Setting time of cement is nothing but the time required for the stiffening of the cement paste.
The setting time of cement has been classified as an initial setting and final setting time. The setting time is not constant for all types of cement; it varies.
Initial Setting Time
When the cement starts to lose its plasticity (semi-liquid state) after mixing with water is known as initial setting time. The time gap between the water addition and the starting of plasticity loss is known as the initial setting time. Generally, the initial setting time of cement is in the range of 30 to 45 minutes.
Final Setting Time
When the cement completely loses its plasticity, it is known as the final setting time.
The time gap between the water addition to complete loss of cement plasticity is known as the final setting time. The maximum setting time of cement should not exceed 10 hours.
How does the cement setting differ from hardening?
Hardening is entirely different from the final setting. In both the initial and final setting times, we can penetrate a needle to a certain depth. But after hardening, it can’t be done.
When shrinkage cracks formation completely goes off after pouring concrete, the application of load could not make any more cracks state known as hardening.
Confusing?
Take an example of Glue Gel (fevikwik gel), the time when you open the cap, the glue starts to lose its semi-solid state, which is called initial setting time. When you place that gel to the correct location, it completely loses its semi-solid state, which is known as the final setting time. After a certain period, it hardens and functions as a binding material known as a hardening state. 🙂
Necessity of Finding the Setting Time
From the glue gel example, we could understand that if we open the glue gel gap to paste, we have to do that before the glue gel dries. Otherwise, the glue loses its stickiness and will not join the paper.
In the same way, before the cement paste loses its plasticity, we must utilize them. Or else the concrete will not become a robust homogenous element and could disintegrate in a short period.
Determining cement setting time helps plan the transport of the mixed concrete and pouring on-site without bleeding and segregation else the mixed concrete becomes harder.
Stages of Setting of Cement
Hydrolysis and Hydration Stage – The hydrolysis is the first stage. The constituents of cement such as C3S, C2S, 3CA1, and 4CAFe hydrate immediately after adding a certain amount of water, which initiates the formation of cement mortar paste.
Colloidisation Stage – In the second stage, the cement mortar becomes a form of a paste which blends and sticks the other concrete or mortar ingredients.
Crystallization Stage – It is the final stage of the cement setting; the cement particles hydrate totally and turn as the stable element form.
Setting Time of Cement Test (By Vicat Apparatus)
The initial and final setting time of the cement can be determined by the method mentioned in IS code 4031 part 5 by the Vicat Apparatus ( IS code 5513 )
Apparatus Required
Vicat Apparatus consists of a plunger and two different needles, which used to penetrate the cement specimen up to the depth of 5mm to 7mm and above.
Weighing balance – Up to measure 1000g with 1g accuracy
Stop Watch
Measuring Cylinder 200 ml
Glass Plate
Vicat Mould
Tray
Trowel
Sample Preparation
Before conducting this test, the required amount of water should be determined, which gives the right consistency of cement paste (P) by the cement consistency test.
Now take 400g of cement sample and add water by 0.85 times of required water for standard consistency.
Water Content (W)
If required water content is W, then w = 0.85 x Standard Consistency
W = 0.85 x 30% (assuming 30% of cement weight water is required for standard consistency).
W = 25.5% (we should add only 25.5 grams of water)
Here we are taking 400g, so required water = 400 x 25.5% = 102 grams of water.
Add required water (102 grams) to the cement. Now start the stopwatch and note the time as T1 (0:00 ).
Mix the cement well to attain the right consistency.
Fill the paste into the Vicat mould smoothly and remove the excess paste. This process should be done within 3 to 5 minutes.
Then place the mould on the Vicat apparatus and set the needle on the top surface of the cement paste.
Now release the needle and allow it freely fall on the specimen for penetration.
Initially, it penetrates the cement specimen completely. Repeat the penetration every 2 minutes until the needle fails to penetrate the sample upto 5mm from the bottom mould. Note the time as T2.
Initial Setting Time = T2 – T1
Final Setting Time
Now remove the needle and attach the annular needle with a circular edge to determine the final setting time.
Now allow the needle to penetrate the cement specimen. In the beginning, the needle and the circular edge makes an impression.
After a certain period, the circular edge will fail to make an impression and note the time as T3.
Final Setting Time = T3 – T1
Test Precautions
Release the needle gently.
Execute the test at room temperature.
The needle should be appropriately cleaned each time.
Record the time of the initial and final setting accurately.
Ensure that there is no vibration occurrence in the testing area.
The specimen should be slightly moved each time to avoid the penetration happening at the same point.
