Consistency and setting time of standard 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

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

 

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.