Setting Expansion Of Gypsum

Setting Expansion Of Gypsum

Write a short note on setting expansion.
Answer:

Setting expansion is of two types:

1. Normal settng expansion
2. Hygroscopic setting expansion.

1. Normal Setting Expansion:

All gypsum products show a linear expansion during setting, due to the outward thrust of the growing crystals. Crystals growing from the nuclei not only intermesh but also intercepteach other during growth.

In dentistry, setting expansion may be both desirable and undesirable depending on the use. It is undesirable in impression plaster, dental plaster, and stone as it will result in an inaccurate cast or change in the occlusal relation if used for mounting. Increased setting expansion is desired in the case of investment materials as it helps to compensate the shrinkage of the metal during casting.

• Control of Setting Expansion:
  • Mechanical mixing reduces setting expansion when compared to hand-mixed stone.
  • Increase in W/P ratio reduces the setting expansion.
  • Modifirs generally reduce the setting expansion.
  • Potassium sulphate 4% solution reduces setting expansion from 0.5 to 0.06%.
  • Sodium chloride and borax also decreases setting expansion.

2. Hygroscopic Setting Expansion:

When a gypsum product is placed underwater before the initial set stage, a greater expansion is seen. This is due to hygroscopic expansion. When expansion begins, externally available water is drawn into pores forming in the setting mass and this maintains a continuous aqueous phase in which crystal growth takes place freely.

Under dry conditions, this additional water is not available and as expansion occurs the aqueous phase in the mix is reduced to a fim over the growing crystals. It is greater in magnitude than normal setting expansion.