Consistency and setting time of standard Gypsum paste

Abstract:

There is a specific amount of water should be added to gypsum to have a paste of standard consistence, which called consistency.

Fig 1- using of gypsum in construction in ceiling and decoration

Introduction:

This experiment is divided into two parts: 

1. Determination of the best value of water gypsum ratio to form a paste of standard strength gypsum to use in the preparation test samples setting time .
2. Determination of the setting time for the Gypsum paste.

Objectives

1. Finding the best value of water - gypsum ratio.
2. To determine the setting time of gypsum.

Theory

Standard amount of water is the amount of water which required until the rod penetrates 30 ± 2 mm from the bottom of vicat’s device.

Setting time is the time when we start to add water to the gypsum until the needle of vicat’s device penetrates 3 mm to the bottom of the device, and in this time the gypsum loses its elasticity.

Equipments

• Vicat’s device with needle and rod.
• Stop watch.
• Container.
• Graduated cylinder.
• Sharp edged trowel.
• Digital balance.
• Tray
• 200 g of gypsum as an initial amount
• 100 g of water as an initial amount

Methodology

• Consistency test:

1. 200 gram of gypsum were used for the five trials
2. W-G ratio 50%, 48%, 46%, 44%, 42%.
3. Start the stop watch and Mix the gypsum with measured quantity of water in less than 3 minutes (approximately 2.5 minutes).
4. Put the mixture in vicate apparatus and put the plug on the surface of the mixture and wait until we reach 3 minutes from starting mixing then the plug were left to enter in the mixture.
5. The distance of plugs penetrating from the bottom was seen of the mixture from the entire grade in the vicate apparatus.

These steps were repeated, and in each trial the ratio of the water was changed descending from 50% to 42%, and as the penetrating distance from the bottom of the mixture was 31 mm which it is in the range (30-32) wanted, the test was finished.

• Setting time test:

After the first test was finished the stop watch continued and

1. The plug and needle were substituted.
2. The needle was put on the surface of the mixture, and left to enter in the mixture and repeated the trial in different places on the surface. 
3. When the needle penetrated about 3 mm from the bottom of the mixture, the test finished, and the setting time was determined.

Data and calculation

Weight of the Graduated cylinder: 26 g
Weight of the cylinder + the initial weight of water: 126 g
Weight of the initial amount of gypsum: 200 gWeight of the tray: 99 g
Weight of the initial amount of gypsum + tray: 299 g

 

Table-A: Results of 5 samples in the lap

No. of trial	Weight of Gypsum (g)	Weight of Water(g)	Water percentage %	Penetration from the bottom (mm)
1	200	100	50	0
2	200	96	48	0
3	200	92	46	0
4	200	88	44	25
5	200	84	42	31

Results and discussion

In the Consistency test, it’s clear that at percentage 42% of water, the Penetration from the bottom was 31 mm, which is the required amount (between 30 ± 2) to start the setting time test, and no graph is needed. However, the needed time for the needle to penetrate 3 mm from the bottom was 6 minutes, which called the setting time.

Conclusion

From this experiment the suitable percent of water with gypsum was determined, which equals = 42% to produce standard consistence paste, also setting time was determined, and it lasts for 6 minutes. Since the gypsum hardens quickly, after using this amount of water, and 6 minutes passed, the paste loses its elasticity, which is a very small time compared with the cement mixture.

Sources of errors:                                                                          

1. While mixing water and gypsum the tray wasn’t clean well, and some of the gypsum was lost.
2. The needle was not straight while doing the setting time test
3. Some the paste was lost while putting it in the ring.

Engineering use

Since the gypsum occasionally used in construction purposes, like using it recently in wall boards and tiles, so the mixture of gypsum must be strong as not to fail when it used. engineers provide troubleshooting and advice on maintenance to minimize and avoid the margin of error when the gypsum is used, because it’s fast solidification.