Dental Gypsum And Investments

Dental Gypsum And Investments

Classify gypsum products. Describe the setting reaction and microscopic structure of the set material.

Answer:

Setting Reaction of Gypsum Products:

When 1 gram mol of calcium sulfate hemihydrates reacts with 1.5 gram mol of water, the hemihydrates reverts to dihydrate forming 1 gram mol of calcium sulfate dihydrate by hydration, with liberation of 3900 calories of heat. Here the heat which is evolved in the reaction is equal to the heat originally used in calcinations.

• (CaSO₄)₂.H₂O + 3H₂O →2CaSO₄.2H₂O + unreacted (CaSO₄)2.½H₂O + Heat
• Hemihydrate + Water →  Dihydrate + unreacted hemihydrates + 3900 cal/g mol

Microscopic Structure of Set Material:

• Set material composed of an entangled aggregate of gypsum crystals having a length of 5 to 10 µm. Two different types of microscopic porosities are seen in the mass.
• Microporosity caused by residual unreacted water. These voids are spherical and occur between clumps of gypsum crystals.
• Microporosity results from the growth of gypsum crystals.
• These voids are associated with setting expansion and are smaller than the first type.
• They appear as angular spaces between individual crystals in aggregate.