Investing Materials (2) Flashcards
What is the starting mineral for Gypsum?
Calcium Sulfate Dihydrate
CaSO4:2H2O
What is the product form calcining?
Calcium Sulfate Hemihydrate
CaSO4:1/2H2O
What is the calcining procedure for Plaster?
Open container to air
What is the calcining procedure for Stone?
Closed autoclave under steam pressure
Setting reaction for Gypsum products
Exothermic reaction
Reverse of calcining process
End product is gypsum
Same for all gypsum products
What is a common accelerator for Gypsum products?
Potassium Sulfate
What is a common retarder for Gypsum products?
Borax
What happens to plaster when it sets?
It expands
What causes porosity in dry Gypsum?
Loss of excess water beyond that required for balanced setting reaction
What does porosity do to Gypsum?
Acts to concentrate stress, leading to material fracture
What is the binder in Gypsum?
Calcium sulfate hemihydrate
T/F - Refractory component of Gypsum participates in the setting reaction
False - it does not
What is the most important role in the refractory component of Gypsum?
Provide thermal expansion during heating in burnout furnace
Also provides strength of investment at temperatures above 100 C
Is the relative amount of expansion greater for Cristobalite or Quartz?
Cristobalite
What do manufacturers offer for thermal expansion investments?
Quartz
Cristobalite
Changes in crystal structure cause…
Large thermal expansion over a narrow temperature range
An increase in the P/W ratio causes…
An increase in strength and expansion
What causes increased in expansion and strength in greater P/W ratio
It’s due to a more dense gypsum framework
Why do companies include in heating investments? Why?
Insoluble raw starch
It provides expansion by water absorption and swelling. The starch burns out along with patterns during burnout process
