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Factors Affecting Gypsum Properties: A Summary

This document analyzes factors influencing setting time, expansion, and strength of gypsum products.

Setting Time:

• Water-to-Powder Ratio (W/P): Higher W/P ratios delay setting due to reduced nuclei per volume, slowing crystallization.
• Fineness: Finer hemihydrate particles accelerate setting due to increased dissolution rate and more numerous nuclei.
• Mixing: Extended mixing shortens setting time by exposing more particles to water, forming more nuclei, and distributing them evenly.
• Temperature: Setting is minimally affected between 0-50°C. Above 50°C, setting is retarded, and at 100°C, no setting occurs.
• Impurities: Added gypsum shortens setting time by increasing potential nuclei.
• Retarders & Accelerators: These are the most effective controls for setting time. Retarders (e.g., glue, borax) slow crystallization by reducing hemihydrate dissolution and coating nuclei. Accelerators (e.g., sodium chloride, potassium sulfate) speed up setting by enhancing hemihydrate dissolution, leading to quicker saturation.

Setting Expansion:

• W/P Ratio: Higher W/P reduces expansion due to fewer nuclei per unit volume, leading to less interaction and outward thrust.
• Chemicals (Accelerators & Retarders): Both reduce expansion. Accelerators cause rapid initial crystallization, hindering subsequent growth. Retarders alter crystalline form, resulting in thicker, shorter crystals with less outward thrust.

Strength:

• Excess Water: Wet strength is lower than dry strength due to water reducing cohesion between crystals.
• Drying: Removing excess water increases strength by enhancing cohesion between crystals.
• Porosity: Higher W/P ratios increase porosity and reduce crystal density, impacting strength.
• Mixing Time: Proper mixing enhances strength, but overmixing can weaken it by breaking crystals and hindering interlocking.
• Chemicals (Accelerators & Retarders): Both reduce wet and dry strength by decreasing intercrystalline cohesion.

Tensile Strength: This aspect is not directly addressed in the text.

Summary Table:

Key Message: Understanding the influence of various factors on gypsum properties is crucial for achieving desired performance in construction and other applications.

Factors effecting the setting time

1. W/P ratio: the more water is used for mixing the fewer nuclei they will be per unit volume
   consequently the setting will be prolonged.


2. Fineness: the finer the particle size of the hemihydrate the faster the mix will harden. The rate of
   solution of the hemihydrate will be increased, also the gypsum nuclei will be more numerous and
   therefore a more rapid rate of crystallization will accur.


3. Mixing: the longer the plaster is mixed, the shorter is the setting time. When the powder is
   brought into contact with water some gypsum crystals will be formed as mixing begins more
   particles will be exposed to water and thus form more crystals at the same time the crystals are
   broken up by mixing and they are distributed through out the mixture and result in the formation
   more nuclei for crystallization thus the setting time is decreased.


4. Temperature: there is little change in the setting time between 0-50C but if the temp. exceeds
   50C the setting time will be retarded. As the temp. approaches 100 C no setting will take place.


5. Impurities: if the manufactures add gypsum the setting time will be shortened because of the
   increase in the potential nuclei for crystallization.


6. Retarders and acceleraters: the addition of accelerators and retarders are the most effective
   and practical for control of the setting time.
    Retarder: is the chemical material added to gypsum product to increase the setting time. ex.
   glue borax and gum Arabic. It will reduce the dissolution of hemihydrates and might deposit
   on the nuclei of crystallization and effectively reduce the rate of crystallization so retard the
   setting time.
   Accelerator: is the chemical material added to gypsum product to decrease the setting time.
   ex. Sodium chloride and potassium sulfate in certain concentration. These salts increase the
   rate of dissolution of hemihydrates and thus the saturation of the solution occur more rapidly
   Factors affecting the setting expansion:


7. W/P ratio: the higher the W/P ratio the less the expansion because of fewer nuclei of
   crystallization per unit volume are present than the thicker mixes and since it can be
   assumed that the space between the nuclei will be greater in such case, it allows that there
   will be less growth interaction of the dehydrate crystals with less outward thrust resulting.


8. Addition of chemicals (accelerators and retarders): both will reduce the setting
   expansion. The reduction of the expansion is due to that the initial rate of crystallization is
   so rapid that subsequent growth is resisted with the accelerators. For the retarders the
   crystalline form may be changed and the crystals may become thick and short so the
   thrusting between the crystals is reduced so the expansion is reduced.
   Factors effecting the Strength: The strength of the gypsum increases rapidly as the material hardens after the initial
   setting time. The excess water presents in the set mass affect the strength. Therefore, there
   is wet strength and dry strength. The wet strength is when there is excess left in the mass.
   The dry strength is that when the excess water has been dried in air or in oven in warm
   temperature. The dry strength may be two times greater than the wet strength.

The effect of drying is to remove the excess water between the
crystals. The excess water reduce the cohesion between the crystals
themselves 40% of the strength is due to the cohesive forces between
the crystals in addition to the strength which can be attributed to the
interlocking of the crystals during growth.
The set plaster or stone is porous. The greater w/p ratio the greater
the porosity and the fewer the crystals.
Mixing time also effect the strength. Increase mixing increase the
strength but over mixing will reduce strength because it will break up
the crystals which are formed and will result in less crystal
interlocking.
The addition of accelerators and retarders lowers both the wet and
dry strength. This is due to the reduction in the inter crystalline
cohesion
Tensile strength
Q\factor increase or decrease of sitting time, strength and expansion?

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