Gypsum

Question 1

Definition

Answer 1

gypsum which is produced in
dental purposes is nearly pure calcium sulfate dihydrate
(CaSO4. 2H2O).

Question 2

Classification of gypsum products

Answer 2

Type I Impression plaster
Type II Model plaster (Dental plaster)
Type III Dental stone
Type IV Improved stone (Die stone)
Type V High strength, High expansion dental stone

Question 3

All forms of gypsum products are … same , but differ in …

Answer 3

Chemically, physical properties

Question 4

Dental plaster character

Answer 4

β-hemihydrate (plaster) crystals:
Irregular in shape,
Sponginess,
Large and porous (45%)

Question 5

Dental stone character

Answer 5

α-hemihydrate
steam pressure 120 -130 c
crystals are
smaller more regular which have a
prismatic shape, less porous (14%)

Question 6

Improved stone character

Answer 6

α-hemihydrate
Steam pressure
130 0C in presence of
30% Calcium chloride or
0.5% Sodium succinate

Question 7

α-hemihydrate products are much stronger than that of β-
hemihydrate. The reason

Answer 7

differences is that α- hemihydrate powder requires much less amount of gaugingwater for mixing than does the β-hemihydrate.

Question 8

Setting reaction of gypsum

Answer 8

Calcium sulfate hemihydrate > Calcium sulfate dihydrate + heat
(gypsum)

Question 9

Reaction stages:

Answer 9

1- Fluid stage

2- Plastic stage

3- crumbly stage

4- Solid stage

Question 10

Induction period

Answer 10

The elapsed time before the exothermic heat become evident

indicates the onset of rapid crystallization

Question 11

amount of water necessary to mix the
calcium sulfate hemihydrate

Answer 11

18.61 ml. of
water per 100 grams of powder

Question 12

Measurement of setting time

Answer 12

1. Loss of gloss, indicates the initial set of the mass.
2. Penetration test :
   A.Gilmore needles:
   - Small needle: used for measuring initial setting time.
   - Larger needle: used for measuring final setting time.

b. Vicat Test:

Question 13

Control setting time

Answer 13

1] Factor controlled by the manufacturer:
a. Impurities
b. Fineness

2] Factors under operator’s control
a. W/P ratio: The more water longer will be the setting time.
b. Mixing: The longer and the more faster the mix, the shorter setting time
c. Temperature: If the temperature of water is increased from 20 0C to 50 0C,
the setting time is reduced. Above 50 0C the rate of reaction will slow
down, if the temperature is raised to 100 0C, no reaction takes place.

d. Retarders and accelerators: (Modifiers)

Above the temperature of 100 0C, the reaction is reversed,

Question 14

List common Accelerators and retarders

Answer 14

Potassium sulfate
Gypsum particles
Sodium chloride
Sodium sulfate
,

Borax
Potassium citrate
Acetic acid
Gelatin

Question 15

Slurry water can be used to

Answer 15

reduce the setting time

Question 16

normal setting expansion

Answer 16

During the outgrowth of the crystals from the nuclei of crystallization,
the crystals intercept each other and produce outward stress or thrust
which causes an expansion of the entire mass.

Question 17

Factors controlling the setting expansion

Answer 17

1.Spatulation: Longer hand spatulation time give greater
setting expansion.
2. W/P ratio: The higher W/P ratio, the less is the setting
expansion.
3. Modifiers: Both accelerators & retarders generally reduce
setting expansion. Also, 4% potassium sulfate, sodium
chloride and borax decrease setting expansion.
4. Hygroscopic setting expansion: If the setting process is
allowed to occur under water, the setting expansion is
increased by double of the normal setting expansion

Question 18

types strength of the gypsum products

Answer 18

Wet strength (green strength)
Dry strength

Question 19

Factors controlling the strength of gypsum products:

Answer 19

1. Type of gypsum products: α-hemihydrate products are
   stronger than β-hemihydrate.
2. W/P ratio: The lower W/P ratio, the higher the strength.
3. Modifiers: Both accelerators and retarders reduce the
   strength.