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Dental Materials I > Gypsum Products > Flashcards

Gypsum Products Flashcards

1
Q

What does the heating reaction of gypsum rock result in during the manufacture of gypsum products?

A

Formation of hemihydrate, which is the basis of all gypsum used in dentistry.

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2
Q

What does calcium sulfate and dihydrate turn into during the heating reaction?

A

calcium sulfate and hemihydrate

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3
Q

What is the main difference between plaster, stone and high strength stone?

A

The manner of driving off the water

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4
Q

What are the three types of material derived from the partial dehydration of gypsum rock?

A

plaster, stone and high strength stone

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5
Q

What is the precursor to model/lab stone?

A

Plaster

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6
Q

What is the precursor to dental stone?

A

Hydrocal

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7
Q

What is the precursor to high strength stone?

A

Densite

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8
Q

What is the setting reaction of gypsum products?

A

Nucleated recrystallization, characterized by a continuous solution of hemihydrate, diffusion of calcium and sulfate ions to nucleation centers, and precipitation of microscopic gypsum crystals

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9
Q

Regarding water/powder ratio, what is the effect of less water?

A

Less porosity

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10
Q

What is the effect of less porosity?

A

More strength

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11
Q

What happens to a thick mix (low W/P ratio)?

A

It hardens more quickly b/c nucleation centers are concentrated in a smaller volume. The interaction of the growing solid occurs earlier and is more effective

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12
Q

What is the effect of porosity?

A

Tends to weaken the mass

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13
Q

What is the result of a decrease in W/P ratio?

A

Denser cast

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14
Q

What is the result of raising W/P ratio?

A

Lower strength (more porosity)

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15
Q

What is gypsum used as in materials commonly employed in casting of dental gold alloys?

A

A binder

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16
Q

Gypsum is generally not heated above what temperature?

A

700 degrees C

17
Q

Is gypsum used for base metals or other high melting alloys?

A

No

18
Q

What does Refractory provide in gypsum bonded investments?

A

Provides expansion to compensate for gold shrinkage

19
Q

What are the benefits of a binder?

A

Used to form a coherent solid mass. Provide strength for the investment, Calcium sulfate hemihydrate (gold). Phosphate, ethyl silicate (high temp alloys)

20
Q

What binder is used in gold investments?

A

Calcium sulfate hemihydrate

21
Q

What binder is used in high temp alloys?

A

Phosphate, ethyl silicate

22
Q

What are the forms in which silica exists?

A

Alpha - a low temperature phase

Beta - a high temperature phase

23
Q

What are the effects of increasing amount of silica w/in practical limits on the refractory behavior of gypsum bonded investments?

A

Increased manipulation time
Increased setting expansion
Increased thermal expansion
Decreased compressive strength

Dental Materials I (9 decks)

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