Presented by Guided By Ashutosh Pai. Dr. Amit Jagtap. GYPSUM PRODUCTS Presented by Guided By Ashutosh Pai. Dr. Amit Jagtap.

INTRODUCTION PRODUCTION OF CALCIUM SULFATE HEMIHYDRATE. SETTING OF GYPSUM PRODUCTS. TESTS FOR WORKING, SETTING, & FINAL SETTING TIMES. CONTROL OF SETTING TIME.

NORMAL SETTING EXPANSION. HYGROSCOPIC SETTING EXPANSION. TYPES OF GYPSUM PRODUCTS. PROPORTIONING, MIXING & CARING FOR GYPSUM PRODUCTS. SPECIAL GYPSUM PRODUCTS. INFECTION CONTROL.

INTRODUCTION OBTAINED BY :- As a by product of some chemical operations. Mined. USES :- Molds & casts. Making impressions -> type I. Study models for oral and maxillofacial structures.

INTRODUCTION CONTD . . Important auxiliary materials for dental laboratory procedures. Plaster + fillers (forms of silica) = gypsum dental investments. OTHERS USES :- Artifacts. Widely used in industries. Making walls of plaster (false ceiling).

PRODUCTION OF -- [CaSO4] 1/2 H2O Calcining process – Gypsum is ground & subjected to temperatures of 110°C to 130°C to drive off part of the water of crystallization that is the amount of water needed to convert gypsum to its hemihydrate form. It is done in a vat or a kiln open to air. Dry calcination & wet calcination.

PROCESS CONTD.. As the temperature is raised the remaining water is removed , and products are formed as indicated . 1100 –1300 1300 –2000 2000 –10000 CaSO4+2H2O [CaSO4]1/2 H2O CaSO4 CaSO4 CALCIUM SULFATE HEMIHYDRATE HEXAGONAL ANHYDRATE ORTHOROMBIC ANHYDRATE GYPSUM

Depending on the method of calcination, different forms of hemihydrate can be obtained like :- α- hemihydrate (dental stone). β- hemihydrate (dental plaster). α- modified hemihydrate.(made by boiling gypsum in 30% aqueous solution of calcium chloride and magnesium chloride).It is used primarily for dies.

WHY α & β DESIGNATIONS ? For tradition & convenience. Difference between α & β :- Difference in crystal size. Surface area. Degree of lattice perfection.

Difference between α- hemimhydrate and β- hemihydrate. Type III , IV &V Produced by wet calcination. Requires less water for mixing. Better packing ability. Low surface free energy. Crystal habit of hexagonal calcium sulfate . High apparent density. β – hemihydrate Type I & II Produced by dry calcination. Requires more water form mixing. Less packing ability. High surface free energy. Crystal habit that of hemihydrate. Low apparent density.

DENTAL PLASTER (β – HEMIHYDRATE) DENTAL STONE (α - HEMIHYDRATE)

SETTING OF GYPSUM PRODUCTS The reverse of reaction (1) describes the reaction of calcium sulfate hemihydrate powder with water to produce gypsum :- (CaSO4 ) 2.H2O + 3H2O 2CaSO4 . 2H2O+ unreacted (CaSO4 ) 2 1/2 H2O + heat The heat evolved in the exothermic reaction is equivalent to the heat used originally in calcination. The hexagonal anhydrite reacts very rapidly, whereas orthorombic anhydrite requires more time.

SETTING REACTION PROPOSED THEORIES : Colloidal theory – hemihydrate + water --> colloidal state through sol gel mechanism. In sol state hemihydrate particles are converted to dihydrate and as the measured amount of water is consumed the mass converts to a solid gel.

PROPOSED THEORIES CONTD . . Hydration theory— suggests that rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material. Dissolution – precipitation theory— (most widely accepted theory) based on dissolution of plaster and instant recrystallization of gypsum, followed by interlocking of the crystals to form the set solid.

PROPOSED THEORIES CONTD . . Hemihydrate is 4 times more soluble in water than is the dihydrate near room temp (20°C). Thus the setting reaction can be understood as follows:-- hemihydrate + water Suspension is formed that is fluid and workable. Hemihydrate dissolves until it forms a saturated solution. This saturated solution supersaturated in dihydrate, precipitates out dihydrate.

PROPOSED THEORIES CONTD . . Thus solution is no longer saturated with hemihydrate, so it continues to dissolve. Dissolution of the hemihydrate and precipitation of dihydrate proceeds as new crystals form or further growth occurs on the present crystals. Reaction continues until no dihydrate precipitates.

W/P RATIO The ratio of the water to the hemihydrate powder is usually expressed as the W/P ratio, or the quotient obtained when the weight (or volume) of the water is divided by the weight of the powder. W/P ratio  setting time , strength , setting expansion . example : if 100g is mixed with 60 ml of water, the W/P ration is 0.6.

Some typical recommended ranges: Type II plaster -- 0.45 to 0.50 W/P RATIO CONTD.. Some typical recommended ranges: Type II plaster -- 0.45 to 0.50 Type III stone -- 0.28 to 0.30 Type IV stone -- 0.22 to 0.24 In preweighed bags powder mass variation may be ± 2%

Compressive strength as a function of w/p ratio for five types of gypsum products. American National Standard Institute/ American Dental Association (ANSI/ADA) Specification No 25 for dental gypsum products and the strength values represent the wet strength at 1 hr.

STAGES IN SETTING Upon mixing there is a continuous aqueous(FLUID) phase present exhibiting pseudoplasticity As the reaction proceeds the clump of growing gypsum crystals interact, the mix becomes PLASTIC. The glossy surface disappears growing crystals thrust apart converting the plastic mass into a rigid solid, weak and FRIABLE. The relative amount of solid phase increases and then this friable mass becomes CARVABLE.

TESTS FOR WORKING, SETTING AND FINAL SETTING TIMES. MIXING TIME (MT)— Time from addition of the powder to the water until the mixing is completed. mechanical mixing – 20 to 30 secs hand mixing atleast 1 minute. WORKING TIME (WT)— measured from the start of mixing to the point where the consistency is no longer acceptable for the products intended purpose. Generally, a 3 min working time is adequate.

TESTS FOR WT, ST AND FINAL ST CONTD.. SETTING TIME— The time that elapses from the beginning of the mixing until the material hardens is known as the setting time. Reaction (2) requires a definite time for completion .

TESTS FOR SETTING TIME CONTD.. LOSS OF GLOSS TEST FOR INITIAL SET: The excess water is taken up in forming the dihydrate so that the mix looses its gloss. This occurs at approximately 9 mins . The mass still has no measurable compressive strength.

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