Gypsum Seting Time

Introduction
In civil engineering, gypsum plays a role as a building material that contributes to many products such as cement and plaster. In this experiment, we investigated some properties considering the hardening of gypsum, such as the standard water content and setting time.
Aim of the experiment: – Determination of the proper water content to prepare gypsum paste samples – Determination of the final setting time for gypsum paste.

Theory: Standard water content: the amount of water added to gypsum to form a paste that’d allow the pointer of Vicat’s apparatus to descend to a point (30-32) mm above the base of the mold.
Setting time of Gypsum: the amount of time that passes from the moment of adding water to the gypsum paste (would be already determined); to the moment that Vicat’s needle would descend to reach 3 mm above the base of the mold.
Apparatus: Determination of the water content: – Vicat apparatus – Balance – Metal tray – Graduated cylinder
Determination of the setting time for gypsum: – Vicat’s apparatus (the pointer is replaced by Vicat’s needle of 10 mm diameter.
Determination of the water content: 

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A 200 gm sample of gypsum is prepared, and put over a plate, to be added with a specified amount of water.
The gypsum is mixed thoroughly with water for 3 minutes, to form a moist paste.
A part of the paste is poured in the Vicat’s mold until the mold is filled completely with the paste and the surface of the mold is rimmed.
Vicat’s mold is applied to the base of Vicat’s device. The pointer of the device is adjusted to just touch the surface of the mold, then left to fall freely under the influence of its weight.
The reading of Vicat’s pointer is taken. This reading indicates the depth that the pointer has reached above the base of the mold.
The experiment is repeated using a new sample each time; until the standard water, content is reached (Vicat’s needle would reach a height of (30-32) mm above the base of the mold.

Determination of the setting time for gypsum:

A 200 gm sample of gypsum is prepared. The amount of water determined in the previous experiment is added to the sample.
The gypsum is mixed thoroughly with water for 3 minutes, to form a moist paste, and then part of the paste is poured in the Vicat’s mold until the mold is filled completely with the paste and the surface of the mold is rimmed.
Vicat’s mold is applied to the base of Vicat’s device. The needle of the device is adjusted to just touch the surface of the mold, then left to fall freely under the influence of its weight.
The reading of the device’s pointer is taken until we reach a reading of 3mm.
The time needed for the needle of the device would descend to a height of 3mm above the base of the mold is recorded using a stopwatch.

Conclusion
As we can see from the results, the standard water content needed to prepare a standard gypsum paste = 50%, and we obtained a setting time = 4. 16, which compared to other building materials considered quick hardening. This advantage increases the role of gypsum as a component in the Portland cement and plaster industry. [1]
Engineering use: Gypsum is considered very valuable from the economical and industrial side since it can be found a lot in nature. It can be used to manufacture plaster moldings (ceramic industry and engineering industry). Also, gypsum plays a part in the cement industry (for example Gypsum cement and Portland cement). Many new techniques have been developed to get benefit from gypsum and Portland cement. these techniques use both the advantages of gypsum (early hardening, high early strength, enhanced workability) and Portland cement (improved durability in moist conditions), but are free of the deleterious effect of ettringite and thaumasite, which are formed when gypsum and Portland cement interact. Such properties enhance the workability of cement. [2][3]
Reference

[1]:http://www.sciencedirect.com/science?_ob=ArticleURL=B6V2G-4BRRGD8-2=10=1==search=d=c=C000050221=1=0=10=4e71cbde7aba9b4d1338fea79920ab39
[2]:http://www.sciencedirect.com/science?_ob=ArticleURL=B6TWG40GRJTP7=10=1==search=d=c=C000050221=1=0=10=5c7ed60e8eed63f2ad9c9a5412b00080
[3]: http://www.apitco.org/Profiles/Gypsum%20based%20building%20materials.pdf

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