Gypsum Products Plaster and Stone

Question 1

What is Gypsum

Answer 1

Calcium Sulphate Dihydrate = (CaSO4)2H2O

Question 2

What can gypsum based materials be used as

Answer 2

• Impression materials - limited use today, used in edentulous cases (negative replica)
• Models - poured from an impression to construct partial or full denture (positive replica)
• Dies - positive replica of individual teeth
• Moulds - for dentures
• Refractory investments?*

Question 3

What is a positive replica

Answer 3

When a mould or impression i made of the study object and then the mould is filled with a permanent substance

Question 4

What is a negative replica

Answer 4

Inverse representation of the original specimen surface yh lol

Question 5

How do manufacturers process gypsum

Answer 5

Convert the calcium sulphate dehydrate to Calcium sulphate hemihydrate (just one water molecule)

Question 6

Depending on the treatment process of gypsum what forms can gypsum hemihydrate be in

Answer 6

• Plaster - soft white powder
• Stone - harder, yellow powder
• Improved stone

Question 7

What happens to gypsum powders when mixed with water on setting

Answer 7

They convert back to the dihydrate form

Question 8

What are the differences between different gypsum hemihydrate forms

Answer 8

Chemically identical but physical properties depend on nature of dehydration process or preparation process

Question 9

Describe the differences in physical properties between Gypsum plaster and stone/improved stone

Answer 9

• Plaster - large, irregular, porous particles
• Stone/Improved stone - small, regular and non-porous particles
• Improve stone - expensive and more scratch resistant cf stone

Question 10

What are the differences in the product formed when alpha and beta hemihydrates mix with water on setting

Answer 10

Alpha - forms stronger product due to small, regular, non-porous particles that pack closely together within the set material

Beta - forms weaker product due to large irregular and porous particles that don’t pack closely together that results in large pores in the set material

Question 11

In what ways do alpha and beta hemihydrates differ

Answer 11

Alpha and beta hemihydrates differ in particle/crystal size and surface area

Question 12

What are the advantageous features of using gypsum plaster

Answer 12

Softer
Cheap
Easy to use and shape

Question 13

What are the uses of gypsum plaster

Answer 13

• General purpose material used for mounting models into articulators
• Flasking procedures for complete and partial denture processing
• Basing models

Question 14

What are the uses of gypsum stone

Answer 14

Used where strength is v important

• Dies
• Models of mouth - +ve replica of individual teeth, in construction of crowns, bridge and dentures

Question 15

Describe the composition of gympsum products

Answer 15

Hemihydrate - 75-85%
Unchanged gypsum - 5-8%
Mix of fast set soluble and slow set insoluble anhydrides - 5-8%
Impurities - ~4%
Accelerators/Retarders - ~4%

Question 16

What do the accelerators/retarders in gypsum do

Answer 16

These increase or decrease the solubility of gypsum in water and therefore affect its setting time

Question 17

Name some accelerators for gypsum

Answer 17

2% K2SO4 solution
CaSO4.2H2O
NaCl (<20%)

Question 18

How does Potassium Sulfate gypsum accelerator work

Answer 18

Reacts with water/hemihydrate = Sygenite

This crystallises rapidly and encourages growth of more crystals

Question 19

How does CaSO4.2H2O accelerate setting of gypsum

Answer 19

Provides additional nucleation sites

Question 20

How does NaCl (<20%) accelerate gypsum setting

Answer 20

Provides additional sites for crystallisation - increases reaction rate and reduces observed expansion

Question 21

Name some gypsum retarders

Answer 21

NaCl (>20%)
Borax
Potassium Citrate (gum arabic, acetates)

Question 22

How does NaCl (>20%) act as a retarder

Answer 22

Slows down reaction rate by deposition of NaCL on CaSO4.2H2O crystals and preventing growth

Question 23

AY BAWS CAN I HABE DE NOTE PLZ

Answer 23

All gypsum retarders interfere with crystal formation and affect dimensional change on setting

Question 24

Describe the setting reaction of Calcium sulphate hemihydrate with water

Answer 24

Forms calcium sulphate dihydrate (less soluble)

Exothermic

Question 25

Describe the mechanism of the setting reaction of calcium sulfate hemihydrate with water

Answer 25

- Hemihydrate slowly forms dihydrate - Some CaSO4 hemihydrate dissolve in water due to low solubility and reacts to form dihydrate - Dihydrate's lower solubility in water = unstable supersaturated solution - So CaSO4.2H2O precipitates forming stable crystals - As crystals form, more hemihydrate dissolves in water and process continues until setting

Question 26

What is the role of unchanged gypsum dihydrate in setting mechanism

Answer 26

Acts as a crystallisation nuclei for growth of dihydrate crystals

Question 27

How much water is used for 100g of different gypsum product and why do we use excess

Answer 27

Plaster - 50ml/100g Stone/improved stone - 20ml/100g Excess because otherwise the mix would be too viscous

Question 28

How can increasing the proportion of water used in the setting reaction affect the reaction

Answer 28

Slower setting time - longer for saturation, the mix will be runny and the model will be weaker

Question 29

How can increasing the proportion of powder used in the setting reaction affect the reaction

Answer 29

Difficult to mix and product will result in porosity within the set material

Question 30

What does increasing the spatulation time (mixing) in the setting reaction do to the reaction

Answer 30

Reduction in setting time - break up formed crystals to form new sites for crystal growth, also increases the setting expansion

Question 31

What does increasing the temperature in the setting reaction do to the reaction

Answer 31

- Little change between 0 and 50C - More than 50C = gradual retardation occurs - At 100C = no reaction

Question 32

What methods are there to measure setting time

Answer 32

- Vicat needles | - Gillmore needles

Question 33

What different Gillmore needles are there and how are they used to measure setting time

Answer 33

1/4 lb needle measures initial setting time, mould cannot be moulded but can be carved, needle won't mark material 1 lb needle measure final setting time = when material can be removed from the impression without distortion or fracture

Question 34

What is a visual sign that the material has reached its initial set

Answer 34

Surface of material loses its gloss or shine

Question 35

What is the rough working and setting time for model plaster

Answer 35

2-3 mins working time | 5-10 mins setting time

Question 36

What factors can increase setting expansion

Answer 36

- Powder/liquid ratio | - Spatulation time

Question 37

What causes setting expansion in gypsum products

Answer 37

Crystals impinge on each other as they grow and push each other apart = large empty spaces between crystals = porosity = 0.6% by volume

Question 38

What is hygroscopic expansion

Answer 38

When crystals grow freely in water and expand By immersing material in water while setting the setting expansion increases

Question 39

What happens when a material is left to set in air

Answer 39

If set in air the surface tension of the free water in the material tends to draw the crystals together

Question 40

Name some of the properties of gypsum products

Answer 40

- Once set - little or no dimensional change - Storage - excellent - Compressive strength affected by: too much water (inferior strength), too little water (thick mix, porosity, incomplete reaction) - Dry strength = 2x that of wet strength

Question 41

Describe the tensile strength of different gypsum products

Answer 41

- Plaster tensile strength = v low | - Stone tensile strength - double plaster - crown and bridge models and dies

Question 42

Describe the surface hardness and scratch and abrasion resistance for different gypsum products

Answer 42

Surface hardness: - plaster = very low - improves for stone and even better for improved stone Scratch and abrasion resistance: - Plaster - low resistance - Stone + improved stone - better resistance

Question 43

What is the P/W ratio, initial setting time, linear expansion and compressive strength after 2 and 24hrs for Laboratory Plaster

Answer 43

``` P/W ratio = 100g/45-50ml Initial setting time - 5-10 mins Linear expansion - 0.2-0.4% Compressive strength 2hrs - 10MPa Compressive strength 24hrs - 24MPa ```

Question 44

What is the P/W ratio, initial setting time, linear expansion and compressive strength after 2 and 24hrs for Stone

Answer 44

``` P/W ratio = 100g/25-40ml Initial setting time - 7-15 mins Linear expansion = 0.08-0.1% Compressive strength 2hrs - 30MPa Compressive strength 24hrs - 70MPa ```

Question 45

What is the P/W ratio, initial setting time, linear expansion and compressive strength after 2 and 24hrs for Improved Stone - Densite

Answer 45

``` P/W ratio = 100g/20-40ml Initial setting time - 5-12 mins Linear expansion = 0.05-0.07% Compressive strength 2hrs - 35MPa Compressive strength 24hrs - 80MPa ```