Gypsum Overview and Applications

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Questions and Answers

What is the chemical formula for Calcium Sulfate Dihydrate?

CaSO4⋅2H2O (correct)

CaSO4⋅3H2O

CaSO4

CaSO4⋅1/2 H2O

At what temperature does open calcination of gypsum occur?

110-120°C (correct)

150°C

200°C

100°C

What is the main product of open calcination of gypsum?

Calcium Chloride

Calcium Sulfate

Plaster of Paris (correct)

Calcium Sulfate Hemihydrate

What type of crystals does Plaster of Paris consist of?

Irregularly shaped orthorhombic crystals Signup and view all the answers

Which process is used to produce Calcium Sulfate Hemihydrate from Calcium Sulfate Dihydrate?

Calcination Signup and view all the answers

What is the role of Calcium Chloride in the improved stones process?

It speeds up the setting time Signup and view all the answers

What is the characteristic of the crystals that make up Plaster of Paris?

Very dense crystals Signup and view all the answers

Which of the following is NOT a characteristic of Plaster of Paris?

Low density Signup and view all the answers

What temperature is typically used for closed calcination in the production of gypsum?

110-120°C Signup and view all the answers

What effect does smaller crystals have on gypsum setting time?

Faster setting time Signup and view all the answers

What happens when too much water is added to gypsum powder?

Longer setting time Signup and view all the answers

What is the result of mixing gypsum for too long?

Production of air bubbles Signup and view all the answers

What effect does temperature have on gypsum setting reaction up to 50°C?

No significant change Signup and view all the answers

What does the equation H2O + CaSO4⋅1⁄2 H2O represent?

Formation of calcium dihydrate Signup and view all the answers

Which type of gypsum has the highest setting expansion?

Type V: High Strength Signup and view all the answers

What occurs when gypsum is subjected to a temperature above 100°C?

No reaction Signup and view all the answers

What do accelerators do in gypsum setting?

Remove moisture Signup and view all the answers

Which of the following affects the strength of gypsum products?

Type of gypsum used Signup and view all the answers

What happens if the mixing time is too short?

Improper mixing of particles Signup and view all the answers

What is the setting time defined as?

The time from mixing until the material hardens Signup and view all the answers

What is the effect of a lesser water-to-powder (W/P) ratio?

Longer mixing time Signup and view all the answers

Which factor generally reduces setting expansion?

Use of chemical modifiers Signup and view all the answers

What is the setting expansion for dental stone with a WP ratio of .28 - .30?

0 - .20% Signup and view all the answers

How does increased spatulation affect setting time?

Decreases setting time Signup and view all the answers

Which type of plaster has the highest compressive strength?

Die stone Signup and view all the answers

What effect does higher water-to-powder ratio typically have?

Lower strength Signup and view all the answers

Which setting expansion is associated with high-strength dental stone?

0 - .30% Signup and view all the answers

What happens to the crystals when chemical modifiers are used?

Crystals become flat Signup and view all the answers

Which characteristic is NOT affected by the water-to-powder ratio?

Material color Signup and view all the answers

What effect does a greater water-to-powder (W/P) ratio have on the strength of a cast?

Decreases strength and increases porosity Signup and view all the answers

Which of the following is a property of a good cast?

It should be accurately measured and stable Signup and view all the answers

What happens if spatulation time exceeds one minute?

Strength decreases due to overmixing Signup and view all the answers

What type of disinfecting solution is recommended for casts?

1:10 dilution of sodium hypochlorite Signup and view all the answers

Which of the following statements about dry strength is true?

It is twice the wet strength when excess water is removed Signup and view all the answers

Which aspect is not associated with a good cast?

Low strength to allow for easy handling Signup and view all the answers

What happens to gypsum products when they expand during setting in water?

They become more soluble to water. Signup and view all the answers

Which compound is considered the best accelerator at concentrations greater than 2%?

K2SO4 Signup and view all the answers

What effect do retarders typically have on gypsum products?

Increase setting time Signup and view all the answers

How is the strength of gypsum products characterized?

Through the entanglement of crystals Signup and view all the answers

Which property is twice in wet strength compared to dry strength after 24 hours?

Dry strength Signup and view all the answers

What should be used to store gypsum products to prolong their shelf-life?

In an airtight container Signup and view all the answers

Which of the following options correlates with increased strength in gypsum products?

Increased temperature Signup and view all the answers

What is the effect of higher concentrations of NaCl and Na2SO4 on gypsum products?

Accelerate the setting time Signup and view all the answers

Which of the following is a recommended practice before getting powder from a gypsum product bottle?

Shake the bottle first Signup and view all the answers

Which type of gypsum product has the highest compressive strength?

Die Stone Signup and view all the answers

Study Notes

Gypsum Overview

Gypsum is chemically known as Calcium Sulfate Dihydrate (CaSO4·2H2O).
Commonly used in dental applications, with purity levels approaching 100%.
Produced through calcination, which involves heating gypsum to change its physical properties.

Calcination Processes

Open Calcination: Conducted at temperatures of 110-120°C in a kettle or open bath.
- Produces β-hemihydrate, also known as Plaster of Paris.
Closed Calcination: Achieves a chemical reaction transforming gypsum into calcium sulfate hemihydrate.
- Reaction: CaSO4·2H2O → CaSO4·½H2O + 1½ H2O.

Gypsum Products and Their Properties

Types of gypsum products:
- Impression Plaster: WP Ratio 0.50-0.75; Strength 290-870 psi; Setting expansion 0-0.15%.
- Plaster of Paris: WP Ratio 0.45-0.50; Strength 1300 psi; Setting expansion 0-0.30%.
- Dental Stone: WP Ratio 0.28-0.30; Strength 3000 psi; Setting expansion 0-0.20%.
- Die Stone: WP Ratio 0.22-0.24; Strength 5000 psi; Setting expansion 0-0.10%.
- High Strength Dental Stone: WP Ratio 0.18-0.22; Strength 7000 psi; Setting expansion 0-0.30%.

Setting Characteristics

The setting time is measured from mixing to hardening.
Influenced by water/powder (W/P) ratio: more water leads to longer setting time; less water shortens it.
Factors affecting setting and expansion include:
- W/P Ratio: Higher ratios increase setting expansion.
- Spatulation: Increased mixing time may reduce setting time.
- Chemical Modifiers: Modifiers typically reduce expansion and improve strength.

Setting Reactions

Typical Chemical Equation:
- H2O + CaSO4·½H2O → CaSO4·2H2O + Heat.
Accelerators and retarders influence setting speed:
- Accelerators (e.g., K2SO4) decrease setting time.
- Retarders (e.g., Borax) increase the setting time.

Strength and Hardness

Measured in compressive strength:
- Laboratory Plaster: 27.5 MPa (4000 PSI).
- Dental Stone: 62 MPa (9000 PSI).
- Die Stone: 79 MPa (11500 PSI).
Strength quality is affected by factors such as W/P ratio, spatulation, and the presence of chemical modifiers.

Handling Gypsum Products

Store in airtight containers to extend shelf life.
Proper W/P ratio is crucial to avoid issues like air bubbles or incomplete mixing.
Care guidelines:
- Shake the container before dispensing powder.
- Use dry scoops to avoid moisture contamination.

Properties of a Good Cast

High strength and hardness to prevent fractures.
Accurate measurements necessary for stable setting expansions.
Should be compatible with impression materials and provide good color contrast.

Disinfection of Casts

Disinfect casts using an immersion method in a 1:10 sodium hypochlorite solution for 30 minutes.
In clinical settings, models from non-disinfected impressions should be aseptically wrapped and sterilized.

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Description

This quiz explores the chemical properties of gypsum, including its formulation and various calcination processes. You will learn about different gypsum products used in dental applications, their characteristics, and their specific ratios. Test your knowledge on gypsum's importance in various industrial applications.