produced in the
form of rods or prisms.

a-Hemihydrate (dental stone) is produced

by wet calcinations where gypsum is heated to
about 125°C under steam pressure in an auto
clave.
Because of differences in crystal size, surface area,
and degree of lattice perfection - α-hemihydrate
for dental stone and β-hemihydrate for plaster of
Paris.

If the calcination process occurs under pressure in

a 30% calcium chloride solution or in the presence
of more than 1% of sodium succinate, -
hemihydrate crystals will be shorter and thicker
than those produced in a closed container.

Residual calcium chloride or sodium succinate

is removed by washing the powder with hot water.
This type of gypsum- producing product is called
modified α-hemihydrate or die stone. These
crystals require even less water for mixing.
 α-hemihydrate β-hemihydrate
• Formed when dihydrate is ∙ Formed when dihydrate is
heated under steam heated in an open kettle or
pressure. kiln.
• α-hemihydrate crystals are ∙ β -hemihydrate crystals are
denser and have a characterized by their
prismatic shape. “sponginess” and irregular
shape.
When hemihydrate particles ∙ β -hemihydrate produces a
are mixed with water the less stronger and harder
α- hemihydrate produces a dihydrate structure.
much stronger and harder
dihydrate structure.

• It requires less water as • β-hemihydrate crystals

compared to β- require more water to wet
 SETTING OF GYPSUM
PRODUCTS
The reaction between gypsum products and water
produces solid gypsum, and the heat evolved in the
exothermic reaction is equivalent to the heat used
originally for calcination.
Set gypsum products probably never attain 100%
conversion unless they are exposed to high humidity
for a long time. Therefore, there are unreacted
hemihydrates remaining in the set materials .
• The reaction is exothermic, and whenever 1 g mol
of calcium sulfate hemihydrate is reacted with 1.5
g mol of water, 1 g mol of calcium sulfate dihydrate
is formed, and 3900 calories of heat are developed.

• This chemical reaction takes place regardless of

whether the gypsum material is used as an
impression material, a die material, or a binder in
casting investment.
 SETTING
REACTIONS
There are three theories of gypsum setting.

The colloidal theory proposes that, when mixed

with water, hemihydrate enters into the colloidal
state through a 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.
The hydration theory suggests that rehydrated
plaster particles unite through hydrogen bonding
with sulfate groups to form the set material.
The most widely accepted mechanism is the
dissolution- precipitation theory, which is
based on dissolution of the hemihydrate
particles in water followed by instant
recrystallization to the dihydrate.
This reaction has become possible because the
solubility of hemihydrate in water is four times
greater than that of the dihydrate near room
temperature.
Thus, the setting reactions occur as follows:
❖ When the hemihydrate is mixed with water, a
suspension is formed that is fluid and workable.
❖ The hemihydrate dissolves until it forms a
saturated solution of Ca2+ and (SO4) 2−.
❖ This saturated hemihydrate solution,
supersaturated in dihydrate; precipitates out
dihydrate .
❖ As the dihydrate precipitates, the hemihydrate
continues to dissolve. The process proceeds as
either new crystals form or further growth occurs
on the crystals already present until no further
dihydrate can precipitate out of solution.

❖ There is less than 50% dihydrate present in Type IV

and V stones, about 60% in Type II die materials,
and over 90% in Type I plasters.
 QUANTIFYING SETTING
REACTIONS

The time from addition of powder to the water until

mixing iis completed is called the mixing time.

Mechanical mixing is usually completed in 20 to 30

seconds.
Hand spatulation generally requires at least a minute
to obtain a smooth mixture.

The time from the start of mixing to the point where

the consistency is no longer acceptable for the
product’s intended purpose is the working time,
usually 3 min.
 TESTS FOR SETTING OF GYPSUM PRODUCTS
The tests done for setting of gypsum products are
listed as below
o Loss of gloss test.
o Gillmore’s test.
o Vicat test for setting time.

Loss of gloss test: As the reaction proceeds, the

excess water on the surface is taken up in forming
the dihydrate, so that
its surface the and
gloss mix gains
loses
strength.
 Gillmore’s test:
When the mix no longer leaves an impression when
penetrated by Gillmore needle, which has a tip 2.12
mm in diameter and weighs 113.4 g, the time
elapsed is called the initial setting time.
At this point, the mass still has no measurable
compressive strength and the cast cannot be safely
removed from the impression.

The elapsed time at which a heavier Gillmore

needle, weighing 453.6 g and with a tip 1.06 mm in
diameter, leaves only a barely perceptible mark on
the surface is called the final setting time.
 • Vicat’s test for setting time:

After initial setting the further reaction is

determined by an instrument called Vicat
penetrometer .
❑ The needle with a weighted plunger rod is
supported and held just in contact with the mix .
Soon after the gloss is lost, the plunger is released.

❑ The time elapsed until the needle no longer reach

the bottom of the mix is known as setting time.
• The Vicat penetrometer, with a 3-mm diameter
needle and a total weight of 300 g, has been used by a
number of investigators.

• A metal ring, 8 mm high and 16 mm in diameter, is

filled with freshly mixed material and placed on the
penetrometer base.

• The needle is applied to the surface of the impression

material for 10 seconds, and a reading is taken.

• This is repeated every 30 seconds. The initial set is

that time at which the needle no longer completely
penetrates the specimen to the bottom of the ring.

• The final set is the time of the first of three identical

non maximum penetration readings.
.
 CONTROL OF THE SETTING
TIME
Theoretically, there are at least three mechanisms
that can achieve such control. These include:
Solubility of the hemihydrate- If the solubility of
the hemihydrate is increased, supersaturation of
dihydrate is achieved faster, which accelerates
rate of dihydrate crystal deposition.
Number of nuclei of crystallization- The
greater the number of nuclei of crystallization, the
faster the dihydrate crystals will form and the sooner
the mass will harden. Any preexisting fine dihydrate
particles will also serve as nuclei.
 Rate of crystal growth—Increasing or
decreasing the rate of crystal growth will
accelerate or retard the setting time.
In practice, these mechanisms have been
incorporated in the formulation of the material
by the manufacturer and by manipulation
techniques performed by the operator.

Impurities : Fine gypsum particle residues from

incomplete calcination or addition by the
manufacturer will shorten the setting time
because of the increase in the number of nuclei.
 Fineness :Grinding of hemihydrate particles
during manufacturing increases not only the rate of
dissolution of the hemihydrate solution but also the
number of nuclei. This increase in nuclei density
results in a more rapid rate of crystallization.
 Water/powder ratio : The weight (or volume) of
the water divided by the weight of the
hemihydrate powder is known as the water/
powder ratio.

o The use of a higher W/P ratio decreases the

number of nuclei
per unit volume. Consequently, the setting time is
prolonged.
as the W/P ratio
increases, the
setting time
increases, the
strength of the
gypsum product
decreases, and
the setting
expansion
decreases.
Mixing :
Within practical limits, the longer and the
more rapidly the gypsum product is mixed, the
shorter is the setting time as the crystals are
broken up by the spatulation process, which
results in more nuclei of crystallization.

Temperature:
the difference in solubility between
hemihydrate and gypsum becomes smaller with
increasing temperature, and this condition
lowers the driving force for forming the
dihydrate; it also results in a slower setting
reaction
 Effect of Spatulation on Setting
Time
Material W/P Ratio Spatulation Setting Time
Turns

Model Plaster 0.50 20 14 min

0.50 100 11 min

0.50 200 8 min

Dental Stone 0.30 20 10 min

0.30 100 8 min

 MODIFIERS FOR CONTROLLING SETTING
TIME
Chemical modifiers have been used extensively to
increase or decrease the setting time of gypsum
products; they are called retarders and
accelerators, respectively.

The chemical that increases the rate of

hemihydrate dissolution or precipitation of
dihydrate accelerates the setting reaction.

The most commonly used accelerator is

potassium sulfate, which is particularly effective
in concentrations greater than 2%.
 Slurry water flowing out from a model
trimmer contains numerous fine gypsum
particles that act as nuclei of crystallization
and that can serve as an effective accelerator

At a concentration of 2% of the hemihydrate,

sodium chloride is an accelerator. Sodium
sulfate has its maximum acceleration effect
at approximately 3.4%.
Borax, a known retarder for gypsum setting,
has been shown also to promote the growth
of dihydrate crystals, but only at a
concentration lower than 0.2 mM (about
0.08 g/L).
Retarders: The additives that increase the setting
time are known as retarders.
Retarders generally form an adsorbed layer on the
(a) hemihydrates, hence reducing their solubility,
and
(b) gypsum crystals, thus inhibiting their growth.
Some commonly used retarders include
• Borax
• potassium citrate
• sodium chloride (> 2%).
• Orthorhombic anhydrite is a very stable compound
that reacts very less with water. So it acts as a
retarder