Gypsum for Dental Applications

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the chemical formula for the gypsum used in dental applications?

CaSO4⋅2H2O (correct)

CaSO4⋅1/2 H2O

CaSO4

CaSO4⋅3H2O

What is the result of open calcination of gypsum?

Calcium Carbonate

Calcium Sulfide

Calcium Sulphate Dihydrate

Calcium Sulphate Hemihydrate (correct)

What temperature range is used in open calcination?

100-110°C

130-140°C

120-130°C

110-120°C (correct)

Which characteristic is not associated with Plaster of Paris?

Spherical crystal formation Signup and view all the answers

What is the impact of adding 30% calcium chloride to gypsum?

It leads to the formation of improved stones. Signup and view all the answers

What is the primary process used to produce α-hemihydrate/Dental Stone?

Closed calcination in an autoclave Signup and view all the answers

Which type of gypsum is referred to as Dental Stone?

Type IV and V Signup and view all the answers

What defines the crystal shape of β-hemihydrate compared to α-hemihydrate?

Larger and irregular Signup and view all the answers

Which property is not associated with Type IV Gypsum?

Low expansion Signup and view all the answers

What is the purpose of the Vicat needle in relation to gypsum products?

To measure initial setting time Signup and view all the answers

What is the purpose of the Gillmore needle?

To measure final setting time Signup and view all the answers

What occurs during the exothermic setting reaction of gypsum?

The maximum temperature is reached Signup and view all the answers

How long should gypsum be soaked in water before mixing?

20 seconds Signup and view all the answers

What is the approximate temperature rise during the setting process?

30°C Signup and view all the answers

What is the working time (initial setting time) for a workable mix?

3 minutes Signup and view all the answers

How long does the final setting time typically take?

20 minutes Signup and view all the answers

What is the main purpose of the initial setting time (I.S.T)?

To trim off excess material Signup and view all the answers

What should occur after the final setting time (F.S.T)?

Cast separation Signup and view all the answers

What defines the setting time of gypsum material?

Time from mixing to the semi-hard stage Signup and view all the answers

What is the standard time for mechanical mixing of gypsum?

20-30 seconds Signup and view all the answers

What happens to gypsum products when they are allowed to set while immersed in water?

They expand and become more soluble. Signup and view all the answers

Which substance is considered the best accelerator for gypsum products when used at concentrations greater than 2%?

K2SO4 Signup and view all the answers

What effect does fineness of the gypsum crystals have on setting time?

Finer crystals lead to faster setting. Signup and view all the answers

How does increasing the water-to-powder ratio affect setting time?

It increases the setting time. Signup and view all the answers

Which of the following are classified as retarders that create a surface coating of crystals?

Borax, nitrates, and citrates Signup and view all the answers

What happens when gypsum is subjected to temperatures above 50°C?

Retarders are activated. Signup and view all the answers

How does particle size affect the setting time of gypsum products?

Smaller particles lead to faster setting times. Signup and view all the answers

What is the relationship between the water/powder ratio and setting expansion?

Higher W/P ratio reduces expansion. Signup and view all the answers

What is the setting time effect of using Type I Impression Plaster?

0.13% setting time reduction. Signup and view all the answers

Which of the following is true about the wet strength of gypsum products compared to dry strength?

Wet strength is double that of dry strength after 24 hours. Signup and view all the answers

What is the consequence of having too thick a mixture in gypsum setting?

Inadequate mixing of powder with water. Signup and view all the answers

How should gypsum products be stored to prolong their shelf life?

In an airtight container. Signup and view all the answers

What is the typical mixing time for gypsum products?

2 minutes. Signup and view all the answers

What should be done before using powder from a bottle of gypsum product?

Shake the bottle well. Signup and view all the answers

Which of the following is true about setting expansion?

Setting expansion happens primarily in air. Signup and view all the answers

What is the primary characteristic of Type V High Strength, High Expansion Dental Stone?

High strength and consistent expansion. Signup and view all the answers

Study Notes

Gypsum Overview

Gypsum is composed of calcium sulfate dihydrate (CaSO4⋅2H2O).
Commonly used in dental applications includes dental plaster and dental stone.
Ground gypsum can exist as solid particles or powder.

Calcination Processes

Open Calcination:
- Conducted at 110-120°C (230-250°F).
- Produces β-hemihydrate, known as Plaster of Paris, characterized by irregular orthorhombic crystals.
Closed Calcination:
- Performed in an autoclave under 16 PSI at 120-130°C.
- Yields α-hemihydrate, known as Dental Stone, with smaller, prismatic crystals.

ADA Classification of Gypsum Products

Type 1: Impression/Soluble Plaster - Used for impressions.
Type 2: Plaster Model/Plaster of Paris - Used for models and records.
Type 3: Dental Stone (Class 1) - Stronger and used as working casts.
Type 4: Die Stone - High strength, low expansion, used for precise applications.
Type 5: High Strength High Expansion Dental Stone - Used in investment models.

Gypsum Characteristics

Crystals are differentiated by size, shape, and packing:
- β-hemihydrate has larger, irregular crystals with loose packing.
- α-hemihydrate features smaller, more regularly shaped crystals, tightly packed.

Measuring Setting Times

Initial Setting Time (IST): Time from mixing until reaching a semi-hard state typically within 7-13 minutes.
Final Setting Time (FST): Time until the material achieves its final hardness, usually around 20 minutes.

Exothermic Setting Reaction

Temperature during final hardening can rise up to 30°C, depending on the mass of material used.

Mixing and Working Time

Mixing Time: Approximately 20-30 seconds for mechanical mixing; hand spatulation requires at least one minute.
Working Time: Generally allows around 3 minutes of usable time after mixing.

Chemical Reaction of Gypsum Setting

Setting reaction involves the conversion of calcium sulfate hemihydrate to calcium sulfate dihydrate with heat release:
- H2O + CaSO4⋅1⁄2 H2O → CaSO4⋅2H2O + Heat

Factors Affecting Setting Time

Purity and Fineness: Smaller particles dissolve quicker, leading to faster setting times.
Water-Powder Ratio: More water leads to longer setting times; less water may cause incomplete mixing.
Additives:
- Accelerators (e.g., K2 SO4) can shorten setting time.
- Retarders (e.g., Borax) can prolong it.

Strength and Hardness

Measured compressive strength varies:
- Laboratory plaster: 27.5 MPa (4000 PSI)
- Dental Stone: 62 MPa (9000 PSI)
- Die Stone: 79 MPa (11500 PSI)

Care of Gypsum Products

Store products in airtight containers to maintain integrity.
Shake containers before use and utilize clean, dry utensils to avoid contamination.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz explores the properties and applications of gypsum, particularly in dental practices. It discusses the differences in types such as dental plaster and dental stone, highlighting their strength and surface characteristics. Understand how gypsum is utilized in impression materials and dental solutions.