L9: gypsum products

Question 1

chemical name of of gypsum :

Answer 1

calcium sulphate dihydrate

Question 2

gypsum is supplied as ( before mixing with water ):

Answer 2

calcium sulphate hemihydrate

Question 3

raw gypsum is heated to ___ to produce calcium sulphate hemihydrate

Answer 3

110-130 C

Question 4

What is calcination and at which temperature does it occur ?

Answer 4

110-130C , during it , plaster becomes hexagonal anhydrite and orthorhombic anhaydrite

Question 5

describe the different powder particles of gypsum products :

Answer 5

beta-hemihydrate : plaster , irregular ,spongy , inn pair
alpha-hemihydrate : more regular shaped STONE
modified alpha hemihydrate : more cube shaped , reduced surface area , improved flow

Question 6

how are dental plasters prepared:

Answer 6

heating at 125C and calcination to dry off part of water of crystalization

-beta hemihydrate irregular plaster

Question 7

2 ways in which dental stone is perpared

Answer 7

1.) gypsum heated at 125c under steam pressure in an autoclave to form alpha hemihydrate crystals

gypsumis boiled in solution of CaCl2 (calcination)
giving material a short thick , less porosity powder

Question 8

what is the name of the chemical reaction that takes place when calcium sulphate hemihydrate is mixed with water ?

Answer 8

crystalization

Question 9

formation of calcium sulfate dihydrate is called:

Answer 9

precipitation

-exothermic reaction

Question 10

center of nucleation :

Answer 10

small quantities of calcium sulphate dihydrate

Question 11

how does presence of anhydrite affect setting of gypsum product ?

Answer 11

causes rapid setting as it reacts rapidly with water

Question 12

how does warm and cold water affect setting reaction ?

Answer 12

warm water speeds it up , cold water slows it down

Question 13

uses of type 2 plaster:

Answer 13

study models , flasking , articulation , bite registration

Question 14

uses of type 3,4,5 stone:

Answer 14

working models

dies

master casts

investment material

Question 15

manufacturer controls the setting by:

Answer 15

controlling concentration of nucleating agents in hemihydrate powder ex : uncovered calcium sulphate dihydrate

or by adding potassium sulphate (accelerator)or borax ( retarder)

Question 16

what compound can be added by manufacturer into powder to accelerate reaction ? what compound can be added to retard reaction ?

Answer 16

accelerate : potassium sulphate

retard : borax

Question 17

how can the operator control setting time ?

Answer 17

temperature : difference will not be substantial though because increasing temperature speeds up setting but retards crystallization

increasing W/P ratio retards setting by decreasing concentration of crystallization nuclei

increased mixing decreases setting by breaking up dihydrate crystals