To Determine the Initial and Final Setting Time of Cement Paste by Vicat’s Apparatus with Different Needle Attachments.

To Determine the Initial and Final Setting Time of Cement Paste by Vicat’s Apparatus with Different Needle Attachments.

INTRODUCTION:

It is important to know the initial setting time because of the loss of useful properties of cement if the cement mortar or concrete is placed in moulds after this time. The importance of final setting time lies in the fact that the moulds can be removed after this time. The former defines the limit of handling, and the latter represents the beginning of mechanical strength development.

Apparatus Required:

• Vicat’s apparatus, initial setting needle, final setting needle, mould, glass plate, spatula, bowl, and measuring cylinder.

Materials Required:

• Cement, water, and oil.

THEORY:

Initial and Final Setting Time of Cement: When water is added to cement a chemical reaction known as Hydration takes place leading to the setting and hardening of the cement paste. Setting refers to the stiffening of the cement paste.

Although during setting the paste acquires some strength, for practical purposes it is necessary to distinguish “Setting” from “Hardening”. These two terms by no means represent the same thing. The setting is a chemical action that begins to take place when water is added to the cement and which causes the plastic nature of the cement to disappear slowly; it begins to represent the crystal formation. There are two terms for setting time, the first is the initial setting time and the second is the final setting time. Hardening is the process that starts at the end of the initial set and indicates the growth in strength of the mortar or concrete. Under good curing conditions, the strength increases rapidly.

Method to find out the initial setting of cement.

1. The setting time of cement is measured using Vicat’s apparatus.
2. For the initial set, around a needle with a diameter of 1.13+0.05 mm square in a section with a flat end is used.
3. This needle acting under a prescribed weight is used to penetrate a paste of
   standard consistency placed in the vicat’s mould.
4. The amount of water required for standard paste shall be…… 0.85 x p percent by weight of cement.
5. When the cement paste stiffens sufficiently for the needle to penetrate only to a point 5+1 mm from the bottom, the initial set is said to have taken place.
6. The initial set is expressed as the time elapsed since the mixing water was added to
   the cement to the time when the cement paste starts to set.

Method to find out the final setting of cement.

1. The final setting time is determined by the similar needle fitted with a metal attachment hollowed out to leave a circular cutting edge 5 mm in diameter and 0.5 mm behind the tip of the needle.
2. The final set is said to have taken place when the needle gently lowered to the surface of the paste, makes an impression on it but the circular cutting edge fails to do so.
3. The final setting time is expressed as the time elapsed since the mixing of water was added to the cement to the time when the cement paste started to harden. 4. The initial and setting time of some of the cement is given in the table given below.
   Final setting time = 90+ (1.2 x 1.S.T)

Procedure

1. Take 400 grams of cement and add water content obtained for the factor of normal consistency of cement.
2. Prepare a cement paste of normal consistency and note down the time when the water is added to the cement.
3. Attach the needle of the diameter (1.13+0.05) mm.
4. Place the cement into the mould and remove the excess of it from the surface
   of the mould with the help of a spatula.
5. Place the mould into the Vicat’s apparatus.
6. Leave this paste for about 15 minutes to become stiff.
7. Bring the needle in contact with the paste and release it.
8. Note down the initial reading.
   Now attach the other needle of the final setting and let it penetrate the
   cement paste.
9. Note down the final setting time.
10. Observe the penetration by subtracting the final reading from the initial reading.
11. Repeat this whole procedure for 15 minutes twice and then for 10 minutes till the paste becomes stiff.

Precautions:

1. The experiment should be carried out from 270 C to 320 C
2. The apparatus is free from vibrations
3. The needle for the initial set is not initially bent.
4. The needle should be fixed vertically straight into the movable rod.
5. The needle and plunger should be kept free from adhering cement.
6. The movable rod should be oiled periodically.

Results:

The initial setting time of cement when tested in the lab is found to be

minutes and the final setting time is hours.

Review questions:

1. What is the initial and final setting time of cement?
2. Why the initial setting time of Quick Setting Cement is less than that of Ordinary Portland Cement (OPC)?