PROS 8120 Study Guide - Final Guide: Calcium Sulfate, Potassium Sulfate, Gypsum

• School: University of Iowa
• Department: Prosthodontics
• Course: PROS 8120
• Professor: Marchini

Document Summary

All gypsum materials have some linear expansion upon setting. 70% (most) of the expansion occurs within the first hour. Reasoning: lateral thrusting of one growing crystal off another. Occurs along with a decrease in volumetric density of the calcium dihydrate. Breaks the arms of the crystals = more and more crystals. Changes in the solubility constant decreases driving force in the setting reaction. Hotter temp: increases ion mobility, offsets the change in solubility. Net effect: above 37 degrees an increase in setting time (faster) Blocks the expansion of the calcium sulfate dihydrate crystals. Leads to a soft, porous surface of the stone. Addition of accelerators and retarders to modify rate of dissolution of hemihydrate. Adding 1% calcium sulfate dihydrate (terra alba), accelerates setting. Higher the water/powder ratio, more delayed setting time (more water, thinner, takes longer to set) Strength values increase from plasters to the die stones.