Gypsum Products (about setting time) Quiz 2

Question 1

What is guaging water?

Answer 1

the amount of water mixed with the powder

Question 2

Guaging water includes the water needed for what two things?

Answer 2

water needed for the setting reaction and that needed to wet the powder particles

Question 3

What 3 things determine guaging water amount?

Answer 3

1) particle size aka therefore total surface area of powder particles
2) particle size distribution
3) adhesion between particles (surfactants)

Question 4

particle size distribution determines?

Answer 4

packing efficiency of the powder

Question 5

surfactants are?

Answer 5

surface active agents used to reduce the water (gauging water) requirements

Question 6

Purpose of surfactants?

Answer 6

reduce the water requirements

Question 7

what happens to hemihydrate in water?

Answer 7

it dissolves

Question 8

What happens to dihydrate in water? What does this allow?

Answer 8

it precipitates out. This allows for more hemihydrate to dissolve

Question 9

What allows for the material to thicken when added to water?

Answer 9

dihydrate precipitates out and hemihydrate dissolves. As the mass hardens, needle like clusters are formed

Question 10

What can you use excess water for?

Answer 10

1) form a useable slurry
2) act as a lubricant to reduce strength in the SET product
3) make a porous final product (pores left behind when water evaporate)

Question 11

Why is a wet gypsum product less strong?

Answer 11

some of its material is still in water (less crystals). SO when water finally evaporates, more crystals form reinforcing those already present

Question 12

What is the water/powder ratio? Important why?

Answer 12

# of milliliters of water per 100 grams of powder
*important in determining physical properties of the set gypsum product

Question 13

increase W/P results in increased or decreased strength and setting time?

Answer 13

LONGER setting time (direct relationship) and LOWER strength (indirect relationship)

Question 14

typical W/P ranges for plaster, stone and die stone?

Answer 14

plaster: 0.48-0.50
stone: 0.28-0.30
die stone: 0.25-0.30

Question 15

What is the theoretical W/P?

Answer 15

0.19

Closer to 0.0 is best

Question 16

What is steps in the setting process

Answer 16

1) mixing time
2) working time
3) loss of gloss
4) setting time
5) ready for use

Question 17

mixing time

Answer 17

time from addition of powder to water until mixing is completed. 1 minute for hand mixing or 20-30 for machine

Question 18

working time

Answer 18

time available to use WORKABLE mix. About 3 min

Question 19

Loss of Gloss? How long does it take?

Answer 19

caused by excess water removed from the surface during formation of the dihydrate. Usually 9min from start of MIXING time

Question 20

setting time

Answer 20

time elapsed from the beginning of mixing until the material completly hardens, 13-20 minutes

Question 21

When is a gypsum material “ready for use”?

Answer 21

varies from batch to batch but between 30-60 minutes

Question 22

What are 7 things that control setting time?

Answer 22

1) impurities
2) fineness
3) W/P ratio
4) Mixing
5) Temperature
6) retarders
7) accelerators

Question 23

Impurities

Answer 23

the presence of gypsum particles in stone or plaster act as nuclei of crystallization, thereby reducing setting time

Question 24

What is fineness and what does it affect?

Answer 24

How small a powder particle is. Afiner particle size allows the powder to go into solution faster, crystallization to occur sooner (Shorter setting time)

Question 25

Setting time is altered by W/P ratio how?

Answer 25

more water means fewer nuclei of crystallization per unit of volume. Setting time is increased

Question 26

Setting time is altered by mixing how?

Answer 26

the longer and more rapidly the plaster is mixed, the shorter the setting time

Question 27

Setting time is altered by temperature how?

Answer 27

setting time is slowed (retarded) at temps above 120F; stops around 212

Question 28

What do retarders do? Explain how

Answer 28

increase setting time. act by absorbing to the surface of the hemihydrate powder and reducing its solubility

Question 29

examples of retarders?

Answer 29

citrates, acetates, borates

Question 30

Purpose of accelerators?

Answer 30

To shorten setting time by inreasing solubility of hemihydrate

Question 31

The accelerator must NOT affect?

Answer 31

the solubility of dihydrate

Question 32

typical accelerators?

Answer 32

K2SO4, NaCl, slurry water, increasing speed or mixing time

Question 33

according to theoretical calculations, a net linear ____ of 2% should occur during setting of gypsum

Answer 33

shrinkage

In reality there is expansion

Question 34

In reality, there is no shrinkage of gypsum during setting. There is?

Answer 34

expansion. caused by unimpeded growth of crystals

Question 35

increased expansion occurs with what W/P?

Answer 35

lower W/P ratios

Question 36

Hygroscopic expansion? What does this do to expansion?

Answer 36

if gypsum sets under water, the expansion is MORE THAN DOUBLE

Question 37

is there a chemical different between air expansion or hygroscopic expansion?

Answer 37

No. purely physical

Question 38

Why is there more hygroscopic expansion?

Answer 38

crystallites are not drawn together by surface tension like they are in air

Question 39

hygroscopic expansion is most important in?

Answer 39

investments, which are used for fabrication of cast restorations

Question 40

unimpinged crystal growth causes _______ setting expansion

Answer 40

increased