About Hydration

Calcined plaster reacts with water to become dihydrate gypsum, causing its crystals to interlock and solidify.

CaSO4･1/2H2O+3/2H2O→CaSO4･2H2O

  • 01

    The amount of water (ml, cc, g) per 100 g of calcined plaster is called the water mixing ratio.

  • 02

    When the proportion of α-type hemihydrate gypsum in the calcined plaster increases, the standard water mixing ratio decreases.

  • 03

    For the same calcined plaster, reducing the water-to-plaster ratio shortens the setting time (working time) and increases both strength and expansion.

  • 04

    Lowering the water temperature (slurry temperature) lengthens the setting time (working time) and increases both strength and expansion.

  • 05

    Extending the kneading (mixing) time shortens the setting time (working time) and increases both strength and expansion.

  • 06

    The addition of a third substance other than calcined plaster and water can change the physical properties, such as setting time (working time), strength, and expansion.

  • 07

    Various calcined plaster products are made by combining α-type hemihydrate gypsum and β-type hemihydrate gypsum with the addition of third substances.

  • The solubility of the third substance is important.
    Substances with low solubility, such as colorants, have minimal impact on physical properties when added in small amounts.
    Substances with high solubility can significantly affect the physical properties of plaster, even in small amounts, either alone or in combination.

  • The components of the third substance (type of ions, ion radius, ion charge), pH, the effect on the solubility of gypsum, reactivity to gypsum components, and ion migration speed affect the hydration mechanism of gypsum in a complex way, altering physical properties.

About Maturation

  • When manufacturing calcined plaster industrially, it is generally fired at high temperatures to ensure that no dihydrate gypsum remains and to shorten the firing time.
    This often results in the inclusion of some type III anhydrite.

  • Type III anhydrite is highly reactive and typically converts into hemihydrate gypsum due to moisture in the air.
    It is often sold with excess adsorbed water.
    This condition is known as “Maturation,” and properly matured products have good workability and performance.

  • However, if there is excessive adsorbed water, it becomes over-matured or “cold,” leading to poor workability and rapid setting time, making it difficult to use.
    Therefore, it is recommended to store the product in sealed bags or metal containers after use.

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